EARLY EYE DEVELOPMENT IN VERTEBRATES

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1 Annu. Rev. Cell Dev. Biol : Copyright c 2001 by Annual Reviews. All rights reserved EARLY EYE DEVELOPMENT IN VERTEBRATES Robert L. Chow Program in Developmental Biology, The Research Institute, Hospital for Sick Children, 555 University Avenue, Toronto, Ontario, Canada, M5G 1X8; bobchow@sickkids.on.ca Richard A. Lang Division of Developmental Biology, Department of Ophthalmology, Children s Hospital Research Foundation, 3333 Burnet Avenue, Cincinnati, Ohio 45229; richard.lang@chmcc.org Key Words induction, retina, lens, patterning, specification Abstract This review provides a synthesis that combines data from classical experimentation and recent advances in our understanding of early eye development. Emphasis is placed on the events that underlie and direct neural retina formation and lens induction. Understanding these events represents a longstanding problem in developmental biology. Early interest can be attributed to the curiosity generated by the relatively frequent occurrence of disorders such as cyclopia and anophthalmia, in which dramatic changes in eye development are readily observed. However, it was the advent of experimental embryology at the turn of the century that transformed curiosity into active investigation. Pioneered by investigators such as Spemann and Adelmann, these embryological manipulations have left a profound legacy. Questions about early eye development first addressed using tissue manipulations remain topical as we try to understand the molecular basis of this process. CONTENTS INTRODUCTION Overview of Eye Development Fate Mapping of the Eye Primordia Defining Axes Within the Developing Eye CONSERVATION IN DROSOPHILA EYE SPECIFICATION EYE DEVELOPMENT IS CLOSELY ASSOCIATED WITH NEURAL INDUCTION The First Events of Eye Development Occur Within the Anterior Neural Plate Anterior Neural Plate Transcription Factors are Required for Normal Eye Development GENERATING TWO EYES A Single Eye Field is Bisected by Signals from the Underlying Prechordal Mesoderm /01/ $

2 256 CHOW LANG cyclops is Essential for Bisection of the Eye Field hedgehog is Important for Ventral Neural Specification and May Be Regulated by Cyc DEVELOPMENT OF THE OPTIC VESICLES Establishing Dorsal/Ventral Polarity in the Optic Vesicles Interaction with Surface Ectoderm is Required for Neural Retina Development Lhx2 is Essential for the Transition of Optic Vesicle to Optic Cup FGF Signaling Can Specify Retinal Development Early Patterning of the Neural Retina RPE Specification is Dependent on Signals from Extraocular Mesenchyme Dorsal/Ventral Polarity in the Neural Retina Anterior-Posterior Polarity in the Developing Eye LENS INDUCTION Lens Induction is a Multi-Step Process Pax6 is Essential for Lens Induction Bmp4 and Bmp7 Play Important Roles in Lens Development FGF Receptor Signaling is Required for Lens Induction Sox1, Sox2, and Sox3 Participate in Lens Development fork head Transcription Factors Foxe3 and Lens1 Lie Downstream of Pax The sine oculis Related Transcription Factor Six3 Can Transform Otic Vesicle to Lens Maf Family Members in Lens Development CONCLUDING REMARKS INTRODUCTION Overview of Eye Development The eye is essentially a highly specialized extension of the brain. The first morphological sign of eye development in vertebrates is the bilateral evagination of diencephalon in the early neurula (Figure 1A). In mammals, this is marked by the appearance of the optic pit, whereas in fish and amphibians a bulging of the optic primordia is observed. Continued evagination of the optic primordia leads to the formation of the optic vesicles. These extend towards the overlying, non-neural surface ectoderm that will ultimately give rise to the lens and cornea. Mesenchyme between the optic vesicle and the surface ectoderm (apparent in mammals and chick) is displaced as the two tissues come into close physical contact (Figure 1B). This is a critical period in eye development during which inductive signals between the optic vesicle and the surface ectoderm are thought to exchange. Transmission electron microscopy studies in the rat have shown that during this period of close contact, the optic vesicle and lens placode are tightly associated through a network of collagenous fibrils and cytoplasmic processes (McAvoy 1980). At this stage, the presumptive lens also shows the first morphological signs of development. This is characterized by formation of the lens

3 VERTEBRATE EYE DEVELOPMENT 257 placode, a thickening of the surface ectoderm that comes into contact with the optic vesicle. Molecularly, lens placode formation coincides with the onset of crystallin expression. These protein families are expressed at high levels in a lens preferential manner and are required for generating and maintaining lens transparency (Cvekl & Piatigorsky 1996, Graw 1996, Wistow & Piatigorsky 1988, Wride 1996). Coordinated invagination of the lens placode and the optic vesicle results in the formation of the lens vesicle and a double-layered optic cup and provides the first indication of the final shape of the eye (Figure 1C). The inner layer of the optic cup (facing the lens) forms the neural retina, while the outer layer of the optic cup gives rise to the retinal pigment epithelium (RPE) (Figure 1D). At the ventral extremity of the optic vesicle, the process of invagination forms a groove that runs continuously from the ventral-most region of the neural retina and along the ventral aspect of the optic stalk to the junction with the neural tube (Figure 2C). The point at which the laterally growing edges of the optic cup fuse is known as the choroidal (or optic) fissure. This structure provides a channel for blood vessels within the eye and an exit route for projecting axons (Bron et al. 1997). Formation of a primary lens fiber cell mass from posterior cells of the lens vesicle (i.e., those facing the optic cup) establishes polarity within the lens. This is defined by the presence of an anterior monolayer of proliferating epithelial cells that overlies a posteriorly positioned terminally differentiated fiber cell mass (Figure 1D). Secondary lens fiber cells originate from the equatorial region or transitional zone of the lens in a process that continues throughout adulthood. Fiber cell differentiation is characterized by the onset of fiber cell-specific crystallin expression (e.g., β and γ crystallins in the mouse), elongation, and the loss of organelles including the nucleus, mitochondria and endoplasmic reticulum. Development of the cornea from the surface ectoderm overlying the lens and from migrating, neural crest derived mesenchyme is a highly coordinated, multistep process (Hay 1986). Briefly, it first involves the secretion of a collagen-rich extracellular matrix by the corneal epithelium (ectoderm overlying the lens). This primary stroma attracts a wave of neural crest derived mesenchymal cells from the region surrounding the eye that coincides with its hydration and the migration of a second wave of neural crest derived mesenchymal cells. Eventually, an increased level of thyroxine triggers the dehydration and compaction of the posterior stroma that ultimately leads to the formation of the mature, transparent cornea. The ciliary body and iris are derived from the distal tip of the optic cup at the point where the inner and outer optic cup layers meet (Beebe 1986) (Figure 1C). Although a detailed description of the development of these structures is beyond the scope of this review, it is interesting to note that Otx1, an orthodenticlerelated transcription factor, appears to play an essential role during their development. These structures are missing or reduced in Otx1 loss-of-function mutants, whereas the retina is otherwise normal (Acampora et al. 1996). Within the limits of this review, we have chosen to focus on the better characterized elements of early eye development, and as a consequence, sizeable subject areas have been excluded, including the regulation of lens polarization (Lang 1999,

4 258 CHOW LANG McAvoy et al. 1999), anterior segment development (Beebe & Coats 2000, Kidson et al. 1999, Kume et al. 1998, Pressman et al. 2000), and the mechanism of retinal differentiation (Cepko et al. 1996, Harris 1997, Kageyama et al. 1997). Fate Mapping of the Eye Primordia Fate mapping studies reveal that there is species-to-species variation in the location of the retinal primordium. Studies in amphibians (Brun 1981, Eagleson & Harris 1990, Jacobson 1959, Manchot 1929, Woerdeman 1929) and chick (Couly & Le Douarin 1985, Couly & Le Douarin 1987) have demonstrated that the retinal primordia are positioned in two distinct regions on either side of the midline of the anterior neural plate. Although some degree of lateral cell movement from the anterior neural plate to the anterior neural folds has been observed in amphibians (Brun 1981), studies in Xenopus have shown that cells positioned in the midline of embryos at early neural plate stages do not migrate laterally (Li et al. 1997). In contrast, fate mapping studies performed in zebrafish at the early neural shield stage have suggested that retinal precursors are derived from a single domain in the anteromedial forebrain region (Woo & Fraser 1995). This region is thought to undergo lateral movements as a result of being physically displaced by invading ventral diencephalic precursors (Woo & Fraser 1995). Zebrafish mutants such as trilobite, silberblick and knypek that affect convergent extension movements show defects in this process. Mutations in these genes result in varying degrees of cyclopia, which is thought to result from the inability of ventral diencephalic precursors to divide the presumptive retinal field (Hammerschmidt et al. 1996b, Heisenberg et al. 1996, Heisenberg & Nusslein-Volhard 1997, Marlow et al. 1998, Solnica-Krezel et al. 1996). Defining Axes Within the Developing Eye The developing eye undergoes many complex morphological changes that are sometimes difficult to conceptualize spatially. For example, although the RPE develops from the dorsal optic vesicle (Figures 1B,2A C) as morphogenesis proceeds, presumptive RPE eventually spreads ventrally to completely surround the neural retina (Figure 2D). Because the derivatives of the eye primordium (i.e., optic stalk, neural retina, and RPE) and the patterning events responsible for their formation are essentially determined along the dorsal/ventral axis of the eye, it is convenient to think of the developing eye as a simple neuroepithelial sheet defined by this axis (Figures 2A and 3). As the dorsal regions of the developing eye can also be referred to as distal and the ventral regions as proximal, we herein refer to this axis as the dorso-distal (DD) and the proximo-ventral (PV) axis. Although poorly understood and omitted from our model, one should keep in mind that early patterning events also act along the anterior (nasal) and posterior (temporal) axis of the developing eye (see below). In conceptualizing the developing eye field as a simple PV-DD sheet, the PVmost domain will give rise to the ventral floor of the optic stalk (Figures 2, 3). It

5 VERTEBRATE EYE DEVELOPMENT 259 is arguable that the DD-most region would form the dorsal half of the optic stalk (Figures 2, 3); however, we have chosen to define the DD-most region as that which gives rise to RPE for two reasons. First, while the ventral half of the optic stalk invaginates (Figure 2C), differentiates into glial cells, and forms the tissue through which projecting retinal ganglion cell axons project (Bron et al. 1997), the dorsal half of the optic stalk gives rise to the non-neural tissue that eventually encases the optic nerve. Second, genes that pattern the optic stalk such as Pax2 (Nornes et al. 1990, Torres et al. 1996), Six3 (Oliver et al. 1995), Optx2 (Toy et al. 1998), and Vax1 (Hallonet et al. 1998) are restricted to the PV half of the optic stalk, suggesting that it and not the DD-half of the optic stalk responds to signals involved in patterning of the eye field (see below). While this conceptualization of the PV-DD axis is highly simplified, it does provide a convenient framework for discussing inductive processes within the developing eye (Figure 3). CONSERVATION IN DROSOPHILA EYE SPECIFICATION Much of our understanding of how early developmental genes direct eye formation has come from work on Drosophila eye development. Although the architecture of vertebrate and fly eyes are dramatically different, it is now evident that similar regulatory networks and pathways direct eye development and function in both phyla. This has led to the debated hypothesis that rather than having evolved independently, eyes from different phyla evolved monophyletically from a common ancestral precursor (Callaerts et al. 1997, Fernald 2000, Pichaud et al. 2001). Although the mechanism behind this hypothesis is unclear, it has been proposed that the same regulatory networks and pathways directing eye formation in flies and vertebrates may have been recruited independently throughout evolution, thereby allowing eyes to evolve different strategies for perceiving light (Pichaud et al. 2001). Several transcription factors believed to play a critical role in the early stages of Drosophila eye development have been identified in misexpression studies by their ability to generate ectopic eyes in non-eye imaginal discs. These include eyeless (ey) (Halder et al. 1995) and twin of eyeless (toy) (Czerny et al. 1999), both homologues of the vertebrate Pax6 (Czerny et al. 1999, Quiring et al. 1994), eyes absent (eya), and sine oculis (so) (Bonini et al. 1997, Pignoni et al. 1997), dachshund(dac) (Chen et al. 1997, Pignoni et al. 1997, Shen & Mardon 1997), teashirt (ts) (Pan & Rubin 1998), and optix (opt) (Seimiya & Gehring 2000). A pathway ordering these genes is beginning to emerge (Heberlein & Treisman 2000, Wawersik & Maas 2000) (Figure 4). Genetic and biochemical evidence indicate that Toy may lie at the beginning of this pathway because it can directly regulate the eye-specific enhancer of ey (Czerny et al. 1999). ey has been proposed to be upstream of eya, so, and dac because ey expression precedes the expression of these genes (Bonini et al. 1993, Cheyette et al. 1994, Halder et al. 1995, Mardon et al. 1994). Furthermore, eya and so are not required for ey expression, whereas ey is

6 260 CHOW LANG Figure 4 A self-regulatory network of genes directs Drosophila eye specification. Based on its expression pattern and its ability to generate ectopic eye formation in noneye imaginal discs, twin of eyeless (toy) is believed to act at the apex of the Drosophila eye specification network (indicated by shaded box). Ectopic eye formation, timing of expression and mutant expression studies place eyeless (ey) downstream of toy, followed by eyes absent (eya), sine oculis (so), and then dachshund (dac). Additional studies show that ey, eya, so, and dac can up-regulate one another (reverse arrows). Teashirt and optix appear to function in this network based on their ability to generate ectopic eyes. Drx function in the fly eye has not been established, although its vertebrate homologue, Rx, is essential during early eye development. Notch, Egfr, Wingless, and Hedgehog signaling pathways act upstream and are believed to contribute to the coexpression of genes within Drosophila eye specification network. required for so and eya expression in the eye disc during normal eye development (Halder et al. 1998). dac has been proposed to lie downstream of eya and so based on the observation that dac is not required for eya or so expression and that so and eya are required for dac expression (Chen et al. 1997). While Opt requires so and eya for ectopic eye formation, it does not require ey, suggesting that during ectopic eye formation it can function in a partially different pathway (Seimiya & Gehring 2000). The misexpression studies described above suggest that transcription factors involved in Drosophila eye specification act in a hierarchical manner with toy and ey at the beginning of the pathway. These studies, however, have also shown that misexpression of eya, eya + so, dac, and dac + eya can upregulate ey expression as well as each other s expression (Chen et al. 1997, Pignoni et al. 1997, Shen & Mardon 1997), suggesting the presence of a positive feedback loop within this pathway. This has led to the hypothesis that the transcription factors involved in Drosophila eye specification may function within a self-regulating network that locks into an eye development program (Heberlein & Treisman 2000). The ability of Eya to physically interact with either Dac or So has raised the interesting possibility that

7 VERTEBRATE EYE DEVELOPMENT 261 these factors function as part of a transcriptional complex in vivo (Chen et al. 1997, Pignoni et al. 1997). If a network of transcription factors does control eye specification in the fly, one prediction would be that these genes show overlapping expression domains in the developing fly eye. It has been shown that overlapping expression (although incomplete) occurs between these transcription factors within the eye field between larval stage 1 and 2 (Kumar & Moses 2001). This stage of fly eye development has been shown to coincide with a critical period during which up-regulation of Notch signaling and down-regulation of Egfr signaling in the eye half of the eye-antennal disc is believed to specify eye fates (Kumar & Moses 2001). The obsevation that Hedgehog and Wingless signaling can also participate in eye-antennal fate decisions (Royet & Finkelstein 1996, 1997) has led to the hypothesis that Notch, Egfr, Wingless, and Hedgehog signals function upstream of at least some components of the network of transcription factors controlling eye specification. Signals from these pathways would contribute to the overlapping expression of these transcription factors, which in turn would be predicted to lock into an eye development program (Kumar & Moses 2001) (Figure 4). It will be interesting to see the extent to which regulatory pathways have been shared in eye development throughout evolution. Although beyond the scope of this review, one remarkable finding is the observation that retinal differentiation in vertebrates and ommatidial differentiation in flies both appear to follow a wave of Hedgehog signaling (Neumann & Nuesslein-Volhard 2000). During early eye development, experimental evidence in vertebrates supports the existence of a similar, self-regulatory network of transcription factors controlling eye specification. This has come partly from misexpression studies in which transcription factors believed to play an important early role in vertebrate eye formation, such as Pax6, Six3, Optx2, and Rx, can up-regulate one another s expression and induce ectopic retinal tissue (Andreazzoli et al. 1999, Bernier et al. 2000, Chow et al. 1999, Loosli et al. 1999, Zuber et al. 1999). Several differences exist, however, between the putative regulatory networks directing fly and vertebrate eye specification. For example, Drosophila has undergone a duplication of the ancestral Pax6 gene (Czerny et al. 1999), giving rise to toy and ey, whereas in chick, mice, or humans there appears to be only one Pax6 orthologue. In addition, the role of vertebrate eya homologues, EYA1-4, is unclear. Although EYA1-3 are expressed in the developing eye, mouse Eya1 (Johnson et al. 1999, Xu et al. 1999a), human EYA1 (Abdelhak et al. 1997), and EYA4 (Wayne et al. 2001) are not essential for eye development and instead play essential roles in ear and kidney development (it is interesting to note, however, that missense mutations in EYA1 have been associated with cataractogenesis and anterior segment defects; Azuma et al. 2000). Finally, although the Rx gene plays an essential role early in vertebrate eye formation, its Drosophila homologue, Drx, is not expressed in the eye-antennal disc (Mathers et al. 1997). These observations indicate that not all the components that make up the regulatory network directing eye specification are necessarily conserved in flies and vertebrates.

8 262 CHOW LANG EYE DEVELOPMENT IS CLOSELY ASSOCIATED WITH NEURAL INDUCTION The First Events of Eye Development Occur Within the Anterior Neural Plate While the first morphological indication of eye development occurs with the bilateral evagination of the developing forebrain, the events involved in specifying eye formation begin earlier. Although some early support for this idea came from explant studies in amphibians by Adelmann (Adelmann 1936a,b), the elegant studies of Nieuwkoop on explanted presumptive neuroectoderm from frog late blastulae were especially informative. Normally this type of ectodermal explant (also known as an animal cap) gives rise to non-neural epidermis if cultured alone, but Nieuwkoop discovered that if the cells of these explants were dissociated and then re-aggregated, they differentiated into anterior neural tissue (Nieuwkoop 1963). The important point in these experiments was that these so-called activated explants underwent neural induction in the absence of mesoderm and endoderm (possible inductive sources) and gave rise to anterior brain, olfactory placodes, and eye structures (Nieuwkoop 1963). Recent work probing the molecular basis for neural induction (and revisiting Nieuwkoop s observation) have shown that this process is mediated by molecules such as Chordin, Noggin, Follistatin, Cerberus, and xnr3 (reviewed in Harland 2000, Weinstein & Hemmati-Brivanlou 1999) that antagonize the bone morphogenetic protein (BMP) signaling pathways. When ectodermal explants from Xenopus are microinjected with RNA encoding any of these BMP signaling pathway inhibitors, they behave exactly as Nieuwkoop s explants did and give rise to anterior neural structures including eyes. Because the potential inductive influence of mesoderm or endoderm is absent in these neuralized ectodermal explants, these results demonstrate that the molecular mechanisms directing eye specification lie downstream of neural induction and are inherent to developing anterior neural tissue (symbolized in Figure 3). It should be pointed out, however, that the anterior neural plate is patterned by complex interactions (Papalopulu & Kintner 1996) and, therefore, the derivation of eyes from the anterior neural plate will likely involve more complex mechanisms and interactions. Several of the transcription factors that pattern the anterior neural plate of the early vertebrate embryo such as Pax6, Rx, Six3, and Hesx1/Rpx are essential for normal eye development (see below). Although the expression of these factors lies downstream of neural induction, the precise molecular events that initiate their expression are not well understood. Initiation of Pax6, Six3, and Otx2 (Otx2 expression precedes neural induction) expression occurs in the absence of Rx (Zhang et al. 2000); initiation of Rx, Six3, and Otx2 expression occurs in the absence of Pax6 (Oliver et al. 1995, Stoykova et al. 1996); cell-autonomous initiation of Six3 and Hesx1/Rpx expression occurs in the absence of Otx2 (Rhinn et al. 1998); and initiation of Pax6 and Six3 expression occurs in the absence Hesx1/Rpx (Dattani et al. 1998). Although these observations seemingly contradict others indicating

9 VERTEBRATE EYE DEVELOPMENT 263 that this set of genes can co-activate (see above), it is possible that their transcriptional initiation in the anterior neural plate is regulated independently by currently unidentified factors downstream of neural induction. Although factors that appear upstream of Drosophila eye specification, such as Notch, Egfr, and Hh, have not been implicated in vertebrate eye specification, there is evidence that signaling through Wnts (vertebrate Wingless homologues) may be involved. Studies in Xenopus have shown that misexpression of the Wnt receptor, Frizzled 3 (Xfz3), results in ectopic expression of Pax6, Rx, and Otx2 and leads to ectopic eye formation. Furthermore, inhibition of Wnt signaling by overexpression of a truncated Xfrz3 receptor or intracellular Xfrz3-interacting protein inhibited endogenous eye formation (Rasmussen et al. 2001). These results suggest an early role for Wnt signaling during vertebrate eye development, possibly in the regulation of transcription factors essential for eye formation. Anterior Neural Plate Transcription Factors are Required for Normal Eye Development Otx2 Otx2 is a member of the orthodenticle-related family of transcription factors, which contain a homeodomain of the bicoid class (Simeone et al. 1993). Otx2 expression precedes that of other early eye transcription factors (i.e., Pax6, Six3, Rx) and is expressed in the entire embryonic ectoderm of the mouse prior to gastrulation (Simeone et al. 1993). Although Otx2 is eventually restricted to the anterior end as gastrulation proceeds, double in situ hybridization analysis in Xenopus has shown that in the anterior neural plate, Otx2 is not present in the eye field defined by Rx expression (Andreazzoli et al. 1999). This observation is interesting in light of the finding that Rx misexpression at early neurula stages can repress endogenous Otx2 (Andreazzoli et al. 1999). The down-regulation of Otx2 in the early eye field suggests that it does not participate in early specification. At later stages of eye development, however, Otx2 is expressed in the optic vesicles and eventually becomes restricted to the RPE (Bovolenta et al. 1997, Simeone et al. 1993). In Otx2 / mutants, forebrain and midbrain structures are completely missing, possibly due to a defect in neural induction (Matsuo et al. 1995, Pannese et al. 1995). Therefore, it is difficult to determine the precise role of Otx2 in the developing eye because the defect in these mutants manifests itself well before any morphological signs of eye formation would have occurred. Interestingly, in chimeric embryos in which the neuroectoderm is made up of Otx2 / cells, the expression of the forebrain/eye marker, Six3 and Hesx1/Rpx, is initiated but not maintained (Rhinn et al. 1998), suggesting that Otx2 is not required cell-autonomously for the induction of anterior neural tissue but is required for its maintenance and regional specification (Rhinn et al. 1998). Hesx1/Rpx Hesx1/Rpx encodes a member of the paired-like domain family of transcription factors and is expressed early in the mouse gastrula and anterior neural plate. Hesx1 expression becomes restricted to the developing anterior pituitary

10 264 CHOW LANG (Rathke s pouch) by embryonic day 9.5 (Hermesz et al. 1996). In humans, mutations in HESX1 lead to septo-optic dysplasia, a condition that is characterized by variable phenotypes including optic nerve hypoplasia, pituitary gland hypoplasia, and midline abnormalities of the brain (Dattani et al. 1998). In mice, null mutations of Hesx1 lead to similarly variable anterior CNS defects and pituitary dysplasia, as well as anophthalmia or microphthalmia (Dattani et al. 1998). It has been suggested that Hesx1 is required for establishing the size of the forebrain, perhaps by regulating proliferation in the forebrain primordium. Since Hesx1 expression is later restricted to the pituitary, it has also been suggested that the ocular defects observed in Hesx1 mutants arise as a consequence of the reduction in forebrain size rather than a direct regulatory role by Hesx1 (Dattani et al. 1998). Pax6 Pax6,isa paired box and a paired-like homeobox gene that has maintained an extremely high level of conservation throughout evolution (reviewed in Callaerts et al. 1997, Cvekl & Piatigorsky 1996). Pax6 orthologues have been identified in vertebrates (Hirsch & Harris 1997, Krauss et al. 1991, Li et al. 1997, Martin et al. 1992, Walther & Gruss 1991), flies (Quiring et al. 1994), and squid (Tomarev et al. 1997). Interestingly, Pax6 homologues are also expressed in the sense organ of nematodes (Zhang & Emmons 1995) and in the simple photosensitive ocellus of ascidians (Glardon et al. 1997), which suggests a conserved role for Pax6 in sensory detection. In vertebrates, Pax6 is first expressed toward the end of gastrulation in the anterior neural plate (Del Rio-Tsonis et al. 1995; Grindley et al. 1995; Hirsch & Harris 1997; Li et al. 1994, 1997; Puschel et al. 1992; Walther & Gruss 1991). As neurulation proceeds, Pax6 expression resolves to the DD optic vesicles (Figure 3), as well as to the presumptive lens ectoderm. In the developing optic cup, Pax6 is initially expressed throughout (Grindley et al. 1995), but in the differentiated retina it is expressed strongly only in ganglion and amacrine (Belecky-Adams et al. 1997). In addition to a role in eye formation, Pax6 is essential for normal development in many regions outside the eye, including the nasal epithelium (Hogan et al. 1986), brain (Grindley et al. 1997, Schmahl et al. 1993), pancreas (St-Onge et al. 1997), and lacrimal gland (Makarenkova et al. 2000). Loss-of-function mutations in Pax6 are semi-dominant and in the heterozygous state lead to the Small eye (Sey) phenotype in mice (Hill et al. 1991, Hogan et al. 1986) and rats (Matsuo et al. 1993), which is characterized by cataractogenesis, iris hypoplasia, and microphthalmia. Similar mutations in humans cause the ocular syndrome aniridia (Glaser et al. 1992, Hanson et al. 1993, Jordan et al. 1992, Ton et al. 1991). The idea that Pax6 gene dosage is important for normal eye development is illustrated by studies in which additional copies of the Pax6 locus were introduced in transgenic mice (Schedl et al. 1996). Although a single copy of the Pax6 locus could rescue the Sey (heterozygous) phenotype, mice that had more than two normal copies of the Pax6 locus also displayed defects in eye development (Schedl et al. 1996). This revealed that an excess of Pax6 is detrimental to normal eye development (Schedl et al. 1996) and highlights the importance of Pax6 gene dosage.

11 VERTEBRATE EYE DEVELOPMENT 265 Homozygous mouse or rat Sey mutants are anophthalmic and die at birth (Grindley et al. 1995, Hogan et al. 1986, Matsuo et al. 1993). Optic vesicles are present in Sey/Sey mutants but fail to constrict proximally, resulting in the persistence of a luminal optic stalk. In addition, while the optic vesicles do come into contact with the surface ectoderm in Sey/Sey mutants (Grindley et al. 1995), lens placode thickening does not occur (Grindley et al. 1995, Hogan et al. 1986, Matsuo et al. 1993). Eventually, a structure that resembles the optic cup does form, although there is no indication of differentiation in presumptive neural retina or RPE, and both layers appear identical (Grindley et al. 1995). The ability of Pax6 to induce ectopic eyes supports the idea that it plays a critical role in the early stages of eye development. Misexpression in Drosophila non-eye imaginal discs of eyeless or mouse (Halder et al. 1995), squid (Tomarev et al. 1997), or ascidian (Glardon et al. 1997) Pax6 all lead to ectopic eye formation and reveal the high degree of functional conservation of Pax6 throughout evolution. Pax6 misexpression can also induce fully differentiated ectopic eyes in Xenopus and the ectopic expression of early eye development genes such as Otx2, Rx, and Six3 as well as endogenous Pax6 (Chow et al. 1999). The observation that ectopic eyes form only when Pax6 mrna is microinjected into the dorsal animal pole blastomeres (that give rise to neural and ectodermal tissues) indicates that factors in addition to Pax6 are required for the induction of ectopic eyes. Since Pax6 loss-of-function mutants fail to develop eyes, the use of chimeric studies and conditional gene targeting has been important in determining its role at later stages of eye development (see below). Studies in chimeric mice derived from Sey/Sey and wild-type cells have revealed a cell-autonomous requirement for Pax6 in the lens (as early as embryonic day 9.5) and DD optic vesicle (Collinson et al. 2000, Quinn et al. 1996). In the DD region of the optic vesicle and in the neural retina, wild-type and mutant cells segregate, and there is a failure of the mutant cell patches to differentiate. The use of a conditional Pax6 null allele activated by a Pax6 neural retina enhancer-driven Cre expression has shown that Pax6 plays an essential role later in eye development in maintaining the multipotent state of retinal progenitor cells (Marquardt et al. 2001). In the absence of Pax6 neural retina expression, the only retinal cell types to differentiate are a subset of non-glycinergic amacrine cells. Pax6, however, does not appear to be required for the maintenance of retinal identity in these experiments, as expression of Rx1, Six3, Six6, and Lhx2 was not lost in Pax6-deficient neural retina tissue (Marquardt et al. 2001). Six3/Optx2 Six3 and Optx2 (alternatively named Six6 or Six9) are closely related members of the SIX-homeodomain family (of which sine oculis is the founding member) and share over 90% homeodomain amino acid identity with Optix, another Drosophila SIX-homeodomain gene (Toy et al. 1998). Heterozygous mutations in humans for SIX3 have recently been identified in six cases of holoprosencephaly (Wallis et al. 1999). The presence of severe brain defects and microphthalmia in these cases support the role for SIX3 in the developing anterior neural plate and eye.

12 266 CHOW LANG Six3 and Optx2 have expression patterns suggestive of a role in early eye development. Although both genes are expressed in the gastrula, only Six3 is expressed in the eye field (Bovolenta et al. 1998, Granadino et al. 1999, Jean et al. 1999, Loosli et al. 1998, Oliver et al. 1995, Zuber et al. 1999). At later stages, Optx2 and Six3 are expressed throughout the optic vesicle. Expression of both genes continues in the optic stalk and neural retina (Bovolenta et al. 1998, Granadino et al. 1999, Jean et al. 1999, Loosli et al. 1998, Oliver et al. 1995, Zuber et al. 1999). Whereas Six3 is up-regulated in the lens placode, Optx2 expression appears to be either absent (Jean et al. 1999, Zuber et al. 1999) or down-regulated (Toy et al. 1998) in this region. Evidence from misexpression studies suggests that both Optx2 and Six3 play important roles during retinal determination. In Medaka, misexpression of Six3 results in the conversion of regions of the midbrain and prospective cerebellum to RPE-containing, optic cup like structures, and in the appearance of ectopic Rx, Pax6, and endogenous Six3 expression (Loosli et al. 1999). Furthermore, Six3 misexpression in zebrafish results in an enlargement of the rostral forebrain and an enhanced expression of Pax2 in the optic stalks, consistent with the normal expression of Six3 in this region, and suggesting that it may also play an important role in determining cell fate in the optic stalk (Kobayashi et al. 1998). In chick RPE cells, Optx2 misexpression leads to the expression of Chx10 (see below) and visinin, two neural retinal markers normally excluded from the RPE (Toy et al. 1998). Optx2 misexpression in the eye field of Xenopus embryos results in an expansion of the retinal territory or a dramatic increase in eye size, suggesting that Optx2 may also regulate the proliferation of retinoblasts (Bernier et al. 2000, Zuber et al. 1999). Together, these results suggest important roles for both Six3 and Optx2 in the determination of retinal fates. Rx A essential role in the early events of eye development has been established for the paired-like homeobox gene Rx based on its expression pattern, and gainand loss-of-function phenotypes (Furukawa et al. 1997, Mathers et al. 1997). Rx is first expressed throughout the anterior neural plate, and following neurulation it is expressed most abundantly in the optic vesicles and later throughout the neural retina. By post-natal day 6.5 Rx is restricted to photoreceptor cells and the inner nuclear layer, and by post-natal day 13.5, is no longer detectable. Misexpression of Rx in Xenopus embryos results in the extension of ectopic RPE along the optic nerve region and in the hyperproliferation of the neural retina (Mathers et al. 1997), and in zebrafish results in the expansion of retinal tissue into the forebrain (Chuang & Raymond 2001). These results are consistent with observations that Rx misexpression in Xenopus can expand endogenous Pax6, Six3, and Otx2 expression in the optic region at late neurula stages (Andreazzoli et al. 1999) and suggest that Rx may play an important role in regulating the initial specification of retinal cells and perhaps their subsequent proliferation. Mice homozygous for a null mutation of the Rx gene are anophthalmic (Mathers et al. 1997). In contrast to the phenotype of Sey/Sey mice, in which optic vesicle

13 VERTEBRATE EYE DEVELOPMENT 267 formation does occur (albeit abnormally), optic vesicles fail to develop in Rx / mice. Rx / mutants display a variable forebrain phenotype, which, in extreme cases, leads to the complete loss of forebrain. In less extreme cases, the forebrain is still present, but optic vesicle formation is absent and the region where the optic vesicles would have formed is marked by the absence of Pax6, Six3, and Otx2 expression (Zhang et al. 2000). This observation is particularly interesting with respect to the conditional medaka fish eye mutant eyeless (Winkler et al. 2000) in which optic vesicle evagination also does not occur. In contrast to Rx / mutants, however, the ventral region of the brain (where evagination would have occurred) is patterned by distal eye markers Pax6, Rx1, and Rx2, and the proximal eye marker Vax1 (Winkler et al. 2000). The medaka eyeless mutant demonstrates that the morphological events behind optic vesicle formation can be uncoupled from the events that pattern it. Rx might therefore be predicted to function upstream of eyeless because both patterning and morphogenesis of the optic vesicles are lost in Rx mutants. GENERATING TWO EYES A Single Eye Field is Bisected by Signals from the Underlying Prechordal Mesoderm In the early neural plate, transcription factors critical for eye development (such as Pax6 and Rx) are generally expressed across the entire width of the anterior region. Interestingly, these patterns of expression do not correlate with fate-mapping studies that suggest DD eye fates originate only from the lateral regions of the anterior neural plate. As neurulation continues, however, the expression of some of these transcription factors resolves into two lateral domains. A simple, retrospective interpretation of this observation is that at early neural plate stages, the entire anterior neural plate has the potential to give rise to eye structures. Remarkably, in the late 1800s, sparked by interest in the frequent occurrence of cyclopia in newborns, it had already been hypothesized that a single eye field exists in the early embryo that resolves into two fields as development progresses (Adelmann 1936a). Embryological manipulations provided evidence in support of the single eye field hypothesis. Much of this came from the pioneering work of Adelmann, who, in the late 1920s, demonstrated that transplanted regions of the anterior neural plate midline possess an eye-forming potential (Adelmann 1936a). In addition, Mangold made the striking observation that if the prechordal mesoderm underlying the anterior neural plate of Triton embryos was removed, cyclopic tadpoles would develop (Mangold 1931) (Figure 5A B). This work supported the single eye field hypothesis and also provided some of the first evidence suggesting that the prechordal mesoderm functions to prevent eye formation in the midline. Several recent observations also support the idea that a single eye field exists in the early embryo. Experiments similar to those of Mangold s have recently been repeated in Xenopus (Li et al. 1997) and chick (Pera & Kessel 1997), confirming

14 268 CHOW LANG the proposed inhibitory role of the prechordal mesoderm. Additionally, expression of the transcription factors Pax6 and ET (a member of the T-box family) was examined in these experiments. Pax6 and ET are both normally expressed in a broad domain spanning the dorsal/anterior midline following the completion of gastrulation. During neurulation, expression of these genes is down-regulated in the midline (the future ventral forebrain), which results in the formation of two distinct domains that demarcate the presumptive eye. Removal of the prechordal mesoderm in embryos or in explants of the anterior neural plate results in the persistence of midline expression of Pax6 and ET (Li et al. 1997, Pera & Kessel 1997) (Figure 5C, D). This demonstrated, at the molecular level, the importance of prechordal mesoderm in patterning the anterior neural plate. Furthermore, when donor prechordal plate tissue was grafted beneath the chick presumptive retinal region, Pax6 expression was suppressed (Li et al. 1997, Pera & Kessel 1997). Together, these experiments provide strong evidence supporting the existence of a single eye field early in development that is bisected under the influence of underlying prechordal mesoderm. cyclops is Essential for Bisection of the Eye Field Recent studies in zebrafish have shown that the cyclops (cyc) gene is expressed in prechordal mesoderm and is necessary for the generation of two eyes. Zebrafish with a loss-of-function mutation in cyc are characterized by a deficiency in midline signaling that leads to cyclopia and failure of ventral forebrain formation (Ekker et al. 1995, Hatta et al. 1991, Macdonald et al. 1995). cyc encodes a secreted, Nodal-related member of the transforming growth factor-β (TGF-β) superfamily (Feldman et al. 1998, Sampath et al. 1998). The observation that cyc expression is restricted to the dorsal mesendoderm during gastrulation and to prechordal mesoderm and notochord until early somitogenesis suggests that during normal development, secreted Cyc may signal overlying neural tissue to adopt ventral fates and consequently bisect the eye field (Feldman et al. 1998, Sampath et al. 1998). Although the Cyc receptor has not yet been identified, there is strong evidence that Cyc activity may be regulated by one-eyed pinhead (oep), a novel membraneassociated member of the EGF-CFC family of extracellular factors that includes criptic, cripto, and FRL-1 (Zhang et al. 1998). Mutations in oep result in a failure of prechordal mesoderm to develop and also lead to severe cyclopia (Hammerschmidt et al. 1996a). Although misexpression of cyc mrna cannot rescue oep mutants, misexpression of putative downstream components of the Cyc/Nodal pathway (such as an activated form of the putative Cyc receptor ActRIB or the transcription factor Smad2) can rescue them (Gritsman et al. 1999) (Figure 5E, F). As Oep is membrane associated and expressed in both ventral neuroectoderm and prechordal mesoderm, it has been proposed to function cell-autonomously as a co-factor with prechordal mesoderm-derived Cyc to activate an activin-like signaling cascade. This would in turn initiate ventral forebrain specification and bisection of the eye field (Gritsman et al. 1999) (Figure 3B).

15 VERTEBRATE EYE DEVELOPMENT 269 hedgehog is Important for Ventral Neural Specification and May Be Regulated by Cyc It has been of interest to identify factors within the neuroectoderm that act downstream of Cyc. Members of the Hedgehog (Hh) family of signaling molecules, which have been implicated in the specification of ventral neural tissue are excellent candidates for such factors. hh was originally identified in Drosophila as a segment patterning molecule and has since been implicated in many aspects of Drosophila development, including eye morphogenetic furrow progression (Heberlein et al. 1995, 1993; Ma et al. 1993). In vertebrates, a large body of work has implicated Hedgehog family members in specification of the ventral neural tube (Ericson et al. 1997, Patten & Placzek 2000) and in limb development (Capdevila & Johnson 2000). Like oep, sonic hedgehog (shh), a mammalian hedgehog homologue, is expressed in both the ventral forebrain and underlying prechordal mesoderm (Echelard et al. 1993, Ekker et al. 1995, Krauss et al. 1993, Macdonald et al. 1995, Riddle et al. 1993). In humans, loss-of-function mutations in SHH are associated with holoprosencephaly, a condition that results in abnormal development of midline structures including cyclopia in extreme cases (Belloni et al. 1996, Roessler et al. 1996). Similar phenotypes are also observed in mice lacking Shh; cyclopia presumably occurs as a result of ventral neural tube specification failure (Chiang et al. 1996). Interestingly, shh loss-of-function in the zebrafish mutant, sonic-you, does not display cyclopia, although other defects such as abnormal retinal-tectal projections are observed (Schauerte et al. 1998). One possible explanation for this mild phenotype is that another zebrafish hedgehog family member, tiggy-winkle hedgehog (twhh), may compensate for the loss of shh because the two genes share overlapping expression patterns (Ekker et al. 1995, Macdonald et al. 1995). Gain-of-function experiments demonstrate that Hh activity is likely to play a role in ventralization of the eye fields. For example, misexpression of shh or twhh in wild-type zebrafish embryos results in a reduction of eye pigment, as well as the complete absence of lenses in some cases (Ekker et al. 1995, Macdonald et al. 1995). This coincides with a loss of Pax6 expression and ectopic expression of the PV marker, Pax2, in the DD region of the optic vesicles (Figure 5G, H) (Ekker et al. 1995). This result is consistent with the graded repression of Pax6 by Shh in the developing neural tube (Ericson et al. 1997). A similar phenotype occurs when a dominant-negative subunit of protein kinase A, which has been shown to mimic Hh signaling, is misexpressed in the fish (Hammerschmidt et al. 1996a, Ungar & Moon 1996). Experiments performed in zebrafish support the idea that shh expression in the ventral midline of the neural tube and brain lies downstream of cyc and that Cyc may directly regulate shh. In cyc mutants, shh expression is lost in the ventral diencephalon but not in the underlying prechordal mesenchyme, suggesting that Shh does not regulate its own expression in overlying neural tissue (Barth & Wilson 1995, Krauss et al. 1993). This is further supported by misexpression studies showing that Shh is not sufficient to induce medial floor plate cells (i.e., ventral

16 270 CHOW LANG neural tube) (Schauerte et al. 1998) and is a poor inducer of shh expression in neuroectoderm (Muller et al. 2000). cyc misexpression, however, can act in a cell non-autonomous manner to rescue shh expression in cells of the ventral brain and floorplate of cyc mutants (Sampath et al. 1998). In addition, misexpression of constitutively active or dominant-negative downstream components of the Cyc signaling pathway result in either ectopic up-regulation or down-regualtion, respectively, of shh in the neural ectoderm in a cell-autonomous manner (Muller et al. 2000). The possibility that Cyc directly up-regulates shh expression has been suggested from electroporation studies in chick neural tubes. In these studies, it was shown that there is at least one enhancer present in an upstream region of the shh gene that is able to direct expression to ventral neuroectoderm and is responsive to either Cyc or the downstream Cyc signaling molecules Smad2 and FAST-1 (Muller et al. 2000). Although Shh signaling from the ventral midline may play an important role in ventralizing the eye field, other sources of Hh may also be involved. Evidence for this comes from an analysis of loss-of-function mutants of the fork head domain transcription factor encoded by BF-1/Foxg1, which display an extreme dorsalization of the eyes such that the optic stalk is completely missing and is replaced by an expanded retina (Huh et al. 1999). Although Shh expression was normal in the ventral midline of the diencephalon, Shh expression in the ventral telencephalic neuroepithelium, which first appears between E9.5 and E10, was absent in BF-1 mutants. This relatively late domain of Shh expression may therefore play a critical role in the ventral patterning of the optic vesicle (Huh et al. 1999) in addition to the earlier role of Shh derived from the ventral midline. In summary, the anterior neural plate of the early embryo appears to possess a single eye field that, if not resolved into two, will result in cyclopia. Important ventralizing signals coming from prechordal mesoderm, such as Cyc, may induce hh (sonic, tiggy-winkle) expression in overlying neural midline tissue. Hedgehog, in turn, specifies proximo-ventral eye fates and leads to the repression of genes such as Pax6 that direct DD eye development. Additional elements of these pathways (Figure 3) presumably remain to be identified. DEVELOPMENT OF THE OPTIC VESICLES Establishing Dorsal/Ventral Polarity in the Optic Vesicles Regardless of the source, Hh signaling as described above plays a critical role in specifying the ventral region of the developing eye. Patterning of the ventral optic vesicles is highlighted by the genes such as Pax2 (a paired homeobox gene) (Dressler et al. 1990, Nornes et al. 1990), Vax1 (Hallonet et al. 1998), and tailless (an orphan nuclear receptor) (Hollemann et al. 1998). While several studies have used Pax2 as a downstream marker of Hh signaling (Ekker et al. 1995, Hammerschmidt et al. 1996a, Macdonald et al. 1995), the exact molecular mechanisms responsible for the initiation of Pax2 expression are not known.

17 VERTEBRATE EYE DEVELOPMENT 271 Pax2 plays an important role in establishing proximo-ventral character in the developing eye. This is supported by the observation that a frameshift mutation in Pax2 is present in families with heritable optic nerve colobomas (Narahara et al. 1997; Sanyanusin et al. 1995a,b; Schimmenti et al. 1995, 1997). Similarly, genetargeted mice lacking Pax2 (Torres et al. 1996) and KRD (kidney renal defects) mice harboring a naturally occurring mutation of Pax2 (Favor et al. 1996, Keller et al. 1994, Torres et al. 1996) have similar optic nerve abnormalities, consistent with a role for Pax2 in proximo-ventral eye development. These include coloboma (failure to close) of the ventral fissure of the eyes, an extension of the RPE into the optic nerve region, and a failure of optic chiasma to form. The boundary between the presumptive optic stalk and neural retina in the optic vesicles may form as a result of reciprocal transcriptional repression between Pax6 and Pax2 (Schwarz et al. 2000). Indirect evidence for this comes from the observation that there is a dorsal expansion of Pax2 expression in Pax6 mutants and a ventral expansion of Pax6 expression in Pax2 mutants (Schwarz et al. 2000). Furthermore, gel shift and β-galactosidase-reporter assays have shown that Pax2 and Pax6 can reciprocally bind and inhibit the activity of upstream regulatory regions in the Pax6 and Pax2 genes, respectively (Schwarz et al. 2000). These results have led to the prediction that following the activation of Pax2 expression in the ventral optic region (presumably somewhere downstream of hh), Pax2 and Pax6 expression would initially overlap (Figure 3B, C). Eventually, owing to the reciprocal transcriptional repression of these genes, they would resolve into two distinct expression domains thereby establishing a distinct boundary at the optic stalk neural retina junction (Figure 3D) (Schwarz et al. 2000). InteractionwithSurfaceEctodermisRequired for Neural Retina Development Formation of the neural retina follows a transient period of close contact between the DD region of the optic vesicle and the surface ectoderm/presumptive lens. Early evidence that the surface ectoderm might play a role in neural retina specification was provided by the observation that in tissue explantation experiments, neural retina would not develop if surface ectoderm was removed (Holtfreter 1939). More recent experiments have confirmed this observation (Hyer et al. 1998, Nguyen & Arnheiter 2000) In addition, it was shown that if the amphibian optic cup was rotated by 180 such that the presumptive RPE was now facing the surface ectoderm, it would transform into a secondary neural retina (Detwiler 1953, Dragomirov 1937, Lopashov & Stroeva 1964, Mikami 1939). These observations indicate a critical role for the surface ectoderm in directing neural retina formation. Interestingly, transplantation experiments have indicated that lens-to-retina signaling is important for retinal maintenance in the mature fish eye, and its absence is a step in the evolution of a species of blind cave fish (Yamamoto & Jeffery 2000).

18 272 CHOW LANG Lhx2 is Essential for the Transition of Optic Vesicle to Optic Cup Lhx2 encodes a LIM homeodomain-containing transcription factor essential for eye development. Within the eyes, Lhx2 is expressed in the optic vesicles and is subsequently localized to the neural retina where it becomes restricted to the inner nuclear layer of the mature retina (Porter et al. 1997, Xu et al. 1993). Loss of Lhx2 function in knock-out mice results in anophthamlia. Although optic vesicle formation occurs in these mice, optic cup and lens placode formation fails to occur (Porter et al. 1997). The timing of the Lhx2 mutant phenotype suggests that Lhx2 plays a critical role during the period of close contact between the surface ectoderm and optic vesicle. Little is known, however, about the downstream and upstream targets of Lhx2. The observation that Pax6 expression is not affected in the arrested optic vesicles of Lhx2 / mutants and that Lhx2 expression is not affected in Sey/Sey mutants has led to the suggestion that Lhx2 and Pax6 are independently essential for normal development of the eye (Porter et al. 1997). FGF Signaling Can Specify Retinal Development Considerable evidence suggests that members of the fibroblast growth factor (FGF) family play a role in signaling neural retina formation during the period of close contact. Much of this evidence has come from studies demonstrating that presumptive chick or frog RPE can transdifferentiate into neural retina when cultured in the presence of FGF (Guillemot & Cepko 1992, Opas & Dziak 1994, Park & Hollenberg 1989, Pittack et al. 1991, Reh et al. 1991, Vogel-Hopker et al. 2000). Furthermore, addition of FGF2 neutralizing antibodies to cultured chick optic vesicles can inhibit neural retina development (Pittack et al. 1997). In contrast, neural retina development can be rescued in explants of the optic vesicle in which the overlying surface ectoderm is removed by the addition of FGF-coated beads, FGF-secreting fibroblasts, or injection of FGF retroviral expression vectors (Hyer et al. 1998, Nguyen & Arnheiter 2000). Although activity of exogenously applied FGF suggests a role in neural retina induction, compound null mutants of two Fgfs (Fgf1 and Fgf2) known to be present in the surface ectoderm (de Iongh & McAvoy 1993, Nguyen & Arnheiter 2000, Pittack et al. 1997) have no overt eye phenotype (Miller et al. 2000). One possible explanation is that there are other Fgf family members or non-fgf ligands (Kinoshita et al. 1995) expressed in surface ectoderm. Another more interesting possibility is that non-fgf signaling from the surface ectoderm may activate autocrine FGF signaling within the presumptive neural retina. The upregulation of Fgf15 expression in the neural retina region around the time of close contact (Nguyen & Arnheiter 2000) suggests that this factor is a good candidate for fulfilling this function (Figure 3B).

19 VERTEBRATE EYE DEVELOPMENT 273 Early Patterning of the Neural Retina The earliest known transcription factor expressed in the presumptive retinal region upon close contact with the surface ectoderm is encoded by the paired-like homeobox gene Chx10 (Liu et al. 1994). Chx10 is essential for normal eye development, as mutations in humans and mice lead to microphthalmia, cataracts, and abnormal iris development (Burmeister et al. 1996, Ferda Percin et al. 2000). Chx10 expression first appears in the presumptive neural retina during the period of close contact between the surface ectoderm and optic vesicle. Explant studies support the idea that Chx10 expression in the presumptive neural retina occurs in response to inductive signals from the presumptive lens ectoderm (Nguyen & Arnheiter 2000). The appearance of differentiated retinal cell types (with the exception of bipolar cells) in Chx10 mutants suggests that Chx10 does not play a major role in specifying cell fates within the neural retina. Instead, Chx10 appears to regulate cell proliferation because mouse Chx10 mutants are characterized by a reduction in the proliferation of retinal progenitor cells (Burmeister et al. 1996). In mice, the basic helix-loop-helix transcription factor Mitf (microphthalmia associated transcription factor) is initially expressed throughout the dorsal optic vesicle, but is down-regulated in the presumptive retinal region shortly after the onset of Chx10 expression (Bora et al. 1998). Interestingly, the dorsal RPE region of some mouse Mitf loss-of-function mutants transdifferentiates into neural retina, suggesting that Mitf down-regulation is required for retinal specification (Bumsted & Barnstable 2000). It has been speculated that formation of the secondary, transdifferentiated neural retina in Mitf mutants depends on signals such as BMP7, which are expressed on the dorsal side of the optic cup (Nguyen & Arnheiter 2000, Wawersik et al. 1999). If this were the case, then one might also predict a similar role for BMP signaling during formation of the normal neural retina. This possibility is interesting in light of the observation that Bmp7 / mutants are anophthalmic. These mutants are believed to harbor a defect that manifests itself during the period of close contact between the optic vesicles and surface ectoderm (Luo et al. 1995, Wawersik et al. 1999). RPE Specification is Dependent on Signals from Extraocular Mesenchyme A large body of work has focused on the role of surface ectoderm and FGF in neural retina formation; however, very little is known about the role of the mesenchyme surrounding the eye. The transcription factors Foxc1/Mf1 and Foxc2/Mfh1 are expressed in the extraocular mesenchyme, but neither gene plays an essential role in early eye development (Bellusci et al. 1997, Kidson et al. 1999, Kume et al. 1998). Explant studies in chick suggest that extraocular mesenchyme may play an important role in RPE development (Fuhrmann et al. 2000). In optic vesicle explants lacking extraocular mesenchyme, initiation of RPE markers such as Mitf (which in the chick does not show an initial pan-optic

20 274 CHOW LANG vesicular expression pattern as it does in the mouse) does not occur. Moreover, the expression of the neural retinal marker Chx10 (already up-regulated at the time of explantation) expands throughout the entire explant, suggesting that extraocular mesenchyme is required for both RPE development and for inhibition of neural retina development. The ability of exogenously applied activin A to rescue Mitf expression and RPE development in explants lacking mesenchyme makes it a good candidate for a secreted mesenchymal factor that induces RPE (Fuhrmann et al. 2000). This possibility is supported by the presence of activin type IIA and IIB receptors in the optic vesicle (Stern et al. 1995) and activin βa and βb subunits in extraocular mesenchyme (Dohrmann et al. 1993, Feijen et al. 1994, Fuhrmann et al. 2000). Dorsal/Ventral Polarity in the Neural Retina Several genes display restricted expression domains along the dorso-ventral axis of the neural retina (Figure 3D). Pax2 (Dressler et al. 1990, Nornes et al. 1990) and Vax2 (Barbieri et al. 1999, Schulte et al. 1999) (an Emx-related homeobox gene) are both expressed early in the ventral half, whereas Xbr-1 (Papalopulu & Kintner 1996) (a Xenopus homeobox gene) and Tbx5 (a T box gene) (Gibson- Brown et al. 1998, Isaac et al. 1998, Logan et al. 1998) are restricted to the dorsal half. Misexpression of Vax2 in the chick dorsal neural retina leads to the ectopic expression of the ventral markers Pax2, EphB2, and EphB3 (EphB2 and EphB3 are receptor tyrosine kinases), and the down-regulation of the dorsal markers Tbx5, EphrinB2, and EphrinB3 (Ephrins B2 and B3 are Eph ligands) (Schulte et al. 1999). In contrast, Tbx5 misexpression in the ventral neural retina leads to the down-regulation of both Vax2 and Pax2 and the ectopic induction of EphrinB2 and EphrinB3 (Koshiba-Takeuchi et al. 2000). In both cases, Vax2 and Tbx5 misexpression leads to the abnormal projection of dorsal and ventral retinal ganglion cells, indicating that retinal cell fate has been altered (Koshiba-Takeuchi et al. 2000, Schulte et al. 1999). These experiments suggest that Vax2 and Tbx5 play important roles in specifying ventral and dorsal fates and also highlight the potential role of Ephrin signaling in the determination of dorsal/ventral polarity in the neural retina. Experiments in zebrafish and mice have provided evidence suggesting that retinoic acid (RA) plays a role in establishing ventral polarity within the developing optic cup (Figure 3D). This is illustrated by the ability of RA-soaked beads to induce a secondary choroidal fissure in any region of the zebrafish optic cup (Hyatt et al. 1996). RA treatment also results in an enlarged optic stalk and in the expansion of Pax [b] (a zebrafish homologue of Pax2) expression in dorsal regions of the eye (Hyatt et al. 1996). Interestingly, experiments using retinoid-responsive transgenic reporter mice revealed prominent dorsal neural retina expression at early stages of development (E11.5) that relocates to ventral regions at later stages (E14.5) (Schulte et al. 1999). In contrast to zebrafish data, this study also showed that although RA is important for the morphological changes involved in choroidal fissure formation, it can not ventralize the neural retina (Schulte et al. 1999). While

21 VERTEBRATE EYE DEVELOPMENT 275 these differing results suggest the possibility of species or experimental differences, they still demonstrate an important role for retinoids in defining cell fate along the dorso-ventral axis. This is further supported by expression studies showing the prominent expression of genes involved in retinoid synthesis in the ventral retina (Grun et al. 2000, Li et al. 2000, Mic et al. 2000, Suzuki et al. 2000). A role for RA in establishing ventral characteristics of the mammalian eye has also been demonstrated in RA receptor loss-of-function mutants (Kastner et al. 1994). RA receptors are developmentally important ligand-inducible transcription factors that are divided into two classes: the RARs (which bind to all-trans-ra and 9-cis-RA) and the RXRs (which only bind 9-cis-RA) (Chambon 1996). Rxrα / mice are characterized by a reduction in size of the ventral retina and by the failure of the optic fissure to close, which results in coloboma of the optic nerve (Kastner et al. 1994). Furthermore, in several Rar and Rxr double-knockout mice, these ventral phenotypes are accentuated by additional defects including the bilateral eversion of the ventral retina and the absence of ventral iris (Kastner et al. 1994). Anterior-Posterior Polarity in the Developing Eye The developing optic vesicles and neural retina also display patterning along the anterior (nasal) posterior (temporal) axis. This is evident at the time of optic evagination for the genes encoding the winged-helix transcription factors BF-1/Foxg1 (see above), which is restricted to the anterior half, and BF-2/Foxd2, which is restricted to the posterior half (Hatini et al. 1994). In BF-1-deficient mice, ectopic BF-2 expression is present in the anterior optic vesicle (Huh et al. 1999), suggesting that these two genes may regulate each other in the same manner as Pax2 and Pax6 (see above). Two homeodomain transcription factors of the HMX family, SOHo1 and GH6, are expressed in the anterior neural retina (Deitcher et al. 1994, Stadler & Solursh 1994). Misexpression studies in chick have shown that while either SOHo1 or GH6 can down-regulate the expression of EphA3, which is normally restricted to the posterior neural retina, neither has major impact on the positional identity of the temporal retina as assessed by mapping of retinal ganglion axons to the optic tectum (Schulte & Cepko 2000). Therefore, unlike Tbx5 and Vax2, which appear to be sufficient to specify dorsal/ventral fates, it is unclear whether SOHo1 and GH6 function in specifying anterior/posterior fates in the neural retina. LENS INDUCTION In 1901, Spemann made the profound observation that ablation of the presumptive retinal region of neural plate-stage Rana temporaria embryos resulted not only in the absence of retinal development but also in the loss of lens formation (Spemann 1901). As early fate mapping had shown that the lens was derived from a region outside of the one ablated, Spemann argued that development of the lens depended

22 276 CHOW LANG on presumptive retina. This observation not only illustrated the importance of tissue interactions during eye development but also provided the first experimental evidence leading to the idea of embryonic induction (Hamburger 1988). Spemann s conclusions on lens induction rapidly became controversial, however, with the observation by Mencl (1903) that a mutant salmon without an optic cup could still generate lenses. Indeed, to date there remain a series of studies in which optic cup ablation experiments have produced different outcomes, all leading to the idea that there is genuine species-to-species variation in the timing with which lens induction signals are delivered. Lens Induction is a Multi-Step Process Using modern techniques, Grainger and colleagues have reexamined the role of early tissue interactions in lens induction and have operationally defined several stages that correspond to (a) a period of lens-forming competence in the mid/late gastrula ectoderm; (b) the acquisition of a lens-forming bias throughout the head ectoderm during neurulation; (c) specification of lens cell fate toward the end of neurulation; and (d ) differentiation, an aspect of lens development that continues throughout life (Grainger 1992). A brief period of lens forming competence is maximal in ectoderm of the midto-late gastrula and has been defined based on the ability of ectoderm to form lenses when transplanted to the presumptive lens-forming region of neural plate stage embryos (Servetnick & Grainger 1991). Lens-forming competence may be a cell-autonomous event since its timing appears to remain unchanged in ectoderm that is pre-cultured in isolation (Servetnick & Grainger 1991). The acquisition of a lens-forming bias throughout the head ectoderm is thought to be mediated, in part, by planar signals derived from the anterior neural plate, which are qualitatively different from those provided by the optic vesicle (Grainger 1992, Grainger et al. 1997). Evidence for this comes from experiments demonstrating that competent ectoderm displays a stronger lens-induction response when grafted to the lens-forming region of neural plate stage embryos than when grafted to the lens-forming region overlying the optic vesicle (Henry & Grainger 1990). The lens-inducing activity of the anterior neural plate may be enhanced by dorsal mesoderm but inhibited by ventral mesoderm (Henry & Grainger 1990), consistent with the proposal that lens induction is the result of cumulative inductive signals from several tissue types (Jacobson 1966). Grafting experiments similar to those performed by Lewis (Lewis 1904) showed that the optic vesicle is able to induce ectopic lenses in head ectoderm outside of the presumptive lens region, but cannot do so in the flank, suggesting that the entire head ectoderm of neural tube stage embryos has a lens-forming bias (Grainger et al. 1997). Lens specification is defined as the ability of ectoderm to form lens tissue when cultured in isolation and is usually characterized by the formation of small, crystallin-expressing, lens-like structures referred to as lentoids. In Xenopus, lens specification is established just as the optic vesicles come into contact with the

23 VERTEBRATE EYE DEVELOPMENT 277 overlying lens ectoderm (Henry & Grainger 1990). In mouse and chick, this has been reported to occur before close contact (Karkinen-Jaaskelainen 1978), although a more recent report suggests that specification occurs after the contact between the optic vesicle and the surface ectoderm is established (Furuta & Hogan 1998). The embryological manipulations described in this section indicate that there are multiple stages to lens induction. The current challenge is to determine which molecular events accompany these distinct stages. Pax6 is Essential for Lens Induction Pax6 expression in the presumptive lens region of vertebrates is first detected at early neurula stages (Grindley et al. 1995; Li et al. 1997, 1994; Walther & Gruss 1991). In Xenopus, this expression appears shortly after that of Pax6 in the anterior neural plate and is clearly visible as a distinct band of expression lateral and adjacent to the region of Pax6 expression in the anterior neural plate (Li et al. 1997). Pax6 expression continues throughout the surface ectoderm overlying the optic vesicles, but as the lens begins to differentiate, it becomes restricted to the proliferating lens epithelial cells (Grindley et al. 1995). This is consistent with the involvement of PAX6 in the activation of crystallins such as αb-, αa-, δ1-, and ζ -crystallins expressed in the lens epithelium (Cvekl et al. 1995a,b, 1994; Richardson et al. 1995) and in the repression of the fiber cell specific βb1- crystallin (Duncan et al. 1998). Expression of Pax6 in the developing lens is directed, in part, by a 341 base pair enhancer located in the 5 region 4.6 kb upstream of the Pax6 gene (Williams et al. 1998). This enhancer is highly conserved in the Pax6 gene of mouse, humans, and puffer fish (Fugu) (Kammandel et al. 1999, Williams et al. 1998), raising the possibility that it has played an important role during the evolution of lenslike structures. In reporter transgenic mice, this enhancer first begins to direct expression in a few cells of the surface ectoderm overlying the optic vesicles at around the time of close contact (Williams et al. 1998). Following separation of the lens vesicle, enhancer-mediated expression continues in surface ectoderm destined to become corneal epithelium (Kammandel et al. 1999, Williams et al. 1998). At later stages, enhancer-dependent expression is observed in the epithelia of the lacrimal gland and conjunctiva (Kammandel et al. 1999), and in the fully formed lens, expression becomes restricted to the undifferentiated epithelium (Kammandel et al. 1999, Williams et al. 1998). At all stages, the expression pattern coincides with the normal expression pattern of Pax6 in the lens placode and its derivatives (Grindley et al. 1995). Several observations suggest a cell-autonomous requirement for Pax6 in the developing lens. In animals chimeric for Sey/Sey and wild-type cells, Sey/Sey cells are excluded from the presumptive lens at E9.5 (Collinson et al. 2000) and from the maturing lens at E12.5 (Quinn et al. 1996). Furthermore, in explant recombination experiments using the rat Pax6 mutant rsey, presumptive lens ectoderm from rsey/rsey animals did not form lenses when cultured with wild-type optic vesicles,

24 278 CHOW LANG whereas wild-type presumptive lens ectoderm did when recombined with either wild-type or rsey/rsey optic vesicles (Fujiwara et al. 1994). This experiment demonstrates an essential, autonomous component for Pax6 in the developing lens and also shows that rsey mutant optic vesicles retain their lens-inducing capacities. In Xenopus ectodermal explants, Pax6 misexpression can lead to the cellautonomous formation of β-crystallin-expressing lentoids in the absence of neural or mesodermal tissues, which are possible lens-inducing sources (Altmann et al. 1997). The majority of ectopic lenses in Pax6-misexpressing explants and embryos are not fully formed, suggesting the requirement of additional lens differentiating factors (Altmann et al. 1997). Conditional mouse mutants in which Pax6 function in the prospective lens ectoderm is lost at the onset of lens placode formation have confirmed the requirement for Pax6 in lens development (Ashery-Padan et al. 2000). These mutants were characterized by the absence of lens placode formation, as well Six3 and Prox1 expression in the tissue that would have given rise to the lens placode. This suggests that Six3 and Prox1 lie downstream of Pax6 placode in the lens pathway (Figure 6). Expression of Sox2, however, was present in the surface ectoderm overlying the optic vesicles at E10. Although Sox2 expression is lost in the surface ectoderm of Sey/Sey unconditional mutants at E9.5, it remains unclear whether Pax6 is essential for the low level of Sox2 expression in the preplacode (Figure 6) (Furuta & Hogan 1998, Wawersik et al. 1999). Figure 6 A model for the genetic pathway regulating lens induction. The arrows indicate genetic interactions. Where the arrows are gray, this indicates a possible pathway (i.e. inferred from expression pattern or misexpression studies) rather than a demonstrated pathway element (black). At the apex of the hierarchy is the pre-placodal phase of Pax6 expression (Pax6 pre-placode ). It is understood that the later phase of Pax6 expression in the lens placode (Pax6 placode ) is dependent upon earlier activity of Pax6. The genetic pathway described reflects this interaction. Pax6 placode is apparently regulated by at least two enhancers. One has been identified and characterized (ectoderm enhancer), whereas the existence of the other (enhancer 2) is implied. Because FGF and Bmp7 signaling cooperate to maintain the placodal phase of Pax6 expression, it follows that their input to the pathway must be upstream of Pax6 placode. Previous analysis has shown that the early phase of Pax6 expression is unaffected in Bmp7 null mice, and thus FGF and Bmp7 signaling must converge on the pathway downstream of Pax6 pre-placode. Based on the absence of Foxe3 expression in mice carrying a deletion of the Pax6 ectoderm enhancer (designated Pax6 EE/ EE ), Foxe3 is downstream of Pax6 placode. The similar phenotypes (reduced lens lineage proliferation and lens vesicle closure and separation failure) of the dysgenetic lens (Foxe3) and Pax6 EE/ EE mutant mice argue that Foxe3 is upstream of these cellular responses. Although we do not understand the genetic relationship between Foxe3 and Sox2, it is clear from this and previous analyses that Sox2 lies downstream of Pax6 placode. Because Sox2 (but not Pax6) expression is diminished in Bmp4 null mice, Bmp4 signaling must contribute to the pathway between Pax6 placode and Sox2.

25 VERTEBRATE EYE DEVELOPMENT 279 Studies of the Sey Neu mutant have defined two distinct phases of Pax6 expression. The Sey Neu allele is a point mutation, and as a result, it is still possible to follow transcription of Pax6 using in situ hybridization. Transcription in the head ectoderm is undisturbed in the absence of functional Pax6 product, but Pax6 expression later in the presumptive lens region is absent (Grindley et al. 1995). This indicates that the late phase of Pax6 expression (defined as Pax6 placode ) depends on the activity of Pax6 in the head ectoderm (a phase defined previously as Pax6 pre-placode ) (see Wawersik et al. 1999). The genetic pathway for lens induction thus includes two distinct phases of Pax6 expression (Figure 6). Based on an understanding of the activity of the Pax6 upstream ectoderm enhancer, it is reasonable to propose that this element is responsible, at least in part, for the placodal phase of Pax6 expression. This has been borne out by genetargeting experiments in which the enhancer has been deleted (P. Dimanlig & R.A. Lang, submitted). Interestingly, lens development in homozygous deletion mice (designated Pax6 EE/ EE ) was not prevented completely but was inhibited

26 280 CHOW LANG at all stages. The lens placode was thinner, the lens pit smaller than usual, and the level of proliferation in the lens lineage was diminished. However, Pax6 immunoreactivity was still detectable in the lens placode, although at a lower level than in wild-type. This suggests that besides the upstream ectoderm enhancer, at least one other transcriptional control element regulates the placodal phase of Pax6 expression. Thus we can extend the model of the genetic pathway controlling lens development to include at least two placodal phase enhancers (Figure 6). We predict that a continuation of studies that have identified transcription control elements for the Pax6 gene (Kammandel et al. 1999, Williams et al. 1998, Xu et al. 1999b) will identify an additional element with activity in the lens lineage. Bmp4 and Bmp7 Play Important Roles in Lens Development Bmp4 and Bmp7 display overlapping expression patterns within the developing eye and appear to play important roles in the early stages of lens development. The consequence of Bmp7 gene targeting in homozygous mice is a variably penetrant phenotype that ranges from mild microphthalmia to anophthalmia (Dudley et al. 1995). In severely affected null animals, the lens placode fails to form, and the placodal phase of Pax6 expression is lost (Wawersik et al. 1999). This has led to the suggestion that Bmp7 plays an important role upstream of Pax6 during lens placode formation and specification (Wawersik et al. 1999). Furthermore, the timing of this proposed role for Bmp7 in lens induction and in the control of Pax6 expression in the lens placode argues that Bmp7 signaling during this period of lens development is directed toward activating the Pax6 enhancers active in placodal ectoderm (Figure 6). Bmp4 null mouse embryos do not survive past E10.5 (Furuta & Hogan 1998). Despite this, explant culture studies have shown that lens formation from the presumptive lens ectoderm of null mutants could be rescued if cultured in the presence of wild-type optic vesicles (Furuta & Hogan 1998). This indicates that Bmp4 expression in the ectoderm is not essential for lens formation (Furuta & Hogan 1998). Lens formation in Bmp4 null tissue could also be rescued if complete eye rudiments were explanted in the presence of Bmp4. In contrast, lens formation was absent if isolated ectoderm was cultured in the presence of Bmp4. These results suggest that Bmp4 activity combines with at least one other signal to permit optic vesicle-induced lens induction. It is possible that Bmp4 and the proposed second signal stimulate presumptive lens ectoderm simultaneously or equally and that Bmp4 acts within the optic vesicle to activate production of a signal that in turn stimulates lens formation (Furuta & Hogan 1998). Bmp4 and Bmp7 mutant embryos display similar lens phenotypes in that the lens placode does not form and expression of the transcription factor Sox2 (see above) in lens lineage ectoderm is not up-regulated in the normal way. There are some distinct differences, however, between the two mutants. Whereas Pax6 expression in the presumptive lens ectoderm is lost in Bmp7 / mutants just prior to the time lens placode formation should be occurring (Wawersik et al. 1999), the expression

27 VERTEBRATE EYE DEVELOPMENT 281 of Pax6 (as well as Six3, Eya1, Eya2)inBmp4 / mutants is normal at comparable stages (Furuta & Hogan 1998). These observations indicate that Bmp4 and Bmp7 may have non-redundant functions in the developing eye (Wawersik et al. 1999) (Figure 6). FGF Receptor Signaling is Required for Lens Induction It is well established that FGF signaling is important for differentiation within the lens lineage. Both loss- and gain-of-function experiments have shown that FGF signaling is necessary and sufficient for lens fiber cell differentiation (reviewed in McAvoy et al. 1991). Recent results reveal that FGF signaling is also required for the early phases of lens development including lens induction (S. Faber & R.A. Lang, in press) (Figure 6). The involvement of FGF was addressed by expressing a dominant-negative FGF receptor in the cells of interest by establishing transgenic mouse lines in which the mutant FGF receptor was expressed from the Pax6 ectoderm enhancer coupled to the P0 promoter (S. Faber and R.A. Lang, in press). Transgenic lines (designated TFR7) expressing this construct showed defects in lens development that included diminished proliferation at E13.5 and a delay in fiber cell differentiation. Of most interest, however, was the observation of early lens development defects, including a small lens pit and a failure of the lens vesicle to separate from the surface ectoderm. These defects suggest a role for FGF receptor signaling in the inductive phases of lens development. The question of whether FGF signaling is involved in lens induction was pursued by determining whether Bmp7, an established lens induction stimulus (Wawersik et al. 1999), cooperates with FGF receptor activity. Indeed, crosses between Bmp7 null (Dudley et al. 1995) and TFR7 mice produced compound genotype mice with more severe lens development defects. In particular, TFR7, Bmp7 +/ mice showed a very small lens pit and failed to separate and close the lens vesicle. When combined with the observation that compound genotype mice show diminished expression levels of the lens lineage markers Pax6, Foxe3, and Sox2, this indicates a genetic interaction between FGF and Bmp7 signaling. Thus FGF receptor signaling, Bmp7, and the pre-placodal phase of Pax6 expression are likely to converge in activating the placodal phase of Pax6 expression (Figure 6). Analysis of Foxe3 and Sox2 expression in TFR7 Bmp7 +/ mice (S. Faber & R.A. Lang, submitted) implies that these genes lie downstream of FGF and Bmp7 signaling in the genetic control of lens induction (Figure 6). The identity of FGF ligands involved in lens induction remains unclear and represents a future challenge. Good candidates include Fgf8 (Lovicu & Overbeek 1998, Vogel-Hopker et al. 2000) and Fgf15 (McWhirter et al. 1997). Although expressed only at low levels, Fgf8 mrna is detectable in the eye at an early stage of lens development (S. Faber, H. Makarenkova & R.A. Lang, unpublished observation). The ability of Fgf8 to induce fiber cell differentiation when misexpressed (Lovicu & Overbeek 1998, Vogel-Hopker et al. 2000) indicates that it can be active

28 282 CHOW LANG within the lens lineage. Fgf15 is expressed in the optic vesicle and from this point of view may be a good candidate for an FGF family ligand involved in lens induction. Sox1, Sox2, and Sox3 Participate in Lens Development Members of the Group B1 SOX family of transcription factors are implicated in lens development. Sox genes encode transcription factors containing a sexdetermining factor (SRY) related HMG box. They were originally implicated in lens differentiation based on the expression cloning of Sox2 in a screen to detect proteins that could interact with a region of the chicken δ1-crystallin enhancer (Kamachi et al. 1995). SOX2 has been shown to bind cooperatively with Pax6 to the δ-crystallin minimal enhancer DC5, and in transactivation studies, SOX2 and Pax6 can synergize (Kamachi et al. 2001). When transfected into chick embryos, SOX2 alone is not sufficient to induce ectopic lens tissue; however, when co-misexpressed with Pax6, lens tissue is induced cell-autonomously in surface ectoderm outside of the eye (Kamachi et al. 2001). SOX3 misexpression in Medaka is sufficient on its own to induce ectopic lens formation cell-autonomously (Koster et al. 2000). Sox1, 2, and 3 display broad patterns of expression throughout the developing embryo and possess similar expression patterns in the developing lens. During lens development in mice and chick, Sox1 is first expressed during the invagination of the lens placode and later is predominantly expressed in lens fiber cells. Sox2 and Sox3 expression appear prior to lens placode formation in ventral surface ectoderm bordering the presumptive lens region (Kamachi et al. 1998). Tissue ablation experiments have demonstrated that Sox2 and 3 expression in the presumptive lens ectoderm depends on the presence of the optic vesicle (Kamachi et al. 1998). A possible mechanistic explanation for the optic vesicle dependence of Sox2 expression has been provided by analysis of both Bmp4 null mice (Furuta & Hogan 1998) and those in which FGF signaling is suppressed in the presumptive lens (see above). Where either of these signaling pathways is suppressed, Sox2 expression in the lens lineage is diminished. Thus it is reasonable to propose that Bmp4 and one or more FGF ligands directly or indirectly signal Sox2 up-regulation in presumptive lens cells. When combined with the observation that Sox2 is down-regulated in Sey/Sey embryos, these analyses suggest a downstream placement for Sox2 in a genetic pathway regulating lens induction (Figure 6). The ability of SOX2 and SOX3 misexpression to up-regulate Pax6 (Koster et al. 2000, Kamachi et al. 2001) raises the interesting possibility that reciprocal gene regulation by SOX2/3 and Pax6 might help to restrict and maintain the expression of these genes within the lens placode (Kamachi et al. 2001) (Figure 6). Homozygous mutant mice lacking Sox1 display an inhibition of fiber cell elongation that leads to cataract formation and microphthalmia (Nishiguchi et al. 1998). Mutant lenses are characterized by the absence of γ -crystallins but the presence of

29 VERTEBRATE EYE DEVELOPMENT 283 α- and β-crystallins, raising the possibility that SOX1 binding to the γ -crystallin promoter is required in vivo for normal γ -crystallin gene activation. Furthermore, the failure of fiber cell elongation in Sox1 / mice is probably caused by the specific absence of γ -crystallin expression (Nishiguchi et al. 1998). Precedent for this comes from the observation that a point mutation in the gene encoding γ - E-crystallin is responsible for the mouse mutant, Cat Elo, in which lens fiber cell differentiation is abnormal (Cartier et al. 1992). fork head Transcription Factors Foxe3 and Lens1 Lie Downstream of Pax6 Foxe3 and Lens1 are members of a sub-family of the fork head family of transcription factors that have restricted expression patterns limited to the lens lineage (Blixt et al. 2000, Brownell et al. 2000, Kenyon et al. 1999) as well as the developing midbrain (Blixt et al. 2000, Brownell et al. 2000). Foxe3 is first expressed at E8.75 in a small patch of presumptive lens ectoderm, which at this stage represents a subset of the Pax6-positive ectoderm. This is also true at E9.5 when the lens placode has formed, and it becomes obvious at E10.5 when Pax6 is expressed broadly in presumptive lens ectoderm and Foxe3 expression is restricted to the lens pit. Foxe3 expression is absent in Sey/Sey animals, implying that it is positioned downstream of Pax6 during lens induction (Brownell et al. 2000). This result is consistent with the observation that Lens1 can be up-regulated by Pax6 in Xenopus ectodermal explants and that Lens1, itself, can not induce Pax6 expression (Kenyon et al. 1999). In Xenopus misexpression studies, it was found that Lens1 can completely suppress lens differentiation but that genes indicative of presumptive lens ectoderm such as Pax6 and Six3 are still expressed in the presumptive lens region. This suggests that Lens1 may play a role in undifferentiated lens ectoderm/epithelia following lens specification (Kenyon et al. 1999). A mutation in the fork head domain of the Foxe3 (predicted to be involved in DNA binding) is responsible for the dysgenetic lens (dyl) mouse, and this leads to defects in lens vesicle closure and separation, as well as a reduction in the proliferation level of lens epithelial cells (Blixt et al. 2000, Brownell et al. 2000). In the dyl mouse, expression of Prox1, which encodes a transcription factor essential for lens fiber cell differentiation (Wigle et al. 1999) and is normally most abundant in the transitional, equatorial region of the lens, is expressed at high levels throughout the lens epithelium (Blixt et al. 2000, Brownell et al. 2000). This suggests that Foxe3 may function as a repressor of Prox1 expression in the lens epithelium (Blixt et al. 2000, Brownell et al. 2000). The phenotypic resemblance of the dyl mouse with those in which the Pax6 upstream ectoderm enhancer had been deleted (see above) prompted an examination of a possible genetic relationship. This revealed that Foxe3 expression was diminished in the enhancer null mice (P. Dimanlig & R.A. Lang, in press). A more precise genetic pathway can be suggested in which Foxe3 is located downstream of the placodal phase of Pax6 expression and plays essential roles in regulating

30 284 CHOW LANG proliferation and differentiation in the lens epithelium following lens induction (Figure 6). The sine oculis Related Transcription Factor Six3 Can Transform Otic Vesicle to Lens A role for Six3 in lens induction has been suggested from its expression in the developing lens. Interestingly, however, there appear to be species differences in the timing of its expression. In mice, Six3 expression in the developing lens first appears during the formation of the lens placode and is later restricted to the lens epithelium (Oliver et al. 1995). In Medaka fish, Six3 is expressed in the presumptive lens ectoderm and like Optx2, is down-regulated in the lens placode prior to lens differentiation (Loosli et al. 1998). As in Medaka, chick Six3 is expressed in the presumptive lens ectoderm overlying the optic vesicles but persists in the lens placode and is later localized to the lens epithelium (Bovolenta et al. 1998). Misexpression studies in Medaka have shown that Six3 can lead to formation of ectopic lenses (Oliver et al. 1996). In contrast to Pax6-induced ectopic lens formation in Xenopus, Six3 misexpression is characterized by the transformation to the otic vesicle into lens (Oliver et al. 1996). In addition, lineage tracing experiments revealed that Six3 can direct ectopic lens formation in a cell non-autonomous manner, which led the authors to speculate that Six3 misexpression may induce a soluble factor that changes the bias of the otic placode toward a lens fate. The timing of Six3 expression and its dependence on Pax6 expression in the lens placode place it downstream of Pax6 placode (Figure 6). Maf Family Members in Lens Development Several members of the maf family of basic-leucine zipper transcription factors have been implicated in lens induction and differentiation. L-maf is first expressed in the lens placode and can up-regulate several crystallin genes when misexpressed in Xenopus embryos and ectodermal explants or in chick neural retinal cultures (Ishibashi & Yasuda 2001, Ogino & Yasuda 1998). c-maf has been shown to play an essential role in lens differentiation: Homozygous c-maf loss-of-function mouse mutants are microphthalmic, have a reduction in crystallin expression, and have defects in lens fiber cell elongation (possibly as a consequence of reduced crystallin expression) (Kawauchi et al. 1999, Kim et al. 1999, Ring et al. 2000). Xenopus mafb is different from L-maf and c-maf in that it is expressed much earlier in development in the presumptive lens ectoderm and is later restricted to the lens epithelium of the mature lens (Ishibashi & Yasuda 2001). In Xenopus ectodermal explants, XmafB can upregulate Pax6, Lens1, Six3, Sox3, and L-maf, as well as several crystallin genes, although it is unable to induce ectopic lens formation when misexpressed in whole embryos (Ishibashi & Yasuda 2001). These observations suggest that XmafB may play a role in lens induction and in maintenance of the lens epithelium; however, it is unclear how XmafB expression is regulated during these stages of lens development (Figure 6).

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43 Figure 1 Schematic overview of vertebrate eye development. In panels A D, presumptive or differentiated eye tissues are color-coded in the following manner: blue, lens/cornea; green, neural retina; yellow, retinal pigmented epithelium (RPE); purple, optic stalk (for explanation why only the ventral region of optic stalk is colored, see the text section on defining axes in the developing eye); red, ventral forebrain/prechordal mesenchyme; grey, mesenchyme. (A) Formation of the optic vesicle is initiated by an evagination (indicated by arrow) of the presumptive forebrain region resulting in the formation of the optic pit (OP). The optic vesicle region is divided into dorso-distal region (green), which contains the presumpitve neural retina (PNR) and RPE (not shown), and the proximo-ventral region, which gives rise to the presumptive ventral optic stalk (POS); PLE, presumptive lens ectoderm; M, mesenchyme; VF, ventral forebrain; PCM, prechordal mesoderm. (B) Continued growth of the optic vesicle culminates with a period of close contact between the lens placode (LP) and the presumptive neural retina (NR) during which important inductive signal likely exchange: RPE, presumptive retinal pigmented epithelium; VOS, ventral optic stalk; DOS, dorsal optic stalk. (C) Invagination of the optic vesicle results in formation of the lens vesicle (LV) and neural retina (NR) and establishes the overall structure of the eye. The point at which the neural retina and RPE meet gives rise to components of the ciliary body and iris (C/I). (D) Mature eye: C, cornea; LE, lens epithelium; LF, lens fiber cells; I, iris; CB, ciliary body; GCL, ganglion cell layer; INL, inner nuclear layer; ONL, outer nuclear layer; ON, optic nerve.

44 Figure 2 Defining the proximo-ventral/dorso-distal axis in the developing eye. This figure represents a ventral view of four different stages of eye morphogenesis. (A) The early eye field can be conceptualized as a flat sheet in which the proximo-ventral (PV) region adjacent to the midline will give rise to the ventral optic stalk, and the dorso-distal (DD) domain will give rise to the RPE (see text). (B) Optic evagination (arrow) is the first morphological event that alters the shape of presumptive eye tissues. The PV-DD axis within the eye field is beginning to curve at this stage. (C) Optic invagination occurs along a continuous line extending from the presumptive neural retina (green and top arrow) and along the ventral optic stalk (bottom arrow). The PV-DD axis is reflexed as the presumptive RPE folds back over the presumptive neural retina. (D) Closure of the ventral choroidal fissure (asterisks) occurs as the laterally extending edges of the neural retina and RPE (top set of arrows) sweep in a ventral movement and eventually fuse. The laterally extending edges of the dorsal optic stalk (bottom set of arrows) move ventrally and fuse at the midline to become the outer wall of the optic stalk, while the former ventral optic stalk pinches off at the point of closure and becomes the inner wall. Although this representation of the developing eye field could accommodate anterior (nasal) and posterior (temporal) influences present on either side of the PV-DD axis, it has been omitted because little is known about these influences at early stages of eye development.

45 Figure 3 Patterning and inductive events in the developing eye. The developing eye field is represented as a simple sheet defined by proximo-ventral/dorso-distal boundaries. Anterior (nasal) and posterior (temporal) boundaries have been excluded from this simplified representation. (A) As a downstream response to neural induction and possible Wnt signaling (green triangle), many transcription factors important for eye development are expressed in the anterior neural plate in a single eye field. (B) Cyc signaling from underlying prechordal mesoderm (red triangle) is believed to induce ventral midline specification and up-regulation of hh expression. Hedgehog signals from the midline and possibly other sources (purple triangle) specify the ventral optic vesicle. Pax2 and Pax6 reciprocal repression at the PV-DD boundary is believed to occur gradually (B and C) until a sharp boundary (D) forms between neural retina and optic stalk; m, mouse. (C) Signals from the surface ectoderm (possibly BMPs or FGFs; green, lower triangle) are important for neural retina specification, whereas signals from extraocular mesenchyme (possibly BMPs or activins; yellow, upper triangle) are believed to function in RPE specification; ch, chick. (D) Distinct PV-DD patterning occurs within the neural retina possibly under the influence of retinoic acid (RA; green triangle). Note that while Pax2 is normally present in the ventral neural retina, it is down-regulated in later development as depicted in (D).

46 Figure 5 Generating two eyes. (A) Removal of the prechordal mesoderm (Urd.) from Triton embryos at the neural plate stage resulted in the cyclopic embryo shown in (B) (Mangold 1931). Recent experiments using Xenopus anterior neural plate explants demonstrated that in contrast to the control in (C) (arrowhead), removal of the prechordal mesoderm in explants leads to the persistence of Pax6 expression in the midline (D) (arrowhead) (Li et al. 1997). The one-eyed pinhead phenotype in zebrafish that leads to cyclopia (E)(arrowhead) can be rescued by activating the cyc/nodal signaling pathway in oep mutants misexpressing Smad2 (F) (arrowheads); arrow in (F) indicates restoration of the ventral forebrain (Gritsman et al. 1999). (G) Optic vesicle (ov) restricted Pax6 expression in the zebrafish embryo. (H) Sonic hedgehog misexpression in zebrafish leads to the loss of Pax6 expression in the optic vesicle (ov) and (J) the expansion of Pax2 expression. Pax2 expression is (I) restricted to the optic stalks (os) in uninjected controls (Ekker et al. 1995). The asterisks in (I) and (J) indicate Pax2 expression at the midbrain-hindbrain boundary.