Part III Organization of Cell Populations Chapter Since ancient times, people have wondered how organisms are formed during the developmental process, and many researchers have worked tirelessly in search of the answer. Although large amounts of data have been accumulated along the way, a full understanding of the developmental phenomena has not yet been achieved. However, with the development of techniques to analyze these phenomena at the gene level, our knowledge in the area has rapidly increased in recent years. Now, we are becoming increasingly aware that the seemingly diverse biological developmental phenomena involved are in fact related to a number of common basic mechanisms. By referring to specific case examples, this chapter discusses the basic mechanisms of biological development that have been revealed at the molecular level. I. Oogenesis In many animals other than mammals, the materials necessary for continued development to a certain stage need to be pre-stored in the egg. These materials, which are derived from the mother and therefore known as maternal factors, include many substances that play important roles in the early stages of development. The main constituents of maternal factors are mrna and special proteins. These factors are involved in many of the important events that occur during the early stages of development, such as determining which way the embryo faces (e.g., head-tail direction and dorsal-ventral-side direction), determining the fate of embryonic cells (i.e., deciding which tissues or organs they will become), regulating cell growth, etc. Many maternal factors are stored in the oocyte during oogenesis, and many are stored unevenly in the cytoplasm of the oocyte (Fig. -1). The uneven distribution of these factors is closely associated with the determination of the direction of the embryo and the fate of embryonic cells. As an example, maternal factors localized in the egg will become unevenly distributed over embryonic cells (blastomeres) through the division of the egg (cleavage). As a result, they become deeply involved in the determination of these variables. CSLS / THE UNIVERSITY OF TOKYO 191
Figure -1 Oogenesis in fruit flies Oogenesis in fruit flies occurs with the help of cells known as nurse cells. A germ cell divides four times to form sixteen cells, of which only one becomes an oocyte (an immature egg in the process of oogenesis). The rest become nurse cells and facilitate the creation of the oocyte. The cytoplasm of the oocyte and nurse cells is connected, and many proteins and mrna (i.e., the mrna of maternal factors such as nanos and bicoids) that are synthesized in nurse cells are transported through this pathway to the oocyte. Many of these materials are unevenly stored in the oocyte. indicates the direction of transport of materials synthesized in nurse cells. II. Fertilization and Cleavage Besides the important task of combining two sets of genes from the father and mother into one, fertilization plays a number of other significant roles. As an example, in frog embryos, as discussed later, the direction of the body is determined by the site of the oocyte that the fertilizing sperm enters. The entry of the sperm also sets off the process of cleavage. The cell cycle of cleavage during the early stages of development is very rapid. As an example, the average cell cycle in the cell division of frogs is approximately 16 hours, while early embryonic cells complete their cycle in 30 minutes. This is because the cell cycle lacks phases equivalent to the extent of the G1 and G2 phases. In other words, cell division takes place through repetition of the DNA synthesis period (S phase) and the mitotic period (M phase), making the cell cycle very short (see Chapter 9). In the early stages of development, a certain number of cells are thus created by the repetition of cell division within a short space of time. Cleavage patterns are diverse, and include a type with unique cleavage directions as well as one with uneven blastomere sizes (Fig. -2). This diversity is due to the uneven distribution of nutrients in the embryo, or may reflect the need to unevenly distribute maternal factors localized in the cytoplasm (see IV in Chapter 9). Among these, the uneven distribution of maternal factors plays a particularly important role in the developmental process. This can be easily understood by taking the destiny of the blastomeres in nematodes as an example (Fig. -3). In this case, in each cell division, germ cell granules (maternal factors necessary for the embryo to form germ cells) are unevenly allocated to just one of the two cells. Those that receive the granules remain in the germ cell line and finally become CSLS / THE UNIVERSITY OF TOKYO 192
germ cells. All others become somatic cells that form various tissues and organs. Cell lineage describes which cells become the cells of which tissues during the process of a fertilized egg becoming an adult. (see the Column on p.194). Figure -2 Cleavage patterns There are various types of cleavage, which are classified by the direction of division and the size of blastomeres. Figure -3 Cleavage and the fate of blastomeres in nematodes In each cleavage, germ cell granules are unevenly allocated to one of the two cells. Those that receive the granules remain the germ cell line and become germ cells, while all others become somatic cells. III. Determination of Embryo Directionality The direction of the body is determined immediately after fertilization in terms of, for example, which side of the embryo becomes the head/tail, and maternal factors pre-stored in the oocyte play a pivotal role in this process. Since the importance of these factors in development has been studied in depth for fruit flies and frogs, these organisms are used as examples in the discussions below. CSLS / THE UNIVERSITY OF TOKYO 193
Column Cell Lineage of Nematodes Cell lineage, like a family tree, is a line that shows the history of a fertilized egg going through cleavage and subsequently being differentiated into various parts of the body. Since humans consist of an enormous number of cells (60 trillion in an adult), it is impossible to trace the history of all cells from fertilized egg to adult body. However, in nematodes, which measure only 1 mm in length, the number of cells in an adult body is only 959. The initial number is 1,090, but 131 cells are programmed to die during the developmental process (this is known as apoptosis; see Chapter 9). It is therefore easy to create a cell lineage for nematodes, as they consist of such small numbers of cells. Indeed, a cell lineage map for nematodes, from the fertilized egg to all cells that make up a mature individual, has already been created (Column Fig. -1). Furthermore, the complete DNA sequence of nematodes (consisting of approx. 0 million base pairs) has been determined, and it is estimated that approximately 17,500 genes are distributed over six chromosomes. As multicellular organisms on an easily manageable scale, nematodes have therefore been widely used as a material suitable for analyzing the basic mechanisms of organisms at the molecular level, including developmental mechanisms, functional analysis of the neural network and the causes of diseases. Column Figure -1 Cell lineage of nematodes In nematodes, the lineage of all cells from the fertilized egg (zygote) to the 959 somatic cells that make up an adult individual has been determined. CSLS / THE UNIVERSITY OF TOKYO 194
Figure -4 The process from determination of directionality in the fruit-fly embryo to segmentation of the embryo into 14 parts The gene expression pattern observed during the process of bicoid and nano protein expression and the subsequent segmentation of the embryo into 14 parts is shown here. See Column Figure -2 on homeobox genes. Development of Fruit Flies In fruit flies, as shown in Figure -1, a number of maternal factor types that determine the directionality of the embryo (i.e., the head and tail sides) are stored in the oocyte during oogenesis. Among these, the major factors are mrna for two protein bicoid and nanos. These mrnas are stored disproportionately in both sides of the oocyte. After fertilization, they are translated and the concentration gradient of bicoid and nanos is created along the lengthwise axis of the embryo (Fig. -4). In the early stages of development in fruit flies, only the nucleus divides, and the plasma membrane that envelops it is not newly created; the embryo therefore takes on the appearance of a multinucleated cell. At this stage, all the nuclei are in the same cytoplasm. Under these circumstances, when the concentration gradient of bicoid and nanos is created in the embryo, the germ cell nuclei are directly exposed to the gradient. Since bicoid and nanos are factors that regulate gene expression and protein synthesis, the expression of new genes and protein synthesis take place in accordance with their concentration gradients (Fig. -4). The expression of gap genes occurs in accordance with the concentration gradient patterns of bicoids and nanos, and the proteins translated from gap genes are known as transcription factors. As a result, based on the expression pattern of the gap genes, pair-rule genes are expressed in a way that forms seven stripes in the embryo (this mechanism is discussed later). The proteins translated from the pair-rule genes are also transcription factors. Then, based on the expression pattern, segment-polarity genes are expressed in a way that forms 14 stripes in the embryo. As a result of the cascade expression of these gene groups, 14 regions (or compartments) are formed in the embryo, which is CSLS / THE UNIVERSITY OF TOKYO 195
Figure -5 Patterns of a new gene being expressed in accordance with the transcription factor concentration gradient (A) An example of a gene being expressed only in a region where the transcription factor concentration is within a certain range. In this case, upper and lower limits (thresholds) of the transcription factor concentration causing gene expression exist. (B) An example of a gene being expressed in a region where the concentration balance of transcription factors that inhibit gene expression and those that enhance it is within a certain range. The enhancement of gene expression is based on the combined effects of the different promoting factors. the origin of the 14 basic regions of fruit flies (i.e., the segmental structure of insects). The cascade expression of these gene groups takes place in accordance with the concentration gradient patterns of transcription factors (Fig. -5), which are the proteins that regulate the expression of target genes by binding to their regulatory regions (see Chapter 4). Such regulation may enhance or suppress the expression of target genes. Figure -5A shows an example of the expression of a target gene being enhanced only in regions where the transcription factor concentration is within a certain range, and otherwise being suppressed. In this case, the expression of target genes is induced when the concentration is within a certain range. Figure -5B shows an example of the expression of a target region occurring in an area where the action of enhancing transcription factors dominates that of inhibitory transcription factors. During the process from the formation of the concentration gradients of bicoids and nanos to the expression of segment polarity genes, 14 regions are formed in the embryo based on the expression pattern of the genes. As the next step, the fate of the 14 regions (in terms of the organs to be created from each) must be determined. A group of genes called homeobox genes play an important role in this process (Column Fig. -2). (A) (B) CSLS / THE UNIVERSITY OF TOKYO 196
Column Homeobox Genes During the long history of research into fruit flies, many mutants with structural abnormalities have been reported. Recent gene-level analysis has identified a series of genes that cause structural abnormality in the body of fruit flies. These are called homeotic gene complexes (HOM-C), and consist of a group of eight genes (Column Fig. -2A). It has also been found that gene groups equivalent to these exist in four clusters (HoxA-D) in mammals. In addition to this correspondence in the sequence of genes in chromosomes between HOM-C in fruit flies and Hox in vertebrates, their expression patterns in the body are also similar. As an example, the expression pattern of HOM-C in fruit flies (which is expressed in an anteroposterior direction along the body) and that of Hox in vertebrates (which is expressed along the head-tail axis) are very similar (Column Fig. -2B). The series of genes contained in HOM-C and Hox are called homeobox genes, as all proteins translated from such genes have a region consisting of 60 amino acids called a homeodomain. The homeodomain is a region with a special tertiary structure for binding to DNA (Column Fig. -2C). The proteins translated from homeobox genes are transcription factors that regulate the expression of other genes by attaching to them. It is believed that homeobox genes determine which region becomes which organ after the rough regional division that takes place in the embryo. Column Figure -2 Homeobox genes (A) HOM-C in fruit flies and HoxA D in mammals are shown here. indicates the correspondence between the two. (B) The similarity of gene expression between HOM-C in fruit flies and Hox in vertebrates. The homeobox genes in fruit flies are expressed in the body in accordance with the gene sequence on the chromosomes. This is also the case with homeobox genes in vertebrates. As an example in mice, a part of the homeobox genes expressed in the brain and the spinal cord is shown. The arrow shows the directionality of homeobox gene expression along the head-tail axis. (C) The tertiary structure of the region known as the homeodomain, which all proteins translated from homeobox genes have, is shown here. It has three α-helices with which it binds to DNA. (C) (A) (B) CSLS / THE UNIVERSITY OF TOKYO 197
Development of Frogs Next, we discuss development of frog embryo as an example of vertebrate development. The upper hemisphere of the frog egg, which is rich in pigment and therefore dark, is called the animal pole, while the opposite hemisphere, which is white, is called the vegetal pole. Before fertilization, the egg has only animal-vegetal-pole directionality. However, upon fertilization, the future dorsal and ventral sides are determined in the embryo. This is because the surface part of the cytoplasm in the egg moves in a particular direction after fertilization. This in turn causes the movement of the maternal factors that determine the dorsal side of the embryo from the vegetal pole to near the equator on one side of the embryo (Fig. -6). As a result, the side opposite the sperm entry point becomes the future dorsal side. Thus, in the development of frogs too, maternal factors stored locally in the egg play an important role in determining the directionality of the embryo. A protein called Dishevelled is among the important maternal factors that are stored in the vegetal pole of the embryo and turn the region to which they move into the dorsal side. This protein moves to a region where it causes the expression of new genes, thereby inducing that side of the embryo to become the dorsal side (Column Fig. -3). As a result, a region called the organizer, which determines the dorsal side of the embryo, is created on the side that the Dishevelled moves to. The basic structure of the frog s body is formed around this region. Figure -6 Determination of dorsal-ventral directionality Post-fertilization movement in the surface part of the frog egg is shown here. Upon fertilization, part of the cytoplasm in the vegetal pole moves to a point near the equator on the side opposite the sperm entry point. CSLS / THE UNIVERSITY OF TOKYO 198
Column Roles of Maternal Factors in Determining the Dorsal Side of the Frog Embryo Dishevelled suppresses the degradation of a transcription factor called β catenin. This means that β catenin increases in the cytoplasm on the side of the embryo to which Dishevelled moves. The increased amount of β catenin moves into the nucleus and causes the expression of its target, the Siamois gene. Siamois (a transcription factor) causes the expression of its target (the goosecoid gene) in cooperation with other transcription factors known as Smads. The goosecoid gene has a strong effect in inducing the formation of the dorsal side in the embryo. Column Figure -3 Process of Dishevelled determining the dorsal side of the embryo The process of maternal factors determining the dorsal side of the embryo takes place mainly through the regulation of gene expression by transcription factors. During the process, maternal factors and extracellular signals collaborate. IV. Cell Differentiation and Stem Cells During the developmental process, cells derived from one fertilized egg become units with various functions such as muscle cells, neurons and epithelial cells. This phenomenon is called cell differentiation. Embryonic cells in the early developmental stage have the potential to become various cell types. These cells, which have not yet undergone differentiation, are referred to as undifferentiated cells. However, as development proceeds, most cells are differentiated into units with particular functions. Each differentiated cell CSLS / THE UNIVERSITY OF TOKYO 199
type shows the different gene expression pattern and level. (Fig. -7). In other words, in differentiated cells, the expression of certain genes necessary to fulfill their cellular functions is enhanced, whereas the expression of unnecessary genes is suppressed. In differentiated cells, the suppression of unnecessary genes is performed through chemical modification of the genes themselves (or of proteins that bind to them) or through the binding of special proteins to them. Such suppression may be temporary or semi-permanent (see Chapter 4). If the suppression is unlocked thus allowing gene expression the cells regain the potential to be transformed into other cells. Cells that retain the ability to differentiate into many cell types exist in the various tissues of the human body. These are known as stem cells, and are believed to be involved in the repair of damaged tissues. Attempts to collect, grow and differentiate these cells in order to artificially create tissues and organs have recently been made. The aim is to create tissues and organs using stem cells obtained from patients and implant them back into the body. The method of implanting the patient s own tissues avoids immunological rejection and raises fewer ethical issues than other methods (such as techniques that use human embryonic cells). Figure -7 Cell Differentiation A model showing changes in the gene expression pattern and level during cell differentiation. The gene expression patterns occurring in three types of differentiated cell are schematically outlined. Differentiated cells have the same gene set as the fertilized egg, but the gene expression pattern and level differ. CSLS / THE UNIVERSITY OF TOKYO 200
V. Induction and Morphogenetic Movement (A) Induction is a phenomenon that strongly influences the fate of adjacent cells and tissues through its action on them. Induction occurs throughout the course of embryonic development. When induction (which is found in various phases of the developmental process) is considered at the cellular level, a number of common mechanisms are found. The three main methods by which one cell influences and changes the fate of other cells are via secretory substances, molecules on the plasma membrane surface, and gap junctions (Fig. -8). Whichever method is taken, one cell can influence many adjacent cells. In triploblastic animals, whose bodies consist of the three basic layers of endoderm, mesoderm and ectoderm (including many types of animal from planarians to humans), the embryo is roughly divided into three regions in the early stages of development. This determines the fate of each region. By way of example, the respiratory and digestive organs are formed from the endoderm, muscle and connective tissues are formed from the mesoderm, and the central nervous system and skin are formed from the ectoderm. Induction plays a role in the differentiation into the three germ layers. This mechanism is discussed below using mesoderm induction in frogs as an example. In mesoderm induction, the mesoderm is formed in a certain area of the embryo (the equatorial region) as a result of influence from various other parts of the embryo (Fig. -9). This induction is caused by many types of substance secreted from the organizer on the dorsal side, the animal pole, the vegetal pole and ventral side. Through their interaction, the mesoderm region is induced in the intermediate area of the embryo (the equatorial region). Morphogenetic Movement After the determination of the rough arrangement of embryonic regions such as the prospective mesoderm and ectoderm areas following the formation of the organizer (the center of embryogenesis), major morphogenetic movement is seen. First, the formation of three germ layers and morphogenetic movement occurs to create archenterons, which later become the gastrointestinal tract. Second, further morphogenetic movement occurs to create the neural tube. The distribution of embryonic cells is rearranged through these morphogenetic movements, thereby allowing the interaction of embryonic cells that previously existed separately. (B) Figure -8 Induction in adjacent cells (A) There are three methods of causing induction in adjacent cells: a) influence via secretory substances such as growth factors and hormones; b) influence through cell adhesion via proteins and carbohydrate chains that exist on the surface of the plasma membrane; and c) influence from the formation of gap junctions between the cell and adjacent cells. (B) Methods of transmitting induction to surrounding adjacent cells: a) through the diffusion of secretory substances; b) by extending cell protrusions and attaching them to surrounding cells; and c) intercellular transmission via gap junctions. indicates the direction of induction. CSLS / THE UNIVERSITY OF TOKYO 201
Figure -9 Mesoderm induction in frogs Mesoderm induction in frogs is caused by the action of inducers secreted from the surrounding area. indicates the direction of induction from each of the four regions. As a result of the combined induction effects, the mesoderm region is induced in the central area. (A) (B) Figure - Morphogenetic movement (A) Morphogenetic movement occurring in the early embryo. This takes place mainly through the movement and deformation of the epithelial cells that constitute the embryo, and includes: a) the movement of cells that break away (or migrate) from epithelial tissues; b) the bending or invagination movement of the epithelium; and c) the extension movement caused by the rearrangement or flattening of epithelial cells. (B) Examples of morphogenetic movement in sea-urchin and chicken embryos: a) bending and invagination movement in the sea-urchin embryo, and b) locomotion of mesodermal cells in the chicken embryo. In chicken embryos, cells that separate from the central part of the ectoderm move inwardly in the embryo and become mesodermal cells, which then move in the direction of the arrows between the ectoderm and endoderm. The part from which cells break away appears as a streak, which is referred to as a primitive streak. CSLS / THE UNIVERSITY OF TOKYO 202
Figure -11 Movement of mesodermal cells in the chicken embryo A schematic diagram showing the movement of mesodermal cells, seen from the dorsal side (the ectoderm is not shown). The prospective heart mesoderm region is located at the tip of the moving mesoderm. indicates the direction of movement of mesodermal cells. Mesoderm formation, gastrulation and neurulation all take place through a number of basic cell movements (Fig. -). Since the early embryo is made of epithelial tissue (see Chapter 11), morphogenetic movement during this stage mainly involves the movement and deformation of epithelial cells. This includes the bending movement caused by contraction on one side of epithelial cells, invagination movement in which bent epithelial tissues extend inward in the embryo, locomotive movement caused by cells separated from epithelial cells, and extension movement caused by the flattening and rearrangement of epithelial cells. Among these movement types, one that plays a particularly important role in subsequent organogenesis is the large-scale movement of mesodermal cells. This movement in vertebrates is so great that the cells move to the opposite side of the embryo. Taking the chicken embryo as an example, cells that break away from the epithelium of the ectoderm enter the embryo, form mesodermal cells and move to the sides and the anterior part between the ectoderm and endoderm (Fig. - 11). The prospective heart mesoderm regions, which later become the heart, are located at the tip of the moving mesoderm on both sides of the embryo. Neural Induction After mesoderm induction, neural induction occurs. By this process, the neural tube (which later becomes the brain and the spinal cord) is created from the ectoderm. Neural induction is an important event in the development of animals, since the neural tube plays the central role in the construction of the body and CSLS / THE UNIVERSITY OF TOKYO 203
forms the brain and the spinal cord the most important components in the bodies of animals. Neural induction is brought about by the organizer and the mesoderm. First, the organizer sends induction signals to the ectoderm. Then, induction signals are sent from the mesoderm, which has moved below the ectoderm, to the ectoderm itself. Both induction effects are exerted by substances secreted from cells. In response to this induction, which destines the ectoderm to become nerve tissue, the neural tube the origin of the brain and the spinal cord is formed from the ectoderm. The brain is created from the anterior part of the tube, and the spinal cord is created from the posterior part (Fig. -12). Figure -12 Neural induction in a frog embryo Neural induction exerted by the mesoderm to the ectoderm is shown here. Inducers are secreted from the anterior and posterior parts of the mesoderm that have moved inside the embryo, which respectively induce the formation of the brain and the spinal cord from the ectoderm. CSLS / THE UNIVERSITY OF TOKYO 204
Column The Mechanism of Flower Organ Formation As with animals, organogensis in plants is also regulated by various genes. In this column, the formation of flower organs is discussed in connection with homeotic genes. Generally, flowers consist of four organs: calyxes, petals, stamens and pistils (carpels) (Column Fig. -A). These organs can be depicted as a concentric ring structure of four layers (whirls) as in the floral diagram (Column Fig. -4B). It has gradually become clear that the formation of these organs is regulated by the combined expression of the three homeotic gene groups of A, B and C (the ABC model, Column Fig. -4C). In the first whirl, only gene group A is expressed, forming calyxes. In the second whirl, gene groups A and B are expressed, forming petals. In the third whirl, gene groups B and C are expressed, forming stamens. In the fourth whirl, only gene group C is expressed, forming pistils (carpels) (Column Fig. -4C). Antagonism exists between gene groups A and C; if the functions of gene group A are lost, the gene-group-c functions dominantly over the entire flower, and if those of gene group C are lost, the genegroup-a functions dominantly. Therefore, if the functions of gene group B are lost, only gene group A is expressed in the first and second whirls, and only gene group C is expressed in the third and fourth whirls, thereby forming a flower consisting of calyxes, calyxes, carpels and carpels (Column Fig. - 4D). If gene C is lost, only gene group A is expressed in the first whirl, only gene groups A and B are expressed in the second and third whirls, and only gene group A is expressed in the fourth whirl, thereby forming a flower consisting of calyxes, petals, petals and calyxes (Column Fig. -4E). Column Figure -4 The ABC model in flowers (A) An Arabidopsis flower, and (B) an Arabidopsis floral diagram. The flower consists of four calyxes, four petals, six stamens and two carpels. (C) Different organs are formed through the combined effects of the three gene groups A, B and C. (D) A mutant that has lost the functions of gene group B. (E) A mutant that has lost the functions of gene group C. (A) (B) (C) (D) (E) CSLS / THE UNIVERSITY OF TOKYO 205
VI. Organogenesis The triploblastic structure is formed through a process in which the mesoderm enters between the ectoderm and the endoderm. In concurrence with the formation of the triploblastic structure, interaction between the germ layers occurs, thereby initiating organogenesis. Induction phenomena like those shown in Figure -8 are seen when organs are formed through interaction between germ layers. Here, organogenesis is discussed using the heart as an example. The formation of the heart (the first of all organs to begin expressing its functions) is initiated by mesoderm movement. This organ is formed from embryonic regions collectively known as the prospective heart mesoderm. There are two such regions located in the posterior part of the embryo. Together with the mesoderm, these two regions move a long way from the posterior to the anterior part of the embryo. They then merge in the anterior part, creating a tubular heart that gradually changes shape to eventually form a heart consisting of atria and ventricles (Fig. -13A). While moving to the anterior part of the embryo, the prospective heart mesoderm regions are influenced by inducers secreted from the endoderm and ectoderm, thereby being induced to take the path to generate the heart. Through the subsequent expression of homeobox genes, the regions then become destined to form the heart. A number of interesting findings have been made from the identification of the inducers involved in heart formation and the genes that determine embryonic regions to become the heart. Such findings include a structural similarity between the homeobox gene that controls heart formation in vertebrates (the Nkx gene) and the homeobox gene that controls the formation of dorsal blood vessels (equivalent to the heart in mammals) in fruit flies (the Tinman gene) (Fig. -13B). Another such finding is a similarity between the inducers (secretory substances) that cause the expression of these genes in both fruit flies and vertebrates. These findings indicate that the basic mechanism used in the formation of dorsal blood vessels in fruit flies has been well conserved throughout evolution and is found in heart formation in vertebrates. Similar cases have been confirmed in many organs, including the formation of limbs and the nervous system. Homeotic genes also play an important role in plant organogenesis. One such example is the formation of flower organs (see the Column on p.205). CSLS / THE UNIVERSITY OF TOKYO 206
(A) Figure -13 Cardiogenesis (A) The heart is formed by two prospective heart mesoderm regions moving from the left and right sides of the embryo and finally merging. First, a tubular heart is formed, which is then twisted clockwise in a sigmoid curve (this is reversed in the photo, since it was taken from the ventral side) to form the atria and ventricles, thus becoming the heart. The photo shows an amphibian (newt) heart consisting of two atria and one ventricle. (B) The similarities between vertebrates and fruit flies are shown with regard to inducers involved in cardiogenesis, the homeobox genes that determine heart formation, and the timing of expression for these genes. Bone morphogenetic factors and decapentaplegic, which induce the mesoderm to form the prospective heart mesoderm region, belong to the same growth factor group. (B) CSLS / THE UNIVERSITY OF TOKYO 207
Summary Chapter While the developmental processes of organisms are diverse, a number of common basic mechanisms are found. In the various phenomena that occur during the early stages of development (such as the determination of embryo directionality and the destiny of germ cells), substances derived from the mother (maternal factors) stored in the egg play important roles. As development proceeds, the embryo is roughly compartmentalized into regions. The fates of these regions (i.e., determination of which ones will form the various tissues and organs) are then decided by the expression of homeobox genes. The proteins translated from homeobox genes are transcription factors, which have been well conserved in the evolutionary process. These play an important role in forming the bodies of organisms. Throughout the development process, induction occurs in which cells in certain regions influence those in adjacent regions. There are several induction patterns, but in each case, induction causes the expression of new genes in target cells, thereby determining the fate of those cells. Germ layers are formed in many animals, and interaction among these layers causes the formation of tissues and organs. The large-scale morphogenetic movement that occurs in the early stages of development allows the formation of these germ layers and the subsequent interaction among them. It is now possible to compare the genes involved in the development of nematodes and fruit flies with those involved in human development. As a result, it has been shown that genes with a similar structure play similar functions in each. This indicates that the basic mechanisms forming the bodies of organisms have been continuously passed on during the course of evolution and function in a similar way in the formation of the human body. CSLS / THE UNIVERSITY OF TOKYO 208
Problems [1] The embryo of triploblastic organisms consists of three basic structures. Name these structures and the organs and tissues that develop from them. [2] Using fruit flies and frogs as examples, explain how the anteroposterior axis of animals is determined in the early stages of development. [3] Explain the concept of induction using examples. [4] Using the ABC model, explain how double flowers are formed. Also outline which organs in a normal flower form which organs in these cases. (Answers on p.258) CSLS / THE UNIVERSITY OF TOKYO 209