Landolt's club in the amphibian retina: A Golgi and electron microscope study. Anita Hendrickson

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1 Landolt's club in the amphibian retina: A Golgi and electron microscope study Anita Hendrickson Landolt's club process has been studied in the adult newt retina. In Golgi preparations this process has been shown to arise from the dendritic trunk of a bipolar cell and, to pass through the external synaptic layer without discernible synapse, apparently to terminate as an enlarged, bulb at the level of the external junctional zone (external limiting membrane). This confirms Landolt's original observations. The electron microscope reveals, in addition, that this process open lacks intervening glia between its own cell membrane and that of adjacent photoreceptors. Morphologically the club process differs somewhat from a typical dendrite. The process passes through the external junctional zone and terminates as a 9+0 cilium which extends between the receptor inner segments. I n 1871, Landolt 1 described a clubshaped cell process (Kolbenfaden) which lay between the nuclei of the photoreceptors in newt and frog retinas. By osmium maceration techniques he was able to isolate this process attached to its parent cell, the nucleus of which was found within the internal nuclear layer. On the basis of staining affinities he concluded the club process was neuronal rather than glial in nature, for it stained much more like the bipolar cell dendrites of the external synaptic layer than the glial cells. This process became known as "Landolt's club." Ranvier- confirmed these findings and identi- From the Department of Biological Structure, University of Washington, Seattle, Wash. This investigation was supported by United States Public Health Service Grants No's. 5T1 CM 136 and HE This work is a portion of that submitted to the University of Washington Craduate School in partial fulfillment of the requirements for the Ph.D. degree. 484 fied the cell body as that of a bipolar cell. Subsequent authors of descriptive papers based on sectioned and stained retinas did not mention this process, even when the material was from amphibian retina. 3 Ramon y Cajal 1 was able to demonstrate Landolt's club by means of whole cell Golgi impregnation not only in the newt, salamander, and frog, but also in the retinas of the sparrow and lizard. Polyak 5 described structures similar to Landolt's club in the retinas of chimpanzee, rhesus monkey, and human; however, in these primate retinas clubs were not as numerous as in lower vertebrates. Both authors interpreted the Landolt's club as an enlarged apical dendrite of a true bipolar cell. Polyak further specified the bipolar cell type in primates as the "mop" variety, which synapses with both rods and cones. The specific bipolar cell type giving rise to Landolt's club has not been stated for retinas of other species. More recently, Landolt clubs have been demonstrated in the chicken 0 and

2 Volume 5 Number 5 Landolfs club in amphibian retina 485 dogfish shark, 7 also with the use of Golgi impregnation. Published electron microscope studies on this cell process are very few. Cohen 8 illustrated the process at the level of the external junctional zone* in the pigeon but did not discuss it further. In the course of an electron microscope study of the adult newt retina by the present author, unusual relationships and morphological features of Landolt's club became apparent. Because of this unusual morphology as well as the possible implication for photoreceptor neurophysiology, these findings are now being presented. A brief report has already been made.' 1 Materials and methods Eyes of adult newts (Diemictyhis v. viridescens) were fixed for whole cell Golgi impregnation with Stell's modification of the Cajal. method, 10 embedded in Epon-Araldite and cut at 80 fj- on a sliding microtome. For light and electron microscopy the posterior half of the adult eye was fixed in a final concentration of 3.3 per cent osmium tetroxide buffered with 0.06 M s-collidine, 11 dehydrated in ethanol, embedded in Araldite, and sectioned with glass knives. Thin sections were stained with lead, 1 '- Electron micrographs were taken on an. RCA-2C microscope using special stabilized power supplies and stigmator. Thicker sections for light microscopy were cut at 1.5 f- and stained with azure II methylene blue-. 13 Observations Fig. 1 shows an impregnated bipolar cell prepared by the procedure of Cajal as modified by Stell. 10 The cell body (P) lies in the outermost part of the internal nuclear layer just under the external synaptic layer (ESL), or "outer plexiform layer." The latter is marked by a few impregnated dendrites of other biopolar cells. A single, thick, straight axon passes deep into the inner nuclear layer, but its synaptic termination is not visualized. Apically the cell *The term "external limiting membrane" has been replaced by "external junctional zone" in this paper. Although it has been shown by both light and electron microscopy 1 * 1 JB that this region is not a separate "nienibrune," but rather a line of terminal bars, the older term persists in usage. This author proposes the more accurate, descriptive designation "external junctional zone" as a replacement and as such it will be used in this paper. Fig. 1. Golgi preparation of a bipolar cell from an adult newt retina. The perikaryon (P) lies out of the plane of focus, as does the axon (A) running into the inner nuclear layer (INL). The dendrite trunk (T) arises from the cell body and passes into the external synaptic layer (ESL) where it bifurcates into two types of processes. The small twisted branches are dendrites (D) synapsing with the basal regions of the photoreceptors (R) which are not stained. A single, thicker process runs between the receptors and terminates in a clublike enlargement. This is Landolt's club (L). (Cajal-Golgi preparation, Epon-embedded, SOjt section, x.l.,500.) possesses two types of processes. The more numerous type consists of the "classical" dendrites (D) of the bipolar cell. These thin, twisted or curled nerve fibers originate from an apical stalk, but almost immediately they turn and run perpendicular to the long axis of the bipolar cell within the external synaptic layer. They end in synaptic contact with the base of the photoreceptors. One apical process follows a strikingly different course (L). This fiber, thicker

3 486 Hendrickson Investigative Ophthalmology October 1966 than the others, arises from the same source as the thin dendritic processes, but it continues to lie in the longitudinal plane of the bipolar cell. It passes between the bodies of the photoreceptorsj sometimes showing one or two bends, and terminates in a bulbous or club-shaped enlargement at the level of the external junctional zone. This second bipolar cell process is "Landolt's club." For comparison, an impregnated glial (Muller) cell is shown in Fig. 2. Notice especially the difference between the single, thick Landolt club and the multitu- Fig. 2. Golgi preparation of a single Muller (glia) cell for comparison with the bipolar cell of Fig. 1. Within the internal nuclear layer (INL) the glial cytoplasm envelops several unstained bipolar perikaryions (P). At the level of the external synaptic layer (ESL) the cytoplasm spreads out laterally and then extends to the external junctional zone [external limiting membrane] (EJZ) in numerous slender columns or sheets, quite unlike the single thick Landolt club process. (Cajal-Golgi preparation, Epon-embedded, 80^ section, xl,500.) dinous thin processes of a single glial cell above the ESL. Although both kinds of processes traverse the external nuclear layer, in Golgi preparations identification is easily made between the two. In a sectioned and stained newt retina, as observed in the light microscope, Landolt processes are much more difficult to see, but in Fig. 3 at least the enlarged clubshaped endings can be discerned, lying between the elongated nuclei of the photoreceptors just below the external junctional zone (arrows). With the increased resolution of the electron microscope, many additional details of the Landolt club are revealed. When a low magnification electron micrograph of a tangential section through the external junctional zone is examined carefully (Fig. 4), it can be seen that several types of cells exist. One is the circular profiles of the photoreceptor myoid and nuclear region (R). The photoreceptor cytoplasm is rich in membranes and ribonucleoprotein particles, and it contains a nucleus; there are no mitochondria in the receptors at this retinal level. Between the circular cytoplasmic profiles are found the glial cells (G) whose irregular cytoplasm pervades the background and participates in formation of the prominent terminal bars of the external junctional zone (EJZ). Glial mitochondria are sausage-shaped, with a dark matrix and a few stubby cristae. No nucleus is ever seen in these cells at this retinal level. A third cytoplasmic type (L), contained in large (4 /.i) round cellular profiles, is characterized by a distinctly different mitochondrial morphology. These mitochondria are long and slim with a pale matrix against which the short cristae stand out clearly. The cristae are more numerous in these mitochondria than in the glial mitochondria; dense mitochondrial granules can be seen infrequently in both mitochondrial types. In lead stained sections there are pronounced granules (300 A in diameter) in the cytoplasm, which otherwise contains only a few scattered smooth membranes. No nucleus is ever visible, nor

4 Volume 5 Number 5 Landolt's club in amphibian retina 487 Fig. 3. Light micrograph of the adult newt retina. Below the external junctional zone (EJZ) between the photoreceptors, the densely stained cytoplasm of Landolt's club can be seen (arrows). HC denotes a probable horizontal cell. (Osmium-fixed, Araldite-embedded. 2t* section, stained with azure II methylene blue. x400,) is continuity ever seen with either the photoreceptors or the glial cells. This cytoplasmic type, distinct from the glial cell and photoreceptor, has now been identified as that of Landolt's club process. In the newt eye such processes are large and very numerous, which facilitates their study in thin sections. In addition, the mitochondria within these processes are sufficiently different from those of the surrounding cell types to serve as an internal marker so that, in electron micrographs, identification of Landolt's process can be made reliably at all levels. (Compare these two types of mitochondria in Figs. 4, 6, 7, and 9.) Moreover, in the present study, Landolt processes have been traced in electron microscopic serial sections from club enlargement to their parent cell, confirming and extending the information gained from whole cell impregnation. The bipolar cell nucleus giving rise to Landolt's process usually lies in the second or third layer of nuclei within the internal nuclear (bipolar) layer, immediately under the irregular horizontal cell layer (Fig. 3). The bipolar cell perikaryon contains only a narrow rim of cytoplasm about the nucleus, but apically the cytoplasm increases in amount. This cytoplasm is not remarkable in content, nor does this cell appear to differ from the surrounding bipolar cells. The particular cell which has been studied in serial sections from axon to club process (and from which series Fig. 5 is taken) sends a single basal process into the internal synaptic region. This is presumably the axon, although it could not be traced to its synaptic ending. As the bipolar apical cytoplasm or "dendrite trunk" reaches the external synaptic layer (Fig, 5, T), it almost immediately gives off small branches which run perpendicular to the trunk (Fig. 5, arrows). These are the "classical" dendrites which appear as thin, twisted processes in Golgi impregnation, and which terminate in synapse with the basal areas of the neighboring photoreceptors. Their cytoplasm is characterized in electron micrographs by particulate glycogen granules, small, dense mitochondria, and 200 to 300 A microtubules

5 488 Hendrickson Investigative Ophthalmology October 1966 Fig. 4. Low magnification electron micrograph of the newt retina just below the external junctional zone (EJZ). In this tangential section three cell types are identified. The receptors (R) are nucleated and lack mitochondria at this level. The glial cells (G) are irregular in shape and contain mitochondria with few cristae. The circular Landolt clubs (L) contain smaller mitochondria with numerous cristae of irregular shape, just below the external junctional zone are small areas of cytoplasm (arrows) one containing a centriole (C). (Osmium-collidine fixed, lead stained. xl6,000.)

6 Volume 5 Number 5 Landolfs club in amphibian retina 4S9 HC Fig. 5. Electron micrograph of the external synaptic layer (ESL) showing the dendritic tmnk (T) and the Landolt club (L) passing diagonally from lower left to upper right. Although not continuous in this picture, it is known from serial sections that this dendrite trunk does give rise to this Landolt process at point marked by X. At the arrows the synaptic dendrites (D) arise from the trunk. It can be seen that there is no intervening glial cytoplasm (C) between the Landolt club and the receptor R 1 while glial cytoplasm intervenes between the club and receptor R z. HC is a portion of a horizontal cell. (Osmium-collidine fixed, lead stained. xl9,000.)

7 490 Hendrickson Investigative Ophthalmology October 1966 which are present, but not well resolved, in Fig. 5. The Landolt process begins at the scleral edge of the external synaptic layer as the continuation of this bipolar apical trunk cytoplasm (Fig. 5, L). No side processes are given off above this level, nor is there any evidence of synaptic relationship to surrounding fibers, or to the photoreceptors. Mitochondria are arranged along the long axis of the process, which also contains participate glycogen granules, smooth membranes and 200 to 300 A microtubules, again not well resolved in Fig. 5. A large Golgi apparatus is often seen in the center of the process, aligned in the longitudinal direction. Frequently, within the external nuclear layer, the cell membrane of the club process comes to lie in close proximity (200 to 300 A separation) to the cell membrane of one or more adjoining receptors. In these regions there is a total lack of the usual intervening sheet of glial cytoplasm (Figs. 5, 6, and 8). The variation in this relationship can be seen in Fig. 6 where, in cross section, two Landolt processes (L 1, L 2 ) are in cell membrane proximity to one receptor (R 3 ) as well as to each other, but are separated from two other receptors (R 1, R-) by glia. Within the external nuclear layer, the Landolt process is approximately 1 /.i in diameter with localized enlargements sometimes being found. At the scleral edge of the external nuclear layer the process expands into a round club-shaped ending, up to 5 /.i across (Figs. 4 and 7, L). Some club enlargements remain in close proximity to the photoreceptor cell membrane (Fig. 8), while others are separated by an interposing layer of glial cytoplasm. This enlargement is densely packed with the same type of mitochondria seen scattered throughout the rest of the cytoplasm, but here they frequently possess an unusual membrane modification. The outer mitochondrial membrane often gives off long extensions into the surrounding cytoplasm which are indistinguishable from the other smooth membranes of the cell cytoplasm; at times the extensions run for considerable distances, at least 1 n (Figs. 7 and 8, arrows). The inner mitochondrial membrane and cristae do not participate in these extensions. In contrast, the surrounding glial and receptor mitochondria are completely lacking in such membrane extensions. Especially at this level, but generally throughout the process, the cytoplasm is rich in > ' 300 A diameter granules. These are presumably glycogen, for these cells stain deeply with periodic acid-schiff after fixation in Lison's fixative for glycogen, but this staining is absent after diastase digestion. Previous descriptions based on Golgi preparations placed the termination of the cell below the external junctional zone, but in electron micrographs additional small cross-sections of cells are seen at a level scleral to the circular clubs (Fig. 4, arrows). It is obvious from the connections designated by arrows in Figs. 9 and 10, that, rather than ending, the process narrows to a neck of less than 1 /.<., which is attached via terminal bars to neighboring glial cells, and then again expands to reach the free surface. At the attachment level or in the cytoplasm scleral to it, two centrioles are found. The proximal is a true centriole, while the distal acts as a basal body, giving rise to a single cilium. In cross section (Fig. 10, insert) the cilium contains a ring of 9 pairs of 200 A tubules, but lacks any tubules in its center. In addition, a second ring of 9 single 200 A tubules or fibrils lies outside of the inner ring, just under the cell membrane. The entire cilium is less than 3 fx in length. In Fig. 10 a ciliary rootlet system (CR) showing 525 A periodicity also occupies the neck region. The cell membrane near the base of the cilium gives rise to complicated infoldings and microvilli which project among the glial micro villi. Discussion There can seem to be little doubt that the bipolar cell process, first described by E. Landolt, 1 does in fact occur in the newt

8 Volume 5 Number 5 Landolt's club in amphibian retina 491 Fig. 6. A cross-section through the middle of the external nuclear layer. Three receptors (R 1 ' 2 > s) are within the field. At this level the receptor nuclei virtually fill the cell, allowing only a narrow rim of cytoplasm. Within the triangle between the receptors are other cell processes; two Landolt clubs (L 1 ', L 2 ) and at least one glial process (G), Both Landolt clubs are separated from receptors R' and R? by narrow layers of glial cytoplasm (GC) while R s and these same two processes are in cell-to-cell membrane proximity. Note the difference in morphology between the mitochondria of the glial and Landolt processes. {Osmium-collidine fixed, lead stained. x40,000.)

9 492 Hendrickson Inuestigfitive Ophthalmology October 1966 ( "J*. Fig. 7. Electron micrograph of the external junctional zone. Just below the line of terminal bars (TB) the club enlargements of two Landolt processes (L) are apparent. These are packed with mitochondria of the Landolt type; at the arrow can be seen one of the membrane extensions frequently observed in these mitochondria. {Osmium-collidine fixed, lead stained. xl6,000.) retina, and, as subsequently reported by various authors, in other amphibia, reptiles, sharks, birds, and mammals as well. Since detection of such a cell process depends to a large extent on whole cell impregnation, gaps still remain in assessing the distribution of this process in retinas in general or in any specific group of animals in particular. How is this process to be classified? It arises from the apical cytoplasm of a bipolar neuron, and its morphology, if not its ultimate destination, caused both Cajal 4 and Polyak 5 to term it a true dendrite. In electron micrographs from the region of the external synaptic layer, Landolt's club does not differ strikingly in cytoplasmic content from other bipolar dendrites, but there are variances in the number of organelles. In the "classical" dendrites, 200 to 300 A

10 Volume 5 Number 5 Landolt's club in amphibian retina 493 / a U \ m Fig. 8. Higher magnification of the external junctional zone region. One of a pair of Landolt clubs (L 1 ) lies next to, and in direct membrane proximity to, a receptor (R), The terminal bar (TB), however, occurs between the receptor and an unidentified cell process (U), probably glial. The Landolt cytoplasm contains glycogen (g) and scattered smooth membranes (m). Mitochondria! membrane extensions are found within both club processes (arrows). (Osmiumcollidine fixed, lead stained. x40,000.)

11 494 Hendrickson Investigative Ophthalmology October 1966 ) Fig. 9. Electron micrograph of the connection between the club enlargement and termination of the Landolt process (L). The mitochondria-rich club enlargement narrows abruptly to form a slender neck (arrow) which is bound to neighboring glia (G) by terminal bars (TB). This neck region normally contains two centrioles, only one of which is apparent in this picture (C). (Osmium-collidine fixed, lead stained. x25? 000.) microtubules are a common cytoplasmic feature even after osmium fixation, while they are present, but fewer in number, in the Landolt process. Mitochonclrial morphology does not vary significantly between bipolar cell body, dendrite, or Landolt process, but the Landolt process contains many more of these organelles, especially in the club enlargement. Particulate glycogen is common to all bipolar cell processes. Morphological distinctions are only part of the criteria for classifying nerve processes; functional consideration of conduction direction and synapses are equally important. On these latter points little can be said, for it is not known in what sense the Landolt club is functional. There is no evidence from neurophysiology bearing on conduction direction and thus far no electron microscope evidence for synaptic connections. Although diligently sought, no evidence of fused cell membranes or membrane specializations with or without syn-

12 Volume 5 Number 5 Landolt's club in amphibian retina 495 Fig. 10. The ciliary ending of the Landolt process. The single cilium arises from the distal centriole (C), which bears a satellite (S). Rootlets of 525 A periodicity (CR) can be seen coursing through the cytoplasm near the cilium. The neck or connecting region is indicated by an arrow, {Osmium-collidine fixed, lead stained. x40,000.) Inset. A cross-section of the cilium, showing 9 inner pairs of 200 A tubules and 9 single outer 200 A densities, possibly tubules. There is no central pair. x98,000.) aptic vesicles between Landolt process and photoreceptor has been found. While it is true that cell membrane proximity between Landolt process and receptor is common, this is generally not considered proof of a functional synapse without synaptic vesicles, membrane densities, or fusions. 10 Neuronal cell membranes lacking glial separation occur both in the optic nerve fiber layer 17 and between double cones, 8 ' l8 ' l9 yet no information appears to be transmitted at these sites. Membrane attachments in the form of terminal bars 15 do exist between Landolt processes and glial cells within the external junctional zone, but they have not been observed between photoreceptor and Landolt process at the same level. The concentration of mitochondria at this level, however, does suggest a localized high demand for energy-rich phosphates, but for what specific function is not clear. The occurrence of mitochondrial membrane expansions has previously been described by Robertson 20 in several types of tissue, including dendrites, to support a theory of mitochondrial origin from other cell membranes. Continuity of the outer mitochondrial membrane with smooth-surfaced membranes of the cytoplasm occurs frequently in the Landolt club, but has not been observed thus far in the mitochrondria of adjacent cell types. If this is a sign of mitochondrial synthesis, the club enlargement would then appear to have a higher mitochondrial turnover than the photoreceptors or glial cells. A further puzzling feature in attempts at classification is the termination of the club process in a single cilium. Such 9+0 cilia have been described in many tissues and are generally considered "sensory" 21 ' - although only in the case of the vertebrate photoreceptor 23 and insect ear 24 is a distinct sensory function indicated. It is known from studies of regenerating newt retina 25 that the Landolt process cilium arises at the time of outer segment formation, and in much the same manner. The divergence comes when the Landolt cilium remains as a simple cilium while the photoreceptor cilium goes on to form the elaborately infolded membranous outer segment, replete with visual pigment. At some unknown time after its formation, the Landolt cilium develops a second ring of 9 single tubules or fibrils outside of the inner ring of 9 tubule doublets while the outer segment connecting cilium of the photoreceptor remains in the original 9+0 organization. Both types of ciliary derivatives retain a rootlet system showing 525 A periodicity 25 and both ciliary basal bodies bear at least one satellite.

13 496 Henclrickson In cestigative Ophthalmology October 1966 Distribution of the Landolt club would seem to rule out a preferential relationship to one photoreceptor type since Landolt clubs occur in duplex retinas such as the newt, 9 rhesus monkey, 5 and human 5 ; the predominantly cone retina of the chicken (i ; and the predominantly rod retina of the shark. 7 Additionally, in the newt they appear to be associated with both rods and cones, although the exact proportion is not known. Morphologically the Landolt process resembles the receptor of the locust inner ear-' or the newt photoreceptor inner segment. Its origin, position and "neural" relations are somewhat analogous to the eccentric cell dendrite of Limulus eye. 2 " However, until further information is available concerning the physiology and function of this intriguing nerve process, more definite statements about its possible role in the retina would be only speculation. The author would like to express her thanks to Drs. John Luft and Douglas Kelly and Mr. William Stell for their support and criticism during this investigation. REFERENCES 1. Landolt E.: Beitrage zur Anatomic der Retina von Frosch, Salamander und Triton, Arch. Mikr. Anat. 7: 81, Ranvier, L. A.: Traite technique d'histologie, ed. 2, Paris, Levi, C: Ulteriori studi sullo sviluppo delle cellule visive negli Anfibi, Anat. Anz. 47: 192, Ramon y Cajal, S.: La Retine des Vertebras, La Cellule 9: 119, Polyak, S. L.: The Retina, Chicago, 1941, University of Chicago Press. 6. Shen, S." C, Greenfield, P., and Boell, E.: Localization of acetylcholinesterase in chick retina during histogenesis, j. Comp. Neurol. 106: 433, Stell, W.: Personal communication. 8. Cohen, A.: The fine structure of the visual receptors of the pigeon, Exper. Eye Res. 2: 88, Henclrickson, A.: The structure of Landolt's clubs in the newt retina, J. Cell Biol. 19: 33A, Stell, W.: Correlation of retinal cytoarchitecture and ultrastructure in Golgi preparations, Anat. Rec. 153: 389, Bennett, H. S., and Luft, J.: s-collidine as a basis for buffering fixatives, J. Biophys. & Biochem. Cytol. 6: 113, Millonig, C: A modified procedure for lead staining of thin sections, J. Biophys. & Biochem. Cytol. 11: 736, Richardson, K. C, Jarett, L., and Finke, E. H.: Embedding in epoxy resins for ultrathin sectioning in electron microscopy, Stain Technol. 35: 313, Arey, L.: Retina, choroid and sclera, in Cowdry, E. V., editor: Special cytology, ed. 2, vol.'3, New York, 1932, PaulB. Hoeber, Inc. Medical Book Division of Harper & Row, Publishers. 15. Fine, B.: Limiting membranes of the sensory retina and pigment epithelium, Arch. Ophthal. fi6: 847, Cohen, A.: Some electron microscopic observations on inter-receptor contacts in the human and macaque retina, J. Anat. 99: 595, Cohen, A.: Electron microscopic observations of the internal limiting membrane and optic nerve fiber layer of the retina of the rhesus monkey (M. mulatto), Am. J. Anat. 108: 179, ' 18. Pedler, C, and Tansley, K.: Fine structure of the cone of a diurnal gecko (Phelsunia inungttis), Exper. Eye Res. 2: 39, Sjostrand, F. S., and Elfvin, L.: Some observations on the structure of the retinal receptors of the toad eye as revealed by the electron microscope, in Proc. 4th Intern. Conf. Electron Microscopy, Stockholm, 1956, p Robertson, J.: Unit membranes: a review with recent new studies of experimental alterations and a new subunit structure in synaptic membranes, in Locke, M., editor: Cellular membranes in development, Twentysecond Growth Symposium, New York, 1964, Academic Press, Inc., pp Sorokin, S.: Centrioles and the formation of rudimentary cilia by fibroblasts and smooth muscle cells, J. Cell'Biol. 15: 363, Allen, R.: Isolated cilia in inner retinal neurons and in retinal pigment epithelium, J. Ultrastmct. Res. 12: 730, Cohen, A.: Vertebrate retinal cells and their organization, Biol. Rev. 38: 427, Gray, E. C: Fine structure of the insect ear, Philosophical Tr. Roy. Soc, Ser. B, 243, Henclrickson, A.: Photoreceptor regeneration in the newt, Triturus viridescens, in Abstracts of papers presented at the First Annual Meeting, Am. Soc. Cell Biol., Ruck, P.: On photoreceptor mechanisms of retinula cells, Biol. Bull. 123: 618, 1962.