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Ocular pigmentation in white and Siamese cats L. N. Thibos, W. R. Levick, and R. Morstyn Ocular pigmentation in white cats with blue and. yellow eyes and in Siamese cats was examined ophthalmoscopically and histologically. Yellow-eyed, white cats had. entirely normal ocular pigmentation. Blue eyes of white cats had. normal pigmentation of the iridial and. retinal pigment epithelia but no stromal pigmentation of the iris or choroid. This deficit is apparently due to the absence of stromal pigment cells, certainly in the iris. As a general rule, the blue eye of white cats had no tapetum. Siamese cats had reduced pigmentation of the iridial and retinal pigment epithelia and no stromal pigmentation of the iris or choroid. The lack of pigmentation is apparently due to the inability of stromal pigment cells to produce pigment, certainly in the iris. We conclude that the abnormality of visual pathways previously described, in the Siamese cat is not due simply to a deficiency of pigment in cells of neural crest origin. Key words: ocular pigments, white cats, Siamese cats, iris pigment, choroid pigment v3iamese and white cats are two breeds which have a deficiency of coat pigmentation and which can also have reduced ocular pigmentation. The comparative details of this hypopigmentation are of interest because the Siamese cat suffers from an abnormal visual pathway 1 " 3 yet the white cat does not. 4 Albino individuals of other mammalian species have pathway abnormalities similar to those in the Siamese cat, and the suggestion has been made that the cause might be specifically related to the amount of pigment in the retinal epithelium. 5 ' 6 From the Department of Physiology, John Curtin School of Medical Research, Australian National University, Canberra, Australia. L. N. Thibos was supported by a Postdoctoral Fellowship of the U. S. Public Health Service. R. Morstyn was a Vacation Scholar of the Australian National University. Submitted for publication Dec. 27, 1978. Reprint requests: Dr. W. R. Levick, Department of Physiology, John Curtin School of Medical Research, Australian National University, P.O. Box 334, Canberra City, A.C.T. 2601, Australia. This report describes the extent of ocular pigmentation in white cats as compared with Siamese and normally pigmented cats. Several of the animals used in these experiments were also subjects in neurophysiological investigations of the visual pathways which are described elsewhere. 4 Methods Ophthalmoscopic observations of six adult white cats (A through F) of undetermined genetic constitution were made after each animal was prepared for neurophysiological recordings. The pupil was dilated with atropine drops (1%), and a zero-power contact lens was fitted. Additional observations were made on six white, five Siamese, and one solid black cat (cats G through S). Color photographs of the fundus and iris were taken with a Zeiss fundus camera and Kodacolor 400 film. Upon completion of neurophysiological experiments, a dose of about 30 mg of pentobarbitone per kilogram of body weight was administered intravenously, and the eyes were enucleated. Each eye was then hemisected at the pars plana, the vitreous and lens were discarded, and the remainder was fixed in 10% neutral buffered formal saline. 0146-0404/80/050475+12$01.20/0 © 1980 Assoc. for Res. in Vis. and Ophthal., Inc. Downloaded From: http://jov.arvojournals.org/pdfaccess.ashx?url=/data/journals/iovs/933322/ on 06/14/2017 475 476 Thibos, Levick, and Morstyn Fig. 1. For legend see facing page. Downloaded From: http://jov.arvojournals.org/pdfaccess.ashx?url=/data/journals/iovs/933322/ on 06/14/2017 Invest. Ophthalmol, Vis. Set. May 1980 Volume 19 Number 5 To get satisfactory iris preparations, animals G through R, which were not part of neurophysiological experiments, were given 2 drops of 0.5% physostigmine to contract the pupil. A lethal dose of pentobarbitone was administered, and the eyes were enucleated and fixed as described above. Before the histological preparation was begun, fundus tissue was sandwiched between slices of fixed cat liver to minimize detachment of the retina from the choroid. Tissues were embedded in paraffin, and 5 /u,m sections were prepared with hematoxylin and eosin stain. In some preparations melanin pigment was bleached by treatment with 0.1% potassium permanganate followed by 1% oxalic acid. Photomicrographs were taken with a Zeiss photomicroscope. Results Ophthalmoscopic and microdissection observations. A summary of the gross appearance of the irides and fundi of the 18 subjects is given in Table I. Iris. Color photographs of the living cat iris are given in Fig. 1, A, for a yellow-eyed white cat and in Fig. 1, C, for a blue-eyed white cat. Dissection of the eyes of white cats revealed the posterior surfaces of the irides to be as darkly pigmented as in the ordinary pigmented cat. Side-by-side comparison under transillumination showed that the yellow and blue irides of white cats were each as opaque as the yellow iris of a black cat. The blue iris of a seal-point Siamese cat is shown in Fig. 1, E. The posterior surface of the Siamese iris was not as heavily pigmented as in the ordinary pigmented cat, appearing a dark chocolate brown in the seal point and a lighter brown in the lilac point. Under trans- Ocular pigmentation in cat All illumination the iris had a translucent to diaphanous appearance, quite different in sideby-side comparison with irides of white cats. Fundus. The dominant feature of an ordinary pigmented cat's fundus is the yellowgreen tapetum which is surrounded by very dark pigmentation. Although the precise size and shape of the tapetum varies somewhat from cat to cat,7 it is possible to predict the location of the tapetum with acceptable reliability. We shall refer to this fiducial region where one expects to find a tapetal reflection as the tapetal zone. The statements which follow are based upon complete ophthalmoscopic and microdissection surveys of the entire fundi supported by selected fundus photographs. The fundi of yellow-eyed white cats had the same appearance as those of ordinary pigmented cats. A yellow-green reflection from the tapetal zone and the retinal blood vessels within this region were clearly visible ophthalmoscopically (e.g., upper part of Fig. 1, B). The shape and size of the tapetum was within the normal range.7 The nontapetal zone appeared dark brown (e.g., lower part of Fig. 1, B) due to the presence of pigment in both the retinal epithelium and choroid as observed by microdissection. However, in eight of 12 eyes there were horizontally elongated patches of inferior fundus about 5 to 10 mm wide and 2 to 4 mm high where choroidal pigment was completely missing. Apart from these patches, the choroid was equally heavily pigmented inside and outside the tapetal zone. The fundus of the usual blue-eyed white Fig. 1. Iris and fundus of the living cat eye. Shown are yellow-eyed white cat iris (A) and fundus (B), blue-eyed white cat iris (C) and fundus (D), and seal-point Siamese iris (E) and fundus (F) (left eye of animals Q, R, and S, respectively). The darkly pigmented pupillary ruff of the epithelium, located at the pupil margin, is more evident in A and C than in E. Note the translucent quality of the Siamese iris. Fundus photographs all show the inferior border of the tapetal zone with the retinal blood vessels exiting from the optic nerve head in the upper left of the picture. Each field subtends about 30° of visual angle and each is centered approximately 15° below and 7° nasal to the center of the area centralis, so as to show parts of both tapetal and nontapetal zones. B has the appearance of the ordinary pigmented cat's fundus. D lacks both tapetum and choroidal pigment, thereby revealing choroidal blood vessels. Clumps of retinal epithelial pigment are evident at the bottom of D. Because of the absence of a reflecting tapetum, the exposure for this photograph had to be increased substantially relative to B and F. F has a tapetum, dilute epithelial pigment, but no choroidal pigment; thus the choroidal vessels are visible below the tapetal zone. Downloaded From: http://jov.arvojournals.org/pdfaccess.ashx?url=/data/journals/iovs/933322/ on 06/14/2017 Invest. Ophthalmol. Vis. Sci. May 1980 478 Thibos, Levick, and Morstyn Table I. Summary of macroscopic appearance of ocular pigmentation Cat identification /breed Eye G Bl A W F W Y Iris color Iris epithelium Choroid Tapetum Retina epithelium outside tapetal zone H W R L R L R L R L Y Y Q w Y D W E W L W C W w- w R W SS By Y By Y Bv B B B B B B Y I J B W p B N SS SS K LS B B B B B B B o 0 o o 0 o o o o o o o o o B o o o o o o o o O G O O O O O O O O B 0 Of O o o o o o o o o o o R s M SS B o W = white cat; Bl = black cat; SS = Seal-point Siamese cat; LS = Lilac-point Siamese cat. Letter O not used for identification of cat. Y = yellow appearance; B = blue appearance; By = blue with yellow sector. Key: • Pigment (or tapetum) present; O pigment diluted; O pigment (or tapetum) absent. '"Absence of pigment in a portion of inferior fundus. tQuadrantic sector of pigment in temporal midperiphery. cat was strikingly different from that of the ordinary pigmented cat.8- 9 There was no bright yellow reflection from the tapetal zone, nor was there any choroidal pigmentation. Hence in vivo the tapetal zone had the appearance of a jungle of red blood vessels (e.g., upper part of Fig. 1, D). Microdissection of the fixed eyecup showed complete lack of choroidal pigmentation. There were the following exceptions. In three eyes a predominantly blue iris was partly yellow; in two of these eyes the fundus had both tapetum and choroidal pigment. In all the blue-eyed white cats the pigment of the retinal epithelium was present outside the tapetal zone but not within, just as in normal eyes. Pigmentation is shown in the lower part of Fig. 1, D, partly obscuring the choroidal blood vessels. The Siamese fundus in vivo showed a slightly desaturated tapetal reflection (e.g., upper half of Fig. 1, F) and outside the tapetal zone appeared reddish brown10 (e.g., lower part of Fig. 1, F). Microdissection showed that there was no choroidal pigmentation. Visibility of the tapetum was reduced in the fixed eye cup, probably because the absence of choroidal pigment led to increased amounts of scattered light. The distribution of pigment in the retinal epithelium was the same as for the white cats described above. However, side-by-side comparison between fundi of the Siamese and blue-eyed white cats, neither of which had choroidal pigment, showed that the white cat's epithelial pigment was significantly darker. Even at the border of the tapetal zone where the epithelial pigmentation just began, it was observed that the pigment clumps in the white cat were darker than pigment in any part of the Siamese retinal epithelium. This dilution of epithelial pigment was quite definite in the seal-point Siamese and even more obvious in the lilac-point. Histological observations Yellow iris. A cross-sectional view of the yellow iris taken from a black cat is shown in Fig. 2, A, and from a yellow-eyed white cat in Fig. 2, B. The pigmentation of the white cat's iris is the same as for the black cat's iris: both have heavily pigmented epithelial cells plus light brown pigment cells of the stroma. The stromal pigment is in the form of long thin filaments similar to that found in rhesus monkey iris stromal cells11 known to be true melanocytes.12' 13 Thus it is likely that the cat's stromal pigment cells are also melanocytes. A magnified view of individual stromal pigment cells is given in Fig. 3, A. Perikaryal Downloaded From: http://jov.arvojournals.org/pdfaccess.ashx?url=/data/journals/iovs/933322/ on 06/14/2017 Volume 19 Number 5 Ocular pigmentation in cat B 100 y Fig. 2. Cross-sections of irides from black (A), yellow-eyed white (B), blue-eyed white (C), and Siamese (D) cats (animals G, H, I, and K, respectively). Specimens oriented with posterior (epithelial) surface to left, anterior (stromal) to right. Magnification bar (100 ju,m) is common to all. Pigment cells (PC) and pupillary ruff(R) are labeled in A only. Downloaded From: http://jov.arvojournals.org/pdfaccess.ashx?url=/data/journals/iovs/933322/ on 06/14/2017 479 480 Invest. Ophthalmol. Vis. Set. May J980 Thibos, Levick, and Morstyn A B Fig. 3. Tangential sections of irides of yellow-eyed white (B), blue-eyed white (C) and Siamese (D) cats (same animals as in Fig. 2). Presumed pigment cell nuclei (PN) indicated by arrows. Magnification bar (100 /Am) in B applies to B to D only. A, High-magnification view of three pigment cells found at various locations of the iridial stroma in yellow-eyed white cat of B. Downloaded From: http://jov.arvojournals.org/pdfaccess.ashx?url=/data/journals/iovs/933322/ on 06/14/2017 Volume 19 Number 5 Ocular pigmentation in cat Fig. 4. Fundus cross-sections in nontapetal region approximately 5 to 7 mm below the area centralis in black (A), yellow-eyed white (B), blue-eyed white (C) and Siamese (D) cats (animals G, H, I, and J, respectively). Structures indicated are photoreceptors (R), retinal epithelium (E), choroidfCj, sclerafSJ, and blood vessel (B). Magnification bar (100 JLUTI) is common to all. Separation of layers is artefactual. Downloaded From: http://jov.arvojournals.org/pdfaccess.ashx?url=/data/journals/iovs/933322/ on 06/14/2017 481 482 Thibos, Levick, and Morstyn Invest, Ophthalmol. Vis. Set. May 1980 Fig. 5. Fundus cross-sections in tapetal region approximately 3 mm above the area centralis. Animals used and key to structures indicated are as in Fig. 4. Tapetum (T) is also shown. Magnification bar (100 fxm) is common to all. Downloaded From: http://jov.arvojournals.org/pdfaccess.ashx?url=/data/journals/iovs/933322/ on 06/14/2017 Volume 19 Number 5 shape varies from nearly circular to long and slender. A constant feature of these cells is the large, darkly stained, oval nucleus. The mean of the minimum and maximum nuclear diameters determined for 20 cells ranged from 4.75 to 7.25 /am and averaged 5.5 /xm. Such nuclei were found only within the pigmented cells of the stroma. Stromal pigment cells tended to be concentrated on the anterior surface of the iris. A tangential section made parallel to the iris surface therefore revealed a large number of pigment cells for both black and yellow-eyed white cats (Fig. 3, B). The greatest density of pigment cells was found at invaginations and at the margin of these sections, which was the most anterior portion of the tissue. Blue iris. Cross-sections of the blue-eyed white cat's iris showed a normal, heavily pigmented epithelium (Fig. 2, C). No pigmented cells were found in tangential sections of iris (Fig. 3, C). We ruled out the possibility that such cells were present but unpigmented, as suggested by Lauber,14 because none of the stromal nuclei had the quantitative characteristics described above. Pigmentation in the Siamese blue iris was distinctly different from that of the blue-eyed white cat. The layer of pigment within the epithelium was thinner, and the granules were less densely packed, particularly in the lilac-point animal. Consequently individual pigment granules could be observed, and the normal pupillary ruff was hardly evident (Fig. 2, D). The stroma was not pigmented, but contrary to the situation in the blue-eyed white cat, the pigment cells were evidently present. This conclusion is based on the tangential section of Fig. 3, D, which shows many large oval nuclei having the expected quantitative characteristics. Fundus of the yellow eye. The comparison of pigmentation in different eyes is most straightforward if one avoids the margin of the tapetal zone where the thickness of epithelial pigment varies. Accordingly, the data presented below for the tapetal zone are from the region about 3 mm above the area centralis and for the nontapetal zone is from the Ocular pigmentation in cat 483 region about 7 mm below the area centralis. Pigmentation of choroid and retinal epithelium in the nontapetal zone for a black cat is shown in Fig. 4, A, and the yellow-eyed white cat in Fig. 4, B. Epithelial pigment thickness was in the range of 10 to 20 /Am in these cats. A heavy infiltration of pigment obscured the nuclei of the choroidal cells, but bleaching revealed the nuclei to be of variable size and shape with no unambiguous identifying characteristics. The thickness of the choroid was due mainly to blood vessels rather than the thin pigment cells. Within the tapetal zone of the black (Fig. 5, A) and yellow-eyed white (Fig. 5, B) cat fundus the retinal epithelium was devoid of pigment, and the large, roughly circular nuclei of this single-cell layer were evident. The tapetal cells were easily recognized by their large nuclei, elongated perikaryon, regular array, and particular coloration. The choroid was equally heavily pigmented in the tapetal and nontapetal zones. Fundus of the blue eye. Outside the tapetal zone the blue-eyed white cat's fundus (Fig. 4, C) had heavily pigmented retinal epithelial cells. Above the area centralis within the tapetal zone the retinal epithelium was unpigmented (Fig. 5, C) as in the ordinary pigmented cat, but tapetal cells were absent entirely. There was no choroidal pigmentation. Because no unique features of choroidal pigment cells had been found in the ordinary pigmented cat, we were unable to determine whether in the blue-eyed white cat the pigment cells were present and unpigmented or completely absent. The Siamese cat had a thinner than normal layer of pigment in the retinal epithelium in the nontapetal zone (Fig. 4, D). From the edge of the tape turn to the ora terminalis, the maximum thickness of the epithelial pigment layer in the seal-point retina was 5 (xm and in the lilac-point 3 fxm, considerably less than in the ordinary pigmented cat. Within the tapetal zone there was no epithelial pigment and the tapetal cells appeared normal. Choroidal pigmentation was completely absent throughout the fundus, but it could not be Downloaded From: http://jov.arvojournals.org/pdfaccess.ashx?url=/data/journals/iovs/933322/ on 06/14/2017 Invest. Ophthalmol. Vis. Sci. May 1980 484 Thibos, Levick, and Morstyn determined whether the pigment cells were absent altogether or present but devoid of pigment. Discussion The results of this study are best summarized in relation to the postulated embryological source of the various types of ocular pigment cells. This is not known specifically for the cat, but the following description is common to other mammals,15' 16 birds,17 and amphibia.18 Pigment cells of the iris and choroidal stromata are derived from cells which migrate from the neural crest. The tapetal cells of the cat are likely to be modified choroidal pigment cells19 and hence also of neural crest origin. On the other hand, the pigment epithelia of the retina and iris are products of the embryonic eye cup. Our results show that the yellow-eyed white cat has normal ocular pigmentation but the blue-eyed white cat lacks pigment in the iridial and choroidal stromata. The basis of the deficit in the blue eye appears to be the absence of the pigment cell itself. Presumably the neural crest cells either failed to migrate to the ocular tissue or failed to differentiate and survive as uveal pigment cells. The Siamese cat is also deficient in ocular pigment but in quite a different way. First, there is a relative diminution of pigmentation of the iridial and retinal epithelia. Second, the common lack of pigmentation in iridial and choroidal stromata is associated with the presence of unpigmented pigment cells, certainly in the iris and possibly also in the choroid. In summary, the blue-eyed white cat lacks a particular cell type, whereas the Siamese is defective in pigment production. There were exceptions to the above generalizations. Three of 12 blue irides of white cats had yellow sectors, and in one of these eyes there was a patch of choroidal pigmentation. In eight of 12 yellow eyes of white cats the choroidal pigmentation was incomplete. This heterogeneity might indicate the presence of piebald spotting in the eye. It has long been suspected20 that a blue eye would appear if a cat carrying the gene for dominant white also carried the gene pattern for piebald spotting of the eye. White spots would be undetectable in the completely white hair and pink skin caused by dominant white, but the edges of a spot might be revealed by the ocular pigment. Tapetum lucidum. The results obtained from blue-eyed white cats have shown that the usual deletion of pigment from the retinal epithelium within the tapetal zone is not under direct tapetal control, since in these cats the tapetal cells were absent but the distribution of retinal epithelial pigment was normal. Bernstein and Pease19 have suggested that tapetal cells are modified choroidal melanocytes. The Siamese has heretofore been in apparent contradiction to this hypothesis, since choroidal pigment is missing yet the animal has a normal tapetum. The inconsistency is resolved if the present results on iris tissue are generalized to include choroid. We may suppose that unpigmented choroidal pigment cells are likely to be present in the Siamese cat and therefore the derivative tapetal cells would be present as well. If this be true, it would imply that functional tapetal rods, which are the basis of light reflection phenomena,21 are not dependent upon melanin production. Genetics. The all-white coat of the white cat is inherited as a dominant character8'20"22 and therefore cannot be the result of the action of an allele at the albino locus.23 There has been only one report of an albino cat,24 but it is commonly believed that the Siamese breed represents an imperfect form of albinism, as first suggested many years ago.25' 26 The evidence for this view is that (1) Siamese is a recessive characteristic2' resulting in hypopigmentation and (2) the pattern of thermolabile coat pigmentation in the Siamese cat resembles that in the Himalayan rabbit,28' 29 the gene of which is known to be allelomorphic with the albino gene.27 The cause of the lack of pigment in albino animals and some albino humans appear to be a failure of individual melanocytes to produce pigment because of default of the enzyme tyrosinase.l6- 30> 31 A piebald white spot, Downloaded From: http://jov.arvojournals.org/pdfaccess.ashx?url=/data/journals/iovs/933322/ on 06/14/2017 Volume 19 Number 5 on the other hand, results from the failure of neural crest-derived pigment cells to survive.16' 32 Thus one finds amelanotic melanocytes, or "clear cells," in the skin and hair bulbs of albino individuals but not in piebald individuals.16- 32- M Our observation of clear cells in the iris stroma of the Siamese but not in the white cat provides further experimental support for the idea that Siamese is a member of the albino series whereas dominant white is related to piebaldism, and is consistent with the idea first proposed by Wright34 that Siamese and white cats are two breeds deficient in pigment as a result of entirely distinct genetic mechanisms. Optic nerve decussation. We have shown elsewhere4 that the decussation of optic nerve fibers in white cats is indistinguishable from normal. Coupled with the present anatomical results, this means that normal development of the visual pathways may proceed without neural crest-derived pigment cells and rules out the possibility that the Siamese neural abnormality is simply due to the lack of pigment in cells of neural crest origin. The hypothesis that neural development depends upon optic cup pigmentation5-6 may still be consistent with the results of this paper. One argument against this view, however, is that in cats with normal visual pathways the retinal epithelium is unpigmented in the tapetal zone. Examples from the literature7- 35 indicate that this unpigmented region is from V3 to V2 the area of the retina and may include % of the retinal ganglion cells.36 If the adult pattern of epithelial pigment distribution is also present in the developing embryo (an assumption we have verified in fetuses as small as 11 mm crown-rump length), then it is difficult to see how the presence of pigmentation outside the tapetal zone could control the decussation of axons arising from ganglion cells within the tapetal zone. If the pigment hypothesis becomes untenable, then it may be that the embryonic control of optic nerve decussation is related to some more fundamental abnormality in Si- Ocular pigmentation in cat 485 amese, such as a defective enzyme tyrosinase, and that reduced pigmentation is merely one particular reflection of that defect.6 We are grateful to Miss W. Hughes who prepared the histological slides. Mrs. E. van de Pol rendered valuable technical assistance. We appreciated the technical support provided by Messrs. L. M. Davies, R. M. Tupper, and P. C. Kent and members of the Photographic Service. The fundus camera used in this study was generously provided by the Lions Club of Canberra. REFERENCES 1. Guillery RW: An abnormal retino-geniculate projection in Siamese cats. Brain Res 14:739, 1969. 2. Kalil R, Jhavery S, and Richards WR: Anomalous retinal pathways in the Siamese cat: an inadequate substrate for normal binocular vision. Science 174: 302, 1971. 3. Hubel DW and Wiesel TN: Aberrant visual projections in the Siamese cat. J Physiol 218:33, 1971. 4. Levick WR, Thibos LN, and Morstyn R: Retinal ganglion cells and optic decussation in white cats. (Submitted for publication.) 5. Sanderson KJ, Guillery RW, and Shackelford RM: Congenitally abnormal visual pathways in mink {hiustela vision) with reduced retinal pigment. J Comp Neurol 154:225, 1974. 6. LaVail JR, Nixon RA, and Sidman RL: Genetic control of retinal ganglion cell projections. 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Witkop CJ, White JC, and King RA: Oculocutaneous albinism. In Heritable Disorders of Amino Acid Metabolism: Patterns of Clinical Expression and Genetic Variation, Nyhan WL, editor. New York, 1974, John Wiley & Sons, Inc., pp. 177-261. 24. Bamber RC and Herdman EC: Two new colourtypes in cats. Nature 127:558, 1931. 25. Bateson W: Mendel's Principles of Heredity. Cambridge, England, 1909, Cambridge University Press. 26. Castle WE: Siamese, an albinistic colour variation in cats. Am Natur 53:265, 1919. 27. Keeler CE and Cobb V: Allelomorphism of silver and Siamese coat variations in the domestic cat. J Hered 24:181, 1933. 28. Schultz W: Willkiirliche Augenpigmentierung beim Saugetieralbino. Arch Entwcklngsmechn Organismen 109:287, 1927. 29. Iljin NA and Iljin VN: Temperature effects on the color of the Siamese cat. J Hered 21:309, 1930. 30. Fitzpatrick TB and Lerner AB: Biochemical basis of human melanin pigmentation. Arch Dermatol Syphilol 69:133, 1954. 31. Foster M: Mammalian pigment genetics. Adv Genet 13:311, 1965. 32. Silvers WK: Pigment cells: occurrence in hair follicles. J Morphol 99:41, 1956. 33. Becker SW, Fitzpatrick TB, and Montgomery H: Human melanogenesis: cytology and histology of pigment cells (melanodendrocytes). Arch Dermatol Syphilol 65:511, 1952. 34. Wright S: Color inheritance in mammals. J Hered 9:139, 1918. 35. Hughes A: A supplement to the cat schematic eye. Vision Res 16:149, 1976. 36. Hughes A: A quantitative analysis of the cat retinal ganglion cell topography. J Comp Neurol 163:107, 1975. Downloaded From: http://jov.arvojournals.org/pdfaccess.ashx?url=/data/journals/iovs/933322/ on 06/14/2017