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Ocular Pigmentation in White and Siamese Cats

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Ocular Pigmentation in White and Siamese Cats 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 This report describes the extent of ocular which have a deficiency of coat pigmentation pigmentation in white cats as compared with and which can also have reduced ocular pig- Siamese and normally pigmented cats. Sev- mentation. The comparative details of this eral of the animals used in these experiments hypopigmentation are of interest because the were also subjects in neurophysiological in- Siamese cat suffers from an abnormal visual vestigations of the visual pathways which are 1 3 4 pathway " yet the white cat does not. Al- described elsewhere.4 bino individuals of other mammalian species have pathway abnormalities similar to those Methods in the Siamese cat, and the suggestion has Ophthalmoscopic observations of six adult white been made that the cause might be specifical- cats (A through F) of undetermined genetic con- ly related to the amount of pigment in the stitution were made after each animal was pre- retinal epithelium.5' 6 pared for neurophysiological recordings. The pupil was dilated with atropine drops (1%), and a zero-power contact lens was fitted. Additional ob- servations were made on six white, five Siamese, From the Department of Physiology, John Curtin School and one solid black cat (cats G through S). Color of Medical Research, Australian National University, photographs of the fundus and iris were taken with Canberra, Australia. a Zeiss fundus camera and Kodacolor 400 film. L. N. Thibos was supported by a Postdoctoral Fellow- Upon completion of neurophysiological exper- ship of the U. S. Public Health Service. R. Morstyn iments, a dose of about 30 mg of pentobarbitone was a Vacation Scholar of the Australian National Uni- per kilogram of body weight was administered in- versity. Submitted for publication Dec. 27, 1978. travenously, and the eyes were enucleated. Each Reprint requests: Dr. W. R. Levick, Department of eye was then hemisected at the pars plana, the Physiology, John Curtin School of Medical Research, vitreous and lens were discarded, and the remain- Australian National University, P.O. Box 334, Can- der was fixed in 10% neutral buffered formal berra City, A.C.T. 2601, Australia. saline. 0146-0404/80/050475+12$01.20/0 © 1980 Assoc. for Res. in Vis. and Ophthal., Inc. 475 Downloaded from jov.arvojournals.org on 10/01/2021 Invest. Ophthalmol, Vis. Set. 476 Thibos, Levick, and Morstyn May 1980 Fig. 1. For legend see facing page. Downloaded from jov.arvojournals.org on 10/01/2021 Volume 19 Number 5 Ocular pigmentation in cat All To get satisfactory iris preparations, animals G illumination the iris had a translucent to di- through R, which were not part of neurophysiolog- aphanous appearance, quite different in side- ical experiments, were given 2 drops of 0.5% by-side comparison with irides of white cats. physostigmine to contract the pupil. A lethal dose Fundus. The dominant feature of an ordi- of pentobarbitone was administered, and the eyes nary pigmented cat's fundus is the yellow- were enucleated and fixed as described above. green tapetum which is surrounded by very Before the histological preparation was begun, fundus tissue was sandwiched between slices of dark pigmentation. Although the precise size and shape of the tapetum varies somewhat fixed cat liver to minimize detachment of the ret- 7 ina from the choroid. Tissues were embedded in from cat to cat, it is possible to predict the paraffin, and 5 /u,m sections were prepared with location of the tapetum with acceptable reli- hematoxylin and eosin stain. In some preparations ability. We shall refer to this fiducial region melanin pigment was bleached by treatment with where one expects to find a tapetal reflection 0.1% potassium permanganate followed by 1% as the tapetal zone. The statements which oxalic acid. Photomicrographs were taken with a follow are based upon complete ophthalmo- Zeiss photomicroscope. scopic and microdissection surveys of the entire fundi supported by selected fundus Results photographs. Ophthalmoscopic and microdissection ob- The fundi of yellow-eyed white cats had servations. A summary of the gross appear- the same appearance as those of ordinary ance of the irides and fundi of the 18 subjects pigmented cats. A yellow-green reflection is given in Table I. from the tapetal zone and the retinal blood Iris. Color photographs of the living cat iris vessels within this region were clearly visible are given in Fig. 1, A, for a yellow-eyed ophthalmoscopically (e.g., upper part of Fig. white cat and in Fig. 1, C, for a blue-eyed 1, B). The shape and size of the tapetum was white cat. Dissection of the eyes of white cats within the normal range.7 The nontapetal revealed the posterior surfaces of the irides to zone appeared dark brown (e.g., lower part be as darkly pigmented as in the ordinary of Fig. 1, B) due to the presence of pigment pigmented cat. Side-by-side comparison un- in both the retinal epithelium and choroid as der transillumination showed that the yellow observed by microdissection. However, in and blue irides of white cats were each as eight of 12 eyes there were horizontally elon- opaque as the yellow iris of a black cat. gated patches of inferior fundus about 5 to The blue iris of a seal-point Siamese cat is 10 mm wide and 2 to 4 mm high where shown in Fig. 1, E. The posterior surface of choroidal pigment was completely missing. the Siamese iris was not as heavily pigmented Apart from these patches, the choroid was as in the ordinary pigmented cat, appearing a equally heavily pigmented inside and outside dark chocolate brown in the seal point and a the tapetal zone. lighter brown in the lilac point. Under trans- 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 jov.arvojournals.org on 10/01/2021 Invest. Ophthalmol. Vis. Sci. 478 Thibos, Levick, and Morstyn May 1980 Table I. Summary of macroscopic appearance of ocular pigmentation Cat identification /breed G A F H Q B D E L C I p R J M N s K Eye Bl W W W w W W W W W w- w W SS SS SS SS LS Iris color Y Y By Y By B B B B B B B B R Y Y Y Y Bv B B B B B B B B B Iris epithelium L 0 o o o o R o 0 o o o Choroid L Of o o o 0 o o o o R o o o o o o o o o Tapetum L O o o o R o o o o Retina epithelium L O G O O O outside tapetal zone R O O O O 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 same as for the white cats described above.
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