Normal Eye Color in the Chicken

Normal Eye Color in the Chicken NORMAN M. NELSON* U. S. Regional Poultry Research Laboratory, East Lansing, Mich. (Received for publication July 18, 1946) INTRODUCTION TTYE color, or more specifically, iris *-' color, in the chicken has become a subject of interest since the pathological "gray eye" has been recognized as a manifestation of lymphomatosis. In attempting to control lymphomatosis by selection, it is probable that there may be discrimination against the apparently normal pale yellow or normal gray-eyed birds. This review will attempt to clarify the physiological basis for normal eye color, particularly for the common utility breeds of chickens. It has been pointed out that ocular lymphomatosis, frequently referred to as "iritis," "pearl eye," "fish eye," "gray eye," etc., cannot be accurately diagnosed on the basis of color alone (Nelson and Thorp 1943), However, since the poultryman is continually advised to cull all gray-eyed birds, the practical importance of distinguishing between variations in the normal and the pathological condition is readily seen. In order to make this distinction, the pigments which produce eye color and those factors which are known to influence variations in eye color will be described. PIGMENTS AND THEIR VARIATION The American Standard of Perfection (1940) lists the eye color for breeds and varieties of chickens. Within the breeds which are most important economically * Now with Bureau of Animal Industry, Agricultural Research Administration, U. S. Department of Agriculture, Des Moines. 61 in this country, the reddish-bay eye is listed most frequently, and the brown eye would appear to be second in importance in this respect. Duke-Elder (1942) classifies the body pigments as follows: 1. Haematogenous 2. Cellular a. lipochromes b. melanin 3. Metallic 4. Rhodopsin When this classification is applied to the chicken eye, the reddish-bay iris is the result of red (hemoglobin) and yellow (carotenoid) while the brown iris results from the addition of black (melanin) to the two preceding pigments. This discussion will not concern the metallic pigments of other animals nor rhodopsin, a retinal pigment. For the purpose of this paper, the pigments of the iris may be classified into two groups: (1) the residual or permanent pigments and (2) the transient or variable pigments. Of the three pigments which are responsible for the final gross eye color, melanin, the black pigment, would be considered as permanent and nonvariable because of inherent qualities and the carotenoid, or yellow pigment, and the hemoglobin, or red pigment, would be considered variable due to physiological changes. THE RED PIGMENT FACTOR The red component of the gross eye color is due to the superficial vascular r te of the iris. Except in large, healthy males

62 NORMAN M. NELSON these vessels are not ordinarily visible by naked eye examination, but blend with the surrounding yellow-orange pigment to produce the reddish-bay color. However, upon close examination with a magnifying device such as an ophthalmoscope, the capillaries may be seen apart from the general mixture of colors. Ball (1944) stated that the degree of vascularity or redness was due to variations in the diameter of the blood vessels. He further stated, "With regard to the prominence of the vascular system of the iris, it seems certain that there are consistent and normal variations among individual birds." Likewise, his data show that within individuals gross variations in redness did not occur but were scored the same quite consistently over an 85-day period. He made these observations at approximately twoweek intervals. INFLUENCE ON RED PIGMENT The color value of the red is influenced by the degree of injection of the iridic vessels as well as by the hemoglobin content of the blood. In the case of inflammation hyperemia may occur in the early stages. Such conditions as trauma about the head, particularly of the ocular adnexa, tend to dilate the vessels of that locale. Inflammation of the iris (iritis) from any cause would be demonstrated by dilatation of the vessels and occurs in the early stages of ocular lymphomatosis (Nelson and Thorp 1943). A lesser amount of red coloration will be noted in cases suffering from general anemia due to loss of blood volume. Likewise, in blood dyscrasias which demonstrate a low hemoglobin content, the influence of this pigment on the gross eye color will be less; hence, the iris will appear paler than normal. Since in the vascular rete of the iris the blood vessels are in general decreasing in diameter as they converge toward the pupil, it does not follow that the variations in the diameter of the blood vessels are alone responsible for visible differences in redness. Besides the factors already mentioned, the concentration of the carotenoid pigments which surround the vessels as well as the relative distance of the vessels from the surface would effect variations in the intensity of the red hue. YELLOW PIGMENT FACTOR The yellow to orange hue of the chicken iris is due to carotenoid pigments. Hollander and Owen (1939) found that 95 percent ethyl alcohol extracted all the yellow color from the chicken iris and by further chemical tests determined the pigment to be composed of carotene and xanthophyll. Bond (1919) described the histological presence of the pigment: "In some breeds the orange color is due to the presence of large numbers of branching connective tissue cells containing yellow or yellow-brown pigment granules, which lie around the capillaries and muscle fibers. In other breeds of which the Dorking and Orpington are examples, the muscle fibers themselves often contain the yellow pigment granules." Bond did not consider that the histological form of the pigment might vary within a breed because of differences in concentration of the pigment. The writer has observed the yellow pigment bearing cells in frozen sections stained with Sudan IV. In the immature chicken the muscle fibers alone appear to contain the fat supported pigment while in adult birds there are fat globules. In hematoxylin and eosin stained sections there are mesenchymal-like cells around the anterior vessels and these may be lipophores. INFLUENCES ON YELLOW PIGMENT The intensity of the yellow color in the iris appears to be affected by (1) the amount of carotenoid pigment and the

NORMAL EYE COLOR IN THE CHICKEN 63 pigmentation inhibiting factor in the diet, (2) egg production, (3) genetic variation, and (4) pathological conditions. It has been shown by extraction and electrophotometric methods (Nelson 1944) that the concentration of the carotenoid pigments varies in the iris of different chickens. Ball (1944) likewise has scored gross observations on living birds as well as readings made by the dissecting microscope and tissue mounts, and by extraction, methods showed variations of concentration. Palmer (1915) and Blakeslee and Warner (1915) showed that the yellow pigment of the shanks, beak, skin, and egg yolk is affected by continuous egg production. Hammond and Harshaw (1941) found that the shank and skin color is influenced by (1) the quantity of pigmentation-suppressing factor in the diet, (2) quantity of xanthophyll pigment in the diet, and (3) the breed itself. Culton and Bird (1941) demonstrated that certain protein supplements varied in the content of pigmentation-inhibiting factor or factors. Sherwood and Fraps (1941) recognized a "carotene destroying power" of certain animal feeds. For the purposes of this discussion, the "destroying power" of these authors, the "suppressing" factor of Hammond and Harshaw, and the "inhibiting" factor of Culton and Bird will be referred to as an "inhibiting factor." While these observations have been made on locations in the body other than the eye, Hollander and Owen (1939), as previously stated, showed the iris pigment to be similar to these body pigments. Ball writes of pigment-free diet as "preventing deposition" of carotenoid pigments in the iris. He stated further that the order of removal of the yellow pigment cannot be correlated with the vascularity of the tissue. Nevertheless, the degree of vascularity may have an important bearing on the rapidity of _ the deposit or laying down of the pigment. It would seem that the variables described by Hammond and Harshaw and by Culton and Bird would apply also to the carotenoid pigments of the iris. Ball (1944) and Nelson (1944) both demonstrated that "fading" of the iris results from feeding a pigment-free diet, and the former showed that pigment-free laying pullets did not deposit the pigment in the iris as long as they continued laying even though the pigment was contained in the ration. In the non-laying hen the gross eye color is a darker orange and histologically the pigment is seen in intercellular globules as well as in the striated fibers of the dilatator and sphincter muscles. These observations were made by the writer on frozen circular sections of the entire iris, mounted unstained, in glycerol. Bedson and Knight (1924) noted an increase in the yellow pigment over that normally present in the comb skin, beak and shanks, blood plasma, and. body fat in birds suffering from severe anemia. By chemical tests they identified the pigment as xanthophyll. GENETIC INFLUENCE OF THE YELLOW PIGMENT No reference has been found which specifically attributes the absence, the presence, or the variation of yellow pigment in the chicken iris to a genetic factor. However, reference to so-called "genetic gray eyes" is not uncommon. Ball refers to the likelihood of strain differences in iris color. Warren and Gordon (1933), in studying the inheritance of eye color in Rhode Island Red fowls, stated, "The general trend of results may be taken as evidence of some heritability of eye color variation in the Rhode Island Red." However, they indicated that some environmental factors may act to obscure the inheritable tendencies. Later they

64 NORMAN M. NELSON summarize, "Results on the study of eye color did not seem to indicate that variations in eye color are inherited." The pigment under consideration by Warren and Gordon was "primarily the amount of red in the iris." While they did not explain the components of the "red" it seems probable that they were referring to gross eye color which is due, in the reddishbay eye, to a blending or combination of both red and yellow pigments. Slinger and Macllraith (1944) have recently noted a correlation between green-gray irises and black feathers in several flocks of Barred Plymouth Rock pullets. They do not state whether or not the birds of the twenty flocks studied were genetically related. It would seem that the eye pigment in Slinger and Macllraith's study was the yellow of the carotenoids and not melanin as they have assumed. That the complete absence of yellow pigment is due to a genetic factor, as yet has not been described. However, the Cornish as well as Malay fowls have a pearl-gray eye, devoid of any gross yellow coloring, demonstrating that it is a breed or inheritable trait. Likewise, the not infrequent reference by poultrymen to socalled genetic gray eye in other breeds would tend to support this view. If the yellow pigment is present, the variation in its concentration is believed to be physiological in the normal eye and pathological in ocular lymphomatosis. In speaking of the non-pigmented iris, Duke-Elder states, "as the latter is very transparent, the light is reflected from the deep pigmented layer, and therefore, owing to the phenomena of interference, it appears blue, in the same way as the sky, or the veins immediately under a delicately transparent skin." This refers to an iris devoid of pigmentation of the anterior layers. The variations in carotenoid pigment concentration and therefore, in the yelloworange character of the gross eye color appear to be due to the following: age, breed, amount of pigment in the ration, pigmentsuppressing factor in the ration, egg production, and possibly, when completely absent, to a genetic factor. In the normal gray eye neither black nor yellow pigment is present in the body of the iris. Light is reflected by the posterior pigmented epithelium. The effect is a gray or bluish-gray. This eye color is similar to that color in man as described by Walls (1942). In chickens, two breeds, the Malay and the Cornish, normally have a gray iris. PATHOLOGICAL INFLUENCE It is now well known that depigmentation frequently accompanies the ocular manifestation of lymphomatosis (Nelson and Thorp 1943). Duke-Elder states, "It is important to note that in age the pigment diminishes, while chronic disease may result in widespread atrophy; in which case a blue or gray color appears pathologically." In ocular lymphomatosis the iris is thickened by the massive quantities of the round cells and while the method of removal of the pigment has not been satisfactorily explained, its influence in the final gross color is usually diminished. Jaensch and Lerche (1933) believed that there was a direct relationship between the cellular infiltration and the decline of fat in the sphincter region. They speculated that the possible way of removal was by the canal of Schlemm through the anterior chamber. BLACK PIGMENT FACTOR The pars iridica retinae of the posterior surface of the iris is, developmentally, an extension of the first or outside melanin pigmented epithelial layer of the retina, the stratum Pigmentosum (Mann 1928, Polyak 1941).

NORMAL EYE COLOR IN THE CHICKEN 65 INFLUENCE ON BLACK FACTOR The presence or relative absence of melanin pigment on the posterior surface epithelium is apparently controlled by a genetic factor (Bond 1919, Warren 1933, Hutt and Mueller 1943). In the reddishbay or brown eye the melanin pigment of the posterior epithelium is constant and does not, in that location, effect variations in gross eye color. (Its relative absence at this location accounts for the albino eye.) This black screen on the posterior iridic surface functions much the same as the silver on the back surface of a mirror Light is refracted by this surface and passes through the iris, producing a color effect. The color or hue produced depends upon the pigments which are transilluminated. The brown iris differs from the reddish-bay only in that melanin pigment is scattered in the iridic stroma. Thus, the black pigment when blended with red and yellow gives the visual effect of brown or brownish-bay. In the reddish-bay iris melanin pigment is not present in the stroma or body of the iris. The distribution of melanin in the stroma of the iris of the eye produces a similar effect in the human eye and is a hereditary trait (Walls 1942 and Duke-Elder 1942). METHODS OF MEASURING GROSS EYE COLOR IN THE CHICKEN There is now no satisfactory method for measuring gross eye color in the living bird. Since either two or three hues are present in the normal iris, the resultant color is a blending of the hues. The relative concentration of each hue affects the blend or final visual appearance. Several workers have scored the visual differences. No method of measuring the hues separately has been reported, although the yellow pigment has been measured comparatively by Ball, using visual comparisons of the extract with a graduated control. Nelson (1944) measured comparative differences in the extract with the electrophotometer. Several methods have been used in measuring the hue due to carotenoid pigments in studies of egg yolk, shanks, skin, etc., but these methods cannot be applied per se to eye color. DISCUSSION It is apparent that the gross eye color is the result of the blending of the pigments described. The relative concentration of the three pigments effects marked variations in the eye color. The influences on the pigment concentration as described may be hereditary or environmental. Of the latter, physiological influences, such as the dietary factors or egg production, are responsible in the normal eye, while pathological factors, as in ocular lymphomatosis, effect color changes in the diseased eye. It is apparent that no one factor is alone responsible for all deviation from the reddish-bay iris and consequently the practice of indiscriminate culling of birds on eye color seems unwarranted. The poultryman's concern of gray eyes originates from two basically distinct problems and it is likely that the solution will be met only by a dual method of control. For the breeder, all gray-eyed birds may be considered undesirable since the tendency is to adhere to the reddish-bay eye. The problem of controlling lymphomatosis by culling of cases showing the ocular manifestation should be based on symptoms other than color. SUMMARY The three pigments responsible for the final gross eye color in the common breeds of chickens are present in the form of melanin, carotenoids, and hemoglobin in the hues of black, yellow, and red respectively. The factors which mfluence

66 NORMAN M. NELSON variations in the concentrations of these pigments are discussed. Inasmuch as fading of the yellow factor occurs with continual egg production, the practice of culling such birds is unwarranted. a Cases of ocular lymphomatosis should be diagnosed by criteria other than color. REFERENCES American Poultry Assn., 1940. The American Standard of Perfection. Davenport, Iowa, the Association. Ball, R. F., 1944. The effect of the ration upon iris color of single-comb White Leghorns. Poultry Sci. 23:377-385. Bedson, S. P., and E. Knight, 1924. Anemia in hens associated with an increase in the yellow pigment normally present in certain tissues of these birds. J. Path. & Bact. 27: 239-248. Blakeslee, A. F., and D. E. Warner, 1915. Correlation between egg-laying activity and yellow pigment hi the domestic fowl. Am. Naturalist 49: 360-368. Bond, C. J., 1919. On certain factors concerned in the production of eye color in birds. J. Genet. 9:69-81. Culton, T. G., and H. R. Bird, 1941. Effect of certain protein supplements in inhibiting pigment deposition in growing chicks. Poultry Sci. 20: 432-436. Duke-Elder, W. S., 1942. Text-book of ophthalmology, Vol. I. C. V. Mosby Co., St. Louis. Hammond, J. C, and H. M. Harshaw, 1941. Some factors influencing shank and skin color in the growing chicken. Poultry Sci. 20: 437-444. Hollander, W. F., and R. D. Owen, 1939. The carotenoid nature of yellow pigment in the chicken iris. Poultry Sci. 18:385-387. Hutt, F. B., and C. D. Mueller, 1943. Independent identical mutations to albinism in the sex chromosome of the fowl. Am. Naturalist. 57:181-184. Jaensch and Lerche, 1933. Die augerveranderung bei Marekscher geflugellahme. Albrecht von Groef's Arch, fur ophthalmologic Band 131, Heft 4: 359-376. Mann, Ida C, 1928. The development of the human eye. Cambridge University Press, London. Nelson, N. M., 1944. Feed influence on eye color in White Leghorn chickens. Poultry Sci. 23: 541-542. Nelson, N. M., and Frank Thorp, Jr., 1943. Ocular lymphomatosis, with special reference to chromatism of the irides. Am. J. Vet. Res. 4: 294-304. Palmer, L. S., 1915. Xanthophyll, the principal natural yellow pigment of the egg yolk, body fat, and blood serum of the hen. J. Biol. Chem. 23: 261-279. Polyak, S. L., 1941. The Retina. University of Chicago Press, Chicago. Sherwood, R. M., and G. S. Fraps, 1941. Carotene destroying power of certain animal feeds. Abstr. Poultry Sci. 20:472. Slinger, S. J., and J. J. Macllraith, 1944. The correlation between green grey irises and black feathers in Barred Rock pullets. Poultry Sci. 23: 533-537. Walls, G. L., 1942. The vertebrate eye and its adaptive radiation. Cranbrook Inst, of Sci., Bui. 19, The Cranbrook Press, Bloomfield Hills, Mich. Warren, D. C, 1933, Inheritance of albinism in the domestic fowl. J. Hered. 24: 379-383. W arren, D. C, and C. D. Gordon, 1933. Plumage and eye color inheritance in the single-comb Rhode Island Red fowl. J. Agri. Res. 47-897-910.