Skin and Its Derivatives 



505 



riety of glands — mucous, cement, and poison 

 glands of the lower vertebrates; sebaceous, 

 sweat, and milk glands of the higher ver- 

 tebrates. 



A cursory examination of the hair coat 

 of a mammal or the feathers of a bird re- 

 veals in various regions of the body — head, 

 breast, back, tail, et cetera — a striking dif- 

 ference in the size, shape, and structure of 

 the epidermal outgrowths. Even one single 

 region, the head of a bird for example, may 

 display various adaptive modifications of 

 the contour feathers to form ornamental 

 plumes, ear coverts, facial bristles, and eye- 

 lashes. Among feathers the variety of form 

 is almost limitless, yet each is a modification 

 of the same fundamental structure. Of in- 

 terest also is the fact that one and the same 

 hair or feather papilla of a particular re- 

 gion may produce a succession of hairs or 

 feathers which display morphological and 

 color differences pertaining to definite stages 

 of the life history. 



Not only are there marked regional dif- 

 ferences in the epidermal outgrowths, but 

 also variations in the skin proper. Quantita- 

 tive histological studies of mammalian skin 

 have demonstrated regional differences in 

 growth of the epidermis (Loeb and Haven, 

 '29). Straus's study ('50) of the microscopic 

 anatomy of the skin from ten selected re- 

 gions of a female gorilla reveals significant 

 differences in structure at all of the various 

 regions examined. The skin is thickest over 

 the back and thinnest over the chest. The 

 epidermis reaches its greatest thickness on the 

 palms and soles, the corium its greatest thick- 

 ness on the back. In general the total 

 thickness of the skin is a reflection of the 

 thickness of the corium. The above findings 

 hold for mammals in general, including 

 man. 



Spectrophotometric analysis of living hu- 

 man skin (Edwards and Duntley, '39) has 

 shown significant differences in the distri- 

 bution of the pigments (melanin, hemo- 

 globin, carotene) responsible for differences 

 in the color of the skin in the various body 

 regions. Oxyhemoglobin is especially abun- 

 dant in regions of the skin where the ar- 

 terial blood supply is rich and where for 

 the most part the dermal papillae are high. 

 Carotene is more abundant in regions where 

 the stratum corneum is thickest. Female skin 

 contains less hemoglobin and melanin but 

 more carotene than male skin. Minor sex 

 differences in distribution of the pigments 

 also occur. 



Functional Differences. Differences in struc- 



ture in the various body regions may be 

 correlated often with integumental differ- 

 ences in function, bvit there are also in- 

 trinsic physiological differences among cells 

 which are revealed only by their method of 

 response to certain stimuli such as hormones, 

 vitamins, light, and temperature. In man, 

 regional differences in photosensitivity of the 

 skin are believed to be dependent to a great 

 extent on variations in thickness of the 

 horny cell layer and upon the development 

 of the skin capillaries. The horny layer func- 

 tions as a superficial filter absorbing some 

 of the light waves before they reach the liv- 

 ing layers of the epidermis. But it is known 

 that the quantity of pigment, the age and 

 sex of the individual, and the season of the 

 year are important factors, also, in deter- 

 mining the regional and individual differ- 

 ences in skin photosensitivity (Ellinger, '41). 

 A striking demonstration of the existence 

 of intrinsic, qualitative differences among 

 cells of the skin of different body regions 

 is afforded by amphibians at metamorphosis. 

 Under the influence of thyroid hormone, the 

 skin of the tail of a frog tadpole, for in- 

 stance, responds by undergoing degenerative 

 changes while the response of the adjoin- 

 ing skin of the trunk is one of proliferation. 

 Histologically the two areas of skin response 

 are sharply defined and show no transition. 

 The specific response of these tissues is 

 not altered by heterotopic transplantation 

 (Lindeman, '29). The dependence of certain 

 sexual plumage types upon gonadal hor- 

 mones in fowl is equally striking. Extensive 

 studies of morphological and color changes 

 in the feathers of the Brown Leghorn have 

 shown that regional differences in the 

 threshold of response to female sex hormone 

 and to thyroxin are dependent on local dif- 

 ferences in the growth rate of cells of the 

 individual feather germ (Lillie and Juhn, 

 '32). In many animals melanin pigment is 

 produced by melanophores in certain local- 

 ized regions of the integument in response 

 to male hormone stimulation. Among cases 

 described are the bill of the sparrow (Keck, 

 '34); the lores, roof and floor of the mouth 

 of the night heron (Noble and Wurm, '40); 

 the dorsolumbar spots of the hamster (Kup- 

 perman, '44); the scrotum of the 13-lined 

 ground sqixirrel (Wells, '45); the sternal 

 spot of the Australian opossvim (Bollinger and 

 Hardy, '45). In the human female excess 

 melanin is deposited by melanophores in 

 the skin of certain localized areas (eyelids, 

 nipples, areolae, linea nigra) in response 

 to increased amounts of estrogenic substances 



