Skin and Its Derivatives 



515 



feather-forming epidermal cells. This par- 

 ticular response to estrogen does not occur 

 in vitro, and does not occur in vivo until 

 the epidermis of the feather germ has at- 

 tained a certain developmental stage (Trin- 

 kaus, '48). Willier ('50) has interpreted the 

 response of the estrogen-sensitive melano- 

 blasts, in the zone of differentiation of the 

 feather papilla, in terms of the physiologi- 

 cal reaction gradients established by Lillie 

 and Juhn ('32). Since feather papillae of 

 the various tracts and even within the same 

 tract show distinct differences in reaction 

 gradients, the melanophore response as re- 

 corded in the finished feather pattern varies 

 in a conformable manner. According to Wil- 

 lier, the feather papilla is to be regarded as an 

 endocrine receptor endowed with special prop- 

 erties for responding to sex hormones. Dif- 

 ferences in the responsiveness of the feather 

 papilla appear to be genetically controlled. 

 Danforth ('43) finds that a simple altera- 

 tion in genotype, e.g., a mutational change 

 of a single H gene in fowl, suffices to change 

 th(j response of the feather germ from one 

 of sensitivity to estrone and indifference to 

 testosterone, to one in which the tissues re- 

 spond equally to both, i.e., appear to be 

 insensitive to the difference between these 

 two hormones. 



Reactions Between Pigment Cells in Pat- 

 tern Formation. In the formation of melanin 

 pigmentation patterns there is not only a 

 constant reaction between melanophores and 

 their tissue environments, but also reactions 

 between the individual melanophores them- 

 selves. The importance of such reactions in 

 the foi'mation of rhythmic, barred patterns 

 has been emphasized by Nickerson ('44) in 

 his study of the Barred Plymouth Rock and 

 the Silver Campine, two varieties of fowl 

 with distinctly different types of black and 

 white barring patterns. Having established 

 that periodicity is intrinsic to the melano- 

 phore and, further, that melanophores of 

 the white bands are able to produce pigment 

 imder suitable conditions, Nickerson con- 

 eluded that the barring rhythm is controlled 

 primarily through the medium of diffusible 

 substances produced by the active melano- 

 phores within the black band which inhibit 

 pigment formation by precursor melano- 

 phores in their immediate neighborhood 

 (subjacent white band). As growth proceeds 

 and the black band becomes removed from 

 the zone of differentiation of the feather 

 germ, this region will lie beyond the in- 

 hibiting influence and a new black band may 

 now be formed. Certain properties of the 



feather germ, such as growth rate, size 

 of the barb ridges, etc., are necessarily in- 

 volved. 



It should be mentioned that, although 

 Nickerson for good reasons favored the dif- 

 fusion hypothesis, he did not lose sight of 

 the theoretical possibility that, in the syn- 

 thesis of melanin by the active melano- 

 phores, some substance essential for melanin 

 production might be removed from the epi- 

 dermal substratum of the developing barbs 

 of the white region. In either case the bar- 

 ring rhythm would be associated with 

 melanin production by certain groups of ac- 

 tive melanophores. 



To what extent the pigmentary patterns 

 in the hair coat of mammals are influenced 

 by interactions between pigment cells (mel- 

 anophores) awaits investigation. For more 

 than fifty years it has been known that the 

 skin epithelium of a white area of a spotted 

 guinea pig gradually becomes black when 

 in contact with an area of black skin trans- 

 planted from another region of the same 

 individual. Recently in an attempt to ex- 

 plain this well-known phenomenon Billing- 

 ham and Medawar ('48, '50) have asstmied 

 the passage of a hypothetical, cytoplasmic 

 "ingredient" from the contiguous black pig- 

 ment cells into the supposedly "white" pig- 

 ment cells of the skin epithelium of the 

 white area. A "white" pigment cell so trans- 

 formed in turn transforms other contiguous 

 "white" cells. According to this conception 

 the process of pigmentation proceeds in a 

 manner formally equivalent to a virus in- 

 fection. In the light of well-established facts 

 regarding the migration of precursor pig- 

 ment-forming cells in amphibians, birds and 

 other mammals, further and more crucial 

 evidence is imperative to substantiate this 

 view of "infective" transformation of pig- 

 ment cells. 



In larval salamanders, the process of pig- 

 mentation appears to be profoundly affected 

 by interactions between developing pigment 

 cells. Certain experimental studies have 

 shown that pigment cells which have an 

 advantage in age or in rate of development 

 are able to inhibit or suppress the differ- 

 entiation of younger or less rapidly differen- 

 tiating precursor pigment cells (Twitty, '49; 

 Lehman, '50). In fact, Twitty, on the basis 

 of his extensive experimentation with Tri- 

 turus, is of the opinion that influences ex- 

 erted mutually by the pigment cells are of 

 primary importance in their migrations and 

 their arrangement into specific pigmentary 

 patterns. 



