514 



Special Vertebrate Organogenesis 



being independent of sex hormones. This is 

 in contrast to melanoblasts of breeds like the 

 Brown Leghorn which are "sensitive" to sex 

 hormones and dependent upon them for ex- 

 pressing their phenotype (see Willier, '50). 



In amphibians it has been possible, also, 

 to introduce precursor pigment cells of spe- 

 cies exhibiting one characteristic type of 

 pigmentation pattern into individuals ex- 

 hibiting a different type, by exchanging seg- 

 ments of neural folds (including the neural 

 crest). Results agree in showing that the 

 specific patterns of pigment cell distribution 

 and orientation are dependent primarily 

 upon intrinsic genetic differences in the pig- 

 ment cells themselves (Twitty, '49). 



Numerous histological stvidies of melanin 

 granules deposited by melanophores in the 

 epidermal cells of feathers and hairs have 

 revealed that within any one genotype the 

 size, shape, and color of the granules exhibit 

 a remarkable specificity. In the niimerous 

 cases in which melanoblasts have been trans- 

 planted to foreign feather germs, they have 

 always deposited in the feather cells gran- 

 ules characteristic of their own particular 

 genotype. 



Tissue Environment and Pattern Forma- 

 tion. While it has been clearly demonstrated 

 in birds and amphibians that the genotypic 

 constitvition of the pigment cells is a con- 

 trolling factor in phenotypic expression of 

 color and pattern, it is equally clear that 

 the surrounding tissues exert a definite influ- 

 ence on the realization of their potentiali- 

 ties. A number of workers with amphibians 

 have shown the influence of the epidermis, 

 somites, and neural tube upon the melanin- 

 forming capacity and the orientation of these 

 specialized cells into patterns (DuShane, '43; 

 Twitty, '49; Dalton, '50). 



Evidence of the influence of the tissue 

 substrate on melanophore pattern formation 

 is strikingly brought out by grafting neural 

 crest between widely imrelated individuals 

 such as anurans and urodeles (Baltzer, '41, 

 '43; Leuenberger, '42). Under such condi- 

 tions the orientation and the distribution of 

 the grafted melanophores are quite definitely 

 altered. The morphology of the individual 

 melanophores, however, is not changed. They 

 retain their specific characteristic size, color, 

 type of branching, etc., and can be easily 

 distinguished from those of the host. In gen- 

 eral the results of neural crest exchanges 

 among amphibians have shown that altera- 

 tions in the normal pigmentation pattern 

 produced by melanophores of a given geno- 

 type are progressively more pronounced as 



the donor and host become farther apart 

 phylogenetically. This would appear to in- 

 dicate that the arrangement of melanophores 

 into a distinctive pattern is largely a particu- 

 lar kind of response drawn forth by a par- 

 ticular set of physiological conditions ex- 

 isting in their tissue substrates. Changes in 

 one bring about changes in the other; in 

 other words, there is interplay or interaction 

 between intrinsic and extrinsic factors. 



In fowl the position of the feather germ 

 on the body^ — its tract location — determines 

 the specific type of pigmentary response 

 given by melanophores potentially capable 

 of a range of responses, such as black and 

 red or black and white barring, etc. This has 

 been demonstrated convincingly by grafting 

 such melanoblasts to feather germs of vari- 

 ous regions of the body. The final pattern 

 obtained is always typical of the region — 

 wing, breast, saddle, etc. — showing that the 

 locus of differentiation, i.e., the particular 

 feather germ or portion of the feather germ 

 in which the melanophore differentiates, de- 

 termines which of its potencies is realized. 

 It should be emphasized that there is no 

 correlation between the locus of origin of the 

 melanophores of any one genotype and their 

 differential response to feather germs of 

 various body regions. In fact, melanoblasts 

 from the parietal lining of the coelom which 

 would ordinarily never enter feathers will, 

 upon being introduced into the epidermis of 

 developing feathers of various regions, pro- 

 duce all of the intricacies of pattern char- 

 acteristic of homologous feathers of their 

 genotype (Rawles, '45). Experimental evi- 

 dence indicates that, in their undifferen- 

 tiated or melanoblast stages, pigment cells 

 of any one genotype are all alike. This ap- 

 pears to be true, also, for some if not all of 

 the amphibians (Steamer, '46). 



The importance of the tissue environment, 

 the feather papilla, in governing the color 

 pattern response of melanophores of the 

 Brown Leghorn to sex hormones has been 

 fully demonstrated experimentally. It has 

 been known for a long time that a variety 

 of female colored, reddish bands, can be 

 produced in the normally black breast feath- 

 ers of a caponized Brown Leghorn by inject- 

 ing a known quantity of female sex hormone 

 at definite time intervals after plucking 

 (Lillie and Juhn, '32). In the light of later 

 information regarding the developmental po- 

 tentialities of the pigment-forming cells, it 

 wovild appear that in the breast papillae 

 these cells respond to female sex hormone by 

 depositing red melanin granules into the 



