The ultrastructural effects of the I and S loci upon black-red melanin differentiation in the fowl

Abstract

The dominant white and silver mutants of the fowl were examined ultrastructurally and histochemically because of their unique effects upon black-red melanin differentiation—dominant white bleaches eumelanin, silver whitens pheomelanin. Both eumelanin- and pheomelanin-producing melanocytes were studied in birds of the dominant white, silver, and standard genotypes. Eumelanocytes and pheomelanocytes of all three genotypes possessed tyrosinase as revealed by DOPA incubation tests. This shows that the white areas of the two mutants are not due to the absence of a competent pigment cell system. Standard and silver eumelanocytes were melanogenically very active, exhibiting a high concentration of melanosomes. Dominant white eumelanocytes, however, possessed only a threshold number (one-half standard) of irregularly shaped, partially melanized melanosomes. Pheomelanocytes of all three genotypes were melanogenically quiescent during early development containing only a threshold number (one-half eumelanocyte) of unmelanized premelanosomes. Eumelanocytes of all three genotypes possessed dendrites that were capable of transferring melanosomes into keratinocytes. Dominant white melanocytes transferred fewer because less melanosomes were available. Transferred pheomelanosomes of the standard and dominant white hackles were solidly melanized and frequently of a compound nature. Silver pheomelanosomes, however, were only slightly melanized although compound premelanosomes were frequently observed. Standard pheomelanocytes were sulfhydryl positive, and silver pheomelanocytes were sulfhydryl negative as revealed by histochemical tests. This relates sulfhydryl groups to pheomelanin deposition. Based on the results, a model of melanogenesis is proposed which suggests that three differences exist between eumelanin formation and pheomelanin synthesis.