The expression of beta (b) keratins in the epidermal appendages of reptiles and birds

SYNOPSIS. The integuments of extant vertebrates display a variety of epidermal appendages whose patterns, morphology and terminal differentiation (epidermal keratins) depend upon interactions between ectodermal (epidermis) and mesoder-mal (dermis) tissues. In reptiles and birds, appendage morphogenesis precedes terminal differentiation. Studies have demonstrated that appendage morphogenesis influences the expression of the appendage specific keratin genes. However, little is known about the nature of the structural genes expressed by the epidermal appendages of reptiles. How pattern formation and/or appendage morphogenesis influence terminal differentiation of reptilian appendages is not known. The epidermal appendages of reptiles and birds are characterized by the pres-ence of both alpha (a) and beta (b) type keratin proteins. Studies have focused on the genes of avian b keratins because they are the major structural proteins of feathers. The occurrence of b keratin proteins in the scales and claws of both birds and reptiles and their immunological cross-reactivity suggest that the genes for reptilian b keratins may be homologous with those of birds. In bird appendages