How and when the regional competence of chick epidermis is established: Feathers vs. scutate and reticulate scales, a problem en route to a solution

Abstract

Most of the chick body is covered with feathers, while the tarsometatarsus and the dorsal face of the digits form oblong overlapping scales (scuta) and the plantar face rounded non-overlapping scales (reticula). Feathers and scuta are made of β-keratins, while the epidermis of reticula and inter-appendage or apteria (nude regions) express α-keratins. These regional characteristics are determined in skin precursors and require an epidermal FGF-like signal to be expressed. Both the initiation of appendages, their outline and pattern depend on signals from the dermis, while their asymmetry and outgrowth depend on epidermal competence. For example, the plantar dermis of the central foot pad induces reticula in a plantar or feathers in an apteric epidermis, in a hexagonal pattern starting from the medial point. By manipulating shh levels in the epidermis, the regional appendage type can be changed from scuta or reticula to feather, whereas the inhibition of Wnt7a, together with a downregulation of Shh gives rise to reticula and in extreme cases, apteria. During morphogenesis of plantar skin, the epidermal expression of En-1, acting as a repressor both of Wnt7a and Shh, is linked to the formation of reticula. Finally, in birds, the complex formation of feathers, which can be easily triggered, even in the extra-embryonic somatopleure, may result from a basic genetic program, whereas the simple formation of scales appears secondarily derived, as requiring a partial (scuta) or total (reticula) inhibition of epidermal outgrowth and β-keratin gene expression, an inhibition lost for the scuta in the case of feathered feet breeds.