Skin derivatives in vertebrate ontogeny and phylogeny

The skin of vertebrates has numerous and diverse derivatives, either located within the epithelial sheet itself (glands) or extending above its surface (teeth, scales, feathers, hairs, etc.). Many of them have a modular structure and constitute structural-functional units. Ontogenetically, all skin derivatives are of ectomesodermal origin, and their morphogenesis is subject to metabolic control, heterochronies (divergence in the timing of origination and development), and regulation by means of tissue interactions and molecular signaling via similar pathways. The diversification (origination of morphological diversity) of skin derivatives within the same morphological type is explained by the development of new generations of ectomesodermal structures separated by heterochronies and regulated by changes in the gradients of molecular signaling pathways under the influence of environmental factors. Evolutionary relationships between the majority of skin derivatives are obscure, except for teeth and glands associated with sensory organs that have evolved together with these organs. Apparently, many vertebrate skin derivatives (scales, feathers, hairs, and glands) originated as novelties at nodal stages of phylogeny and subsequently evolved convergently or in parallel.