Adaptive evolution of secretory cell lines in vertebrate skin

The embryonic horny layer of vertebrates contains distinctive cell lines that account for the remarkable, diverse secretory performances of their cutaneous apparatuses. Aquatic classes (jawless, cartilaginous and bony fishes) possess single gland cells, scattered among ordinary epidermal cells, that manufacture proteinaceous or mucous substances. This dichotomy is retained in adult amphibians, although the secretory cells in anamnionic tetrapods begin to be arranged in complex glands located in the loose dermis. In the Amniota, intensive keratinisation involving the epidermis results in the loss of the ancestral secretory lines, accompanied by the onset of a novel, lipidproducing gland type, which in mammals is flanked by the exclusive sweat gland apparatus. In this concise up-to-date review, we attempt to phylogenetically narrow the large variety of secretory structures in vertebrate skin into a few basic schemes, in the light of the close relationships between environmental challenges and ecological, ethological as well as physiological roles of the cutaneous apparatus.