Dermatology in the Darwin anniversary. Part 1: Evolution of the integument

The present review highlights the development of the integument and its adnexa from the primitive metazoans to man. The different stages of development represent independent, partially convergent evolutions rather than a continuous evolutionary line. The epidermis of the invertebrates ( sponges, cnidaria, worms, echinoderms and arthropods) always consists of one layer of pluripotent cells. The barrier function of the integument at this level is achieved with physico-chemical barriers, toxin production, fortification of the epidermis in the form of a cuticula, a syncytium or a neodermis. The lower vertebrates (cyclostoma, fishes and amphibians) have a stratified epidermis harboring many secretory cells. In terrestrial amphibians the outermost cell layer of the epidermis is cornified, and the secretory cells are relocated in the dermis. Terminal differentiation and cornification of the epidermis in the birds and mammals result in a more uniform shape of the epithelium. Stem cells are now restricted to some basal regions of the epithelium. In the mammals the glands are located in the deeper layers of the skin. In contrast to other vertebrate integuments the human skin does not possess specialized structures such as feathers, scales or coats. However, waiving specialization allows for unique universality of the human skin compared to other vertebrates.