Skin is living armor that senses, seals, and sweats
Vertebrate skin is soft outer tissue with three headline jobs: protection, regulation, and sensation. It blocks pathogens and water loss, hosts hair even on 'hairless' whales, and can become leather. Biologists in 2024 reported fossil skin about 289 million years old, possibly from an early reptile.
Skin is the body's front line, keeping pathogens out and water in, while also insulating, adjusting temperature, sensing touch and making vitamin D. Badly damaged skin heals with scar tissue. Every mammal has at least some hair, even whales, dolphins and porpoises, and dense fur adds insulation, camouflage or sexual display. The English word originally meant only tanned animal hide; it was borrowed from Old Norse and traces back to an ancient root meaning to cut.
Mammalian skin has two layers. The outer epidermis is a stack of cells, 95 percent of them keratinocytes, alongside pigment-making melanocytes, Merkel cells and immune Langerhans cells. New keratinocytes divide in the deepest layer, the stratum basale, then migrate upward, lock together with junctions called desmosomes, secrete keratin and lipids, and finally lose their nuclei. The epidermis has no blood supply of its own and is fed by diffusion from capillaries below. A thin basement membrane separates it from the dermis, controlling traffic between the layers and storing growth factors for repair.
The dermis gives skin its strength and stretch through collagen and elastic fibres set in a gel of hyaluronan and proteoglycans. Its upper papillary region sends finger-like projections into the epidermis to lock the layers together, while the thicker reticular region beneath houses hair roots, sweat and oil glands, nails, nerve receptors and blood vessels. Beneath lies the subcutaneous tissue, technically not skin, which anchors it to muscle and bone and stores fat. Bacteria such as Staphylococcus epidermidis live on the surface and recolonise disinfected skin from deep in hair follicles.
Other vertebrates do it differently. Fish and most amphibians have living, largely keratin-free epidermis, and many amphibians breathe substantially through it. Sweat and oil glands exist only in mammals, while fish may have poison glands or light organs. Reptile, amphibian and fish colour comes mostly from dermal chromatophores, which let chameleons and flounders change hue. In January 2024 researchers announced the most ancient preserved skin yet found, dating back roughly 289 million years and perhaps belonging to a reptile.
Source: Skin
