The skin of vertebrates can present different types of differentiation, which help the performance of its functions.
The main integumentary differentiations are scales, hair, feathers, nails, claws, hooves and horns, in addition to various types of glands.
Scales are structures in the form of flattened plates, which are arranged on the integument as protective armor. They are present in fish, reptiles and birds.
Cartilaginous fish (dogfish, sharks and rays) have placoid scales similar in origin and structure to a tooth. The outer part of a placoid scale is made up of esmale, formed by the epidermal cells, and the inner part is made of dentin, formed by the dermal cells.
Bone fish have dermal scales, originated by dermis cells and covered by a thin layer of epidermal cells. Reptiles and birds have corneal scales, keratin formations originated by epidermal cells.
By They are filamentous keratin structures present exclusively in mammals. Certain species have abundant hair, which forms an insulating protective coat. In the human species, except for the hair, the hair is short and thin, concentrating in the armpits and around the genitals.
The hairs are made up of dead and compact keratinized epidermal cells. The hair is born inside a small depression of the skin, the hair follicle. At the bottom of the follicle, cells in continuous multiplication make keratin, die and compact, giving rise to the hair.
The cells that originate the hair are nourished and oxygenated by blood capillaries present near the follicle. Each hair is attached to a small erector muscle, which allows its movement, and to one or more sebaceous glands, which are in charge of their lubrication.
Feathers are present exclusively in birds. They are made up of keratin and form similar to mammalian hair. It seems that feathers and fur evolved from the scales that covered the body of the ancient reptiles of birds and mammals.
Nails are flat structures formed by keratin highly compacted, present at the fingertips of mammals and primates such as monkeys, arthropods and man. The nails guarantee firmness to the fingertips. The feet give more balance when walking; in the hands, they help to grasp and manipulate objects.
The nails grow from an epidermal fold located near the fingertips, where epidermal cells divide intensely, accumulating keratin. The keratinized cells die and compact into the nail.