Hair and nails grow from specialized epithelial structures. The hair grows from the hair follicle, and the nail grows from cells at the base of the nail bed (in the region of the lunula).
Hair is grown in a hair follicle. A hair follicle is composed of epithelial cells within the dermis. They are like tubelike invaginations of the epidermis into the dermis. Like the epidermis, the epithelial tissue of the hair follicle produces many more epithelial cells, and these keratinize and die off. These dead cells accumulate within the follicle and form the hair itself. Therefore, our hair consists of piles of dead cells.
This drawing was taken from Hair Biology 1, which can be accessed off the main Hair Biology page. Please note that all epithelial tissue is shown in green... can you see how the hair follicle is simply an invagination of the epidermis?
These piles of dead cells that make up our hair could flake apart and then you get split ends. The follicle that makes the hair could shrink (as it may do with age) and then you would get finer hair. The melanocytes in the hair follicle contribute to the color of the hair, and as these melanocytes slow down with age, hair becomes gray.
You will see in the gland page that sebaceous glands typically secrete into the hair follicle duct (where the hair lies) in order to get their secretions to the apical aspect of the skin.
Also, the arrector pili muscles attach to the base of the hair follicle. When these muscles contract, they pull on the hairs, causing them to stand on end. This was also described in the temperature regulation section (under skin function) of this week's unit.
Keep in mind that hairs are found all over the body, everywhere we have thin (hairy) skin. There are just a few places where we do not have hairs: palms and soles, lips, nipples, and some reproductive area skin (like the labia minora).
Nails are actually called nail plates. These nail plates are also made up of dead, keratinized cells. These cells are produced at the level of the lunula. The nail plate lies on the nail bed. This is the skin directly under the nail, to which the nail is attached-- you know how sensitive the nail bed is if you ever broke a nail pretty far down.You can think of the lunula as acting for the nail like the hair follicle acts for the hair. The lunula and the hair follicle are both made up of epidermal cells that give rise to the cells that die to make up nails and hair.
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