Epidermis

The outer layer, the epidermis, is relatively thin over most areas but is thickest on the palms of the hands and the soles of the feet. Although the epidermis is composed of several sublayers called strata, the  stratum corneum and the basal layer, which is the deepest layer, are of greatest importance.

Structure of the skin and subcutaneous tissue.

The stratum corneum is composed of dead, flat cells that lack a blood supply and sensory receptors. Its thickness is related to normal wear of the area it covers. 

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The basal layer is the only layer of the epidermis that is composed of living cells where new cells are formed. As these cells move toward the stratum corneum to replace the cells that have been sloughed off, they die and become filled with a hard protein material called keratin. The relatively waterproof characteristic of keratin prevents body fluids from evaporating and moisture from entering the body. The entire process by which a cell forms in the basal layer, rises to the surface, becomes keratinized, and sloughs off takes about 1 month. 

Cells and Layers of the Epidermis

In the basal layer, special cells called melanocytes produce a black pigment called melanin. Melanin provides a protective barrier from the damaging effects of the sun’s ultraviolet radiation, which can cause skin cancer. Moderate sun exposure increases the rate of melanin production and results in a suntan. However, overexposure results in sunburn due to melanin’s inability to absorb sufficient ultraviolet rays to prevent the burn. 

Layers of epidermis

Differences in skin color are attributed to the amount of melanin in each cell. Dark-skinned people produce large amounts of melanin and are less likely to have wrinkles or skin cancer. Production of melanocytes is genetically regulated and, thus, inherited. Local accumulations of melanin are seen in pigmented moles and freckles. An absence of pigment in the skin, eyes, and hair is most likely due to an inherited inability to produce melanin. An individual who cannot produce melanin, known as an albino, has a marked deficiency of pigment in the eyes, hair, and skin.

References

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