The Epidermis

The epidermis consists of stratified squamous epithelium. Four cell types are present:

• Keratinocytes produce keratin, a protein that hardens and waterproofs the skin. Mature keratinocytes at the skin surface are dead and filled almost entirely with keratin.

• Melanocytes produce melanin, a pigment that protects cells from ultraviolet radiation. Melanin from the melanocytes is transferred to the keratinocytes.

• Langerhans cells are phagocytic macrophages that interact with white blood cells during an immune response.

• Merkel cells occur deep in the epidermis at the epidermal-dermal boundary. They form Merkel discs, which, in association with nerve endings, serve a sensory function.

Five layers make up the epidermis:

• The stratum corneum contains many layers of dead, anucleate keratinocytes completely filled with keratin. The outermost layers are constantly shed.

• The stratum lucidum contains two to three layers of anucleate cells. This layer is usually apparent only in thick skin (palms of hands and soles of feet).

• The stratum granulosum contains two to four layers of cells held together by desmosomes. These cells contain keratohyaline granules, which contribute to the formation of keratin in the upper layers of the epidermis.

• The stratum spinosum contains eight to ten layers of cells connected by desmosomes. These cells are moderately active in mitosis.

• The stratum basale contains a single layer of columnar cells actively dividing by mitosis to produce cells that migrate into the upper epidermal layers and ultimately to the surface of the skin.

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