4. Epidermis

4. Epidermis

The integument consists of the skin (epidermis and der—mis) and associated appendages (sweat glands, sebaceous glands, hairs, and nails). Considered the largest body organ, the integument comprises approximately 16 % of total body weight. It is a highly specialized organ that functions to protect the body from injury, desiccation, and infection. It also participates in sensory reception, excretion, thermoregulation, and maintenance of water balance.

Epidermis is the outermost layer of the integument. It is astratified squamous epithelial layer of ectodermal origin.

Layers of the epidermis from deep to superficial consist of four strata. Stratum basale (stratum germinativum) is a proliferative basal layer of columnar—like cells that contain the fibrous protein keratin. Stratum spinosum is a multilaminar layer of cuboidal—like cells that are bound together by means of numerous cytoplasmic extensions and desmosomal junctions.

Stratum granulosum consists of flat polygonal cells filled with basophilic keratohyalin granules. Viewed at the electron microscopic level, these cells also contain numerous mem brane—coating granules. Stratum corneum is the superficial stratum of dead cells and consists of several to many layers of flat, anucleated, and cornified (kera—tinized) cells. In the epidermis of the palms and soles, a thin, transitional zone of flat eosinophilic or pale—staining anucleated cells may occur as the stratum lucidum. This layer is found only in regions with a thick strata cor—neum.

Cells of the epidermis: keratinocytes are the most numerous and are responsible for the production of the family of keratin proteins that provide the barrier function of the epidermis.

Melanocytes are derivatives of neural crest ectoderm. They are found in the dermis and are also scattered among the keratinocytes in the basal layers of the epidermis. These dendritic cells produce the pigment melanin in the form melanosomes that are transferred to keratinocytes.

Langerhans cells are dendritic cells but are members of the immune system and function as antigen—presenting cells. They have also been found in other parts of the body, including the oral cavity and lymph nodes.

Merkel cells are found in the basal epidermis and appear function in concert with nerve fibers that are closely associated with them. At the electron microscopic level, their cytoplasm contains numerous membrane—bound granules that resemble those of catecholamine—produ—cing cells.

New words

epidermis – эпидермис dermis – дерма weight – вес to protect – защищать injury – рана

cytoplasmic – цитоплазматический

level – уровень

flat – плоский

palm – ладонь

thick – толстый

pigment – пигмент

melanin – меланин

nerve – нерв

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