The layers of the epidermis are:

• 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 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 membrane-coating granules.

• Stratum lucidum is the transitional zone of flat eosinophilic or pale-staining anucle-ated cells only found in regions with a thick stratum corneum.

■ Stratum corneum is the superficial stratum consisting of several layers of flat, anucle-ated, and cornified (keratinized) cells.

Cell Types

The epidermis contains several cell types:

• 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 cells produce the pigment melanin in the form of melanosomes that are transferred to keratinocytes.

Clinical Correlate

Pemphigus is an autoimmune blistering disorder caused by disruption of desmosomes linking keratinocytes.

Psoriasis results from an increase in the number of proliferating cells in stratum basale plus stratum spinosum. In addition, there is an increase in the rate of cell turnover. This results in greater epidermal thickness and continuous turnover of the epidermis.

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