The epidermis is a constantly renewing, keratinised, stratified squamous epithelium. It provides many of the functions of skin.
Structure
The epidermis is formed by four or five layers. Thick skin contains an extra layer, the stratum lucidum, which thin skin lacks.
- The stratum basale is the deepest layer and lies against the basement membrane. Cuboidal or columnar basal cells are found in this region.
- The stratum spinosum is characterised by polygonal cells with intervening clefts. The cells digitate in these clefts through cytoplasmic extensions, creating the ‘spine-like’ appearance that gives the layer its name.
- The stratum granulosum varies in size between thick and thin skin. The cytoplasm of cells in this layer contains fine grains on light microscopy.
- The stratum lucidum is a thin layer formed by thin, dead cells with no nuclei. This layer is usually not visible in thin skin.
- The stratum corneum is the most superficial of the layers, containing thin dead cells containing keratin. The cells are linked together with a lipid material making identification of individual cells difficult. The most superficial layers are more loosely connected.
Melanocytes, T-lymphocytes and Langerhan’s cells are also found within the epidermis, usually in the basal layer. The melanocytes and Langerhan’s cells have small cytoplasmic processes which travel some distance from the cell body. Merkel cells, important for light touch sensation, are also found along the basement membrane.
Malignancy
Most malignancies of the skin arise from the epidermis. Squamous cell carcinomas arise from abnormal keratinocytes. Merkel cell carcinomas and melanomas arise from the Merkel cells and melanocytes respectively. The cell of origin for basal cell carcinoma is not clear; some believe it arises from cells the skin appendages.
