The epidermis is the superficial protective layer of the skin. It is derived from the ectoderm, and is composed of stratified squamous epithelium that varies in thickness from .007 to .12 mm. All but the deepest layers are composed of dead cells. Areas exposed to high friction have 5 layers; areas not exposed to high friction have 4 layers. Beginning with the innermost layer, the epidermis is composed of the following layers:
- Stratum basale consists of a single layer of cells in contact with the dermis. Four types of cells constitute this layer:
- Keratinocytes are specialized cells producing keratin. As keratinocytes are pushed away from the vascular nutrient and oxygen supply of the dermis, they undergo keratinization: their nuclei degenerate and their cellular content becomes dominated by keratin.
- Melanocytes are specilized epithelial cells which produce melanin.
- Tactile cells are sparse relative to keratinocytes and melanocytes, and are involved in tactile (touch) reception.
- Nonpigmented granular dendrocytes are scattered throughout the stratum basale. They are protective macrophagic cells which ingest bacteria and other foreign debris.
- Stratum spinosum is oftentimes grouped with the stratum basale. This complex is called the stratum germinativum
- Stratum granulosum contains 3 or 4 flattened alyers of cells. These cells contain granules filled with keratohyalin, a chemical precursor of keratin.
- Stratum lucidum appears clear because the nuclei, organelles and cell membranes are no longer visible. It exists only in the lips, soles, and palms.
- Stratum corneum is composed of 25 to 30 layers of flattened, scalelike, dead cells. It has been cornified, which is a process brought on by keratinization, which means that it has been dried and flattened.