The epidermis has four layers: the basal cell layer, spiny cell layer, granular cell layer, and cornified layer (Fig. 1-2). The basal cell layer (stratum basalis) is composed of columnar or cuboidal cells that are in direct contact with the basement membrane, the structure that separates the dermis from the epidermis. The basal cell layer contains the germinative cells, and, for this reason, occasional mitoses may be present.

The three layers above the basal cell layer are histologically distinct and demonstrate differentiation of the keratinocytes as they move toward the skin surface and become “cornified.” Just above the basal cell layer is the  piny cell layer (stratum spinosum), so called because of a high concentration of desmosomes and keratin filaments that give the cells a characteristic “spiny” appearance (Fig. 1-3A). Above the spiny layer is the granular cell layer (stratum granulosum). In this layer, keratohyalin granules are formed and bind to the keratin filaments (tonofilaments) to form large electron-dense masses within the cytoplasm that give this layer its “granular” appearance.

The outermost layer is the cornified layer (stratum corneum), where the keratinocytes abruptly lose all of their organelles and nuclei. The keratin filaments and keratohyalin granules form an amorphous mass within the keratinocytes, which become elongated and flattened, forming a lamellar array of “corneocytes.” The corneocytes are held together by the remnants of the desmosomes (dense bodies) and a “cementing substance” released into the intracellular space from organelles called Odland bodies.