Layers of epidermis

stratum basale

The stratum basale (bah-SAY-lee) consists mainly of a single layer of cuboidal to low columnar stem cells and keratinocytes resting on the basement membrane. Scattered among these are the melano- cytes, tactile cells, and stem cells. As the stem cells divide, they give rise to keratinocytes that migrate toward the skin surface and replace lost epidermal cells.

stratum corneum

The stratum corneum consists of up to 30 layers of dead, scaly, keratinized cells that form a durable surface layer. It is especially resistant to abrasion, penetration, and water loss.

stratum granulosum

The stratum granulosum consists of three to five layers of flat keratinocytes—more in thick skin than in thin skin. The keratinocytes of this layer contain coarse, dark-staining keratohyalin granules that give the layer its name. The functional significance of these granules will be explained shortly.

stratum lucidum

The stratum lucidum7 (LOO-sih-dum) is a thin zone superficial to the stratum granulosum, seen only in thick skin. Here, the keratinocytes are densely packed with a clear protein named eleidin (ee-LEE- ih-din). The cells have no nuclei or other organelles. This zone has a pale, featureless appearance with indistinct cell boundaries.

stratum spinosum

The stratum spinosum (spy-NO-sum) consists of several layers of keratinocytes. In most skin, this is the thickest stratum, but in thick skin it is usually exceeded by the stratum corneum. The deepest cells of the stratum spinosum remain capable of mitosis, but as they are pushed farther upward, they cease dividing. Instead, they produce more and more keratin filaments, which cause the cells to flatten. Therefore, the higher up you look in the stratum spinosum, the flatter the cells appear.