Stratum basale is the deepest of the five layers of the epidermis. The stratum basale is a continuous layer of cells, usually only one cell thick, that is layered directly above the dermis. It is primarily made up of basal keratinocytes, the stem cells of the epidermis. The primary function of keratinocytes is to divide and migrate superficially to the stratum spinosum, the next of the five epidermal layers.
As the newly created keratinocytes move into the stratum spinosum, tight cell-to-cell adhesions, or desmosomes, form between adjacent cells. Within the stratum spinosum layer, keratinocytes begin to produce fibrous protein structures known as keratin. As new cells are produced in the stratum basale, the keratinocytes of the stratum spinosum are constantly being pushed toward the next layer, the stratum granulosum.
When this layer is reached, the keratinocytes are secreting protein and lipid granules that serve as the skin’s waterproof barrier. The cells also bind keratin filaments together. As the keratinocytes move through this barrier, they are cut off from the body’s supply of nutrients, and they begin to lose their nuclei and organelles. By the time the cells reach the stratum lucidum layer, they are dead and void of organelles. The cells in stratum licidum are clear and thin and stack approximately three to five cells thick. The stratum corneum is the outermost layer of the epidermis. It takes approximately 14 days for a skin cell to migrate from the stratum basale to the stratum corneum. These now dead, keratin-thick cells form the barrier that protects underlying tissues from infection, dehydration and stress. The stratum corneum is approximately 20 cells thick, and the outermost cells are shed.