Dead cells constantly flake off the skin surface. They float around as tiny white specks in the air, settling on household surfaces and forming much of the house dust that accumulates there. Because we constantly lose these epidermal cells, they must be continually replaced.
Keratinocytes are produced deep in the epidermis by the mitosis of stem cells in the stratum basale. Some of the deepest keratinocytes in the stratum spinosum also multiply and increase their number. Mitosis requires an abundant supply of oxygen and nutrients, which these deep cells can acquire from the blood vessels in the nearby dermis. Once the epidermal cells As new keratinocytes are formed, they push the older ones toward the surface. In 30 to 40 days, a keratinocyte makes its way to the skin surface and then flakes off. This migration is slower in old age and faster in skin that has been injured or stressed. Injured epidermis regenerates more rapidly than any other tissue in the body. Mechanical stress from manual labor or tight shoes accelerates keratinocyte multiplication and results in calluses or corns, thick accumulations of dead keratinocytes on the hands or feet.
As keratinocytes are shoved upward by the dividing cells below, their cytoskeleton proliferates, the cells grow flatter, and they produce lipid-filled membrane-coating vesicles (lamellar granules). In the stratum granulosum, three important developments occur. (1) The keratinocyte nuclei and other organelles degenerate and the cells die. (2) The keratohyalin granules release a protein called filaggrin that binds the cytoskeletal keratin filaments together into coarse, tough bundles. (3) The membranecoating vesicles release a lipid mixture that spreads out over the cell surface and waterproofs it.
An epidermal water barrier forms between the stratum granulosum and stratum spinosum. It consists of the lipids secreted by the keratinocytes, tight junctions between the keratinocytes, and a thick layer of insoluble protein on the inner surfaces of the keratinocyte plasma membranes. The epidermal water barrier is crucial to retaining water in the body and preventing dehydration. Cells above the barrier quickly die because the barrier cuts them off from the supply of nutrients below. Thus, the stratum corneum consists of compact layers of dead keratinocytes and keratinocyte fragments. Dead keratino- cytes soon exfoliate (fall away) from the epidermal surface as tiny specks called dander. Dandruff is composed of clumps of dander stuck together by sebum (oil).
A curious effect of the epidermal water barrier is the way our skin wrinkles when we linger in the bath or a lake. The keratin of the stratum corneum absorbs water and swells, whereas the deeper layers of the skin do not. The greater surface area of the stratum corneum forces it to wrinkle. This is especially conspicuous on the fingers and toes (“prune fingers”) because they have such a thick stratum corneum and they lack the sebaceous glands that produce water-resistant oil elsewhere on the body.