Structure and Function of the Skin

Basal Cell Layer

The basal cells are the undifferentiated, proliferating cells. Skin stem cells are located in the basal layer in the interfollicular epidermis, and they give rise to keratinocytes. For normal skin homeostasis, daughter cells from the basal cell layer migrate upward and begin the process of differentiation. In normal skin, cell division does not take place above the basal cell layer. It takes about 2 weeks for the cells to migrate from the basal cell layer to the top of the granular cell layer, and a further 2 weeks for the cells to cross the stratum corneum to the surface, where they finally are shed. Injury and inflammation increase the rate of proliferation and maturation ( Fig. 2.1 ).

Stratum Spinosum

The stratum spinosum lies above the basal layer and is composed of keratinocytes , which differentiate from the basal cells beneath them. The keratinocytes produce keratin, a fibrous protein that is the major component of the horny stratum corneum. The stratum spinosum derives its name from the “spines,” or intercellular bridges, that extend between keratinocytes and which are visible with light microscopy. Ultrastructurally, these are composed of desmosomes, which are extensions from keratin within the keratinocyte; functionally, they hold the cells together ( Fig. 2.2 ).

Stratum Granulosum

The process of differentiation continues in the stratum granulosum, or granular cell layer, in which the cells acquire additional keratin and become more flattened. In addition, they contain distinctive dark granules, seen easily on light microscopy, that are composed of keratohyalin. Keratohyalin contains two proteins, one of which is called profilaggrin , the precursor to filaggrin. As its name suggests, filaggrin plays an important role in the aggregation of keratin filaments in the stratum corneum. The other protein is called involucrin (from the Latin for “envelope”), and plays a role in the formation of the cell envelope of cells in the stratum corneum. Ichthyosis vulgaris ( ichthys , Greek for “fish”) is an inherited dry skin condition secondary to deficient filaggrin production, as noted on light microscopy of a skin biopsy by a reduced or absent granular layer ( Fig. 2.3 ).

Granular cells also contain lamellar granules, which are visualized with electron microscopy. Lamellar granules contain polysaccharides, glycoproteins, and lipids that extrude into the intercellular space and ultimately are thought to help form the “cement” that holds together the stratum corneum cells. Degradative enzymes also are found within the granular cells; these are responsible for the eventual destruction of cell nuclei and intracytoplasmic organelles.

Stratum Corneum

A remarkably abrupt transition occurs between the viable, nucleated cells at the top of the granular cell layer and the dead cells of the stratum corneum ( Fig. 2.4 ). The cells in the stratum corneum are large, flat, polyhedral, plate-like envelopes filled with keratin. They are stacked in vertical layers that range in thickness from 15 to 25 layers on most body surfaces to as many as 100 layers on the palms and soles. The cells are held together by a lipid-rich cement in a fashion similar to “bricks and mortar.” The tightly packed, keratinized envelopes in the stratum corneum provide a semi-impenetrable layer that constitutes the major physical barrier of the skin.

The skin microbiome could be considered another outermost layer of the epidermis. With the better sequencing and metagenomics technologies, the role of the microbiome in human health and disease states is being actively investigated. It plays an active role in modulating the hosťs immune response to pathogens.

The epidermis, then, is composed of cells that divide in the basal cell layer (basal cells), keratinize in the succeeding layers (keratinocytes), and eventuate into the devitalized, keratin-filled cells in the stratum corneum.