Physiology of the skin
The skin is a metabolically active organ with vital functions (Table 1), including the protection and homeostasis of the body.
Keratinocyte maturation
Epidermal cells undergo the following sequence during keratinocyte maturation (Fig. 1):
1. Undifferentiated cells in the basal layer and the layer immediately above divide continuously. Half of these cells remain in place, and half progress upwards and differentiate.
2. In the prickle cell layer, cells change from being columnar to polygonal. Differentiating keratinocytes synthesize keratins, which aggregate to form tonofilaments. The desmosomes connecting keratinocytes are composed of the structural molecules cadherins, desmogleins and desmocollins. Desmosomes distribute structural stresses throughout the epidermis and maintain a distance of 20 nm between adjacent cells.
3. In the granular layer, enzymes induce degradation of nuclei and organelles. Keratohyalin granules containing filaggrin mature the keratin and provide an amorphous protein matrix for the tonofilaments. Membrane-coating granules attach to the cell membrane and release an impervious lipid-containing cement, which contributes to cell adhesion and to the horny layer barrier.
4. In the horny layer, the dead, flattened corneocytes have developed thickened cornified envelopes containing involucrin that encase a matrix of keratin macrofibres aligned by filaggrin. The strong disulphide bonds of the keratin provide strength to the stratum corneum, but the layer is also flexible and can absorb up to three times its own weight in water. However, if it dries out (i.e. water content falls below 10%), pliability fails.
5. The corneocytes are eventually shed from the skin surface after degradation of the lamellated lipid and loss of desmosomal intercellular connections.
