The keratinocyte is the major cell type of the epidermis, making up about 90% of epidermal cells. The epidermis is divided into four layers based on keratinocyte morphology:
Keratinocytes originate in the basal layer from the division of keratinocyte stem cells. They are pushed up through the layers of the epidermis, undergoing gradual differentiation until they reach the stratum corneum where they form a layer of dead, flattened, highly keratinised cells called squames. This layer forms an effective barrier to the entry of foreign matter and infectious agents into the body and minimises moisture loss.
Keratinocytes are shed and replaced continuously from the stratum corneum. The time of transit from basal layer to shedding is approximately one month although this can be accelerated in conditions of keratinocyte hyperproliferation such as psoriasis.
Primary keratinocytes are easily grown in large numbers, and their differentiation can be induced under well-defined culture conditions, with a rapid and homogeneous response amenable to careful biochemical analysis.
Finally, grafting of cultured keratinocytes together with other "instructing" cell types (dermal fibroblasts; dermal papilla cells) is sufficient to reproduce the complex differentiation program that occurs in vivo in the epidermis as well as hair follicles.
The vast majority of growing keratinocytes in culture, as well as in the basal layer of the epidermis in vivo, is represented by transit amplifying cells.
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