Keratinocytes are the major cell type of the epidermis, which is the stratified squamous epithelium forming the outermost layer and thus providing the barrier function of the skin. The keratinocytes lie on a highly specialized extracellular matrix structure known as the basement membrane and are organized into multiple layers of cells. These layers are formed into distinct regions or strata that differ both morphologically and biochemically. From the basement membrane outward, they are: the stratum basale, the stratum spinosum, the stratum granulosum, and finally at the skin’s surface, the stratum corneum. Cellular proliferation is restricted to the s. basale and results in the production of replacement progenitor cells, which remain within this layer, and also cells that are committed to undergo the process of terminal differentiation. The latter cells leave the s. basale and progressively migrate through each layer of the epidermis, simultaneously maturing along the differentiation pathway as they go. Finally, they reach the outer surface of the epidermis in the form of fully functional anuclear cells known as corneocytes. The function of these mature cells is the protection of the underlying viable tissues from the external milieu. Initial attempts to grow keratinocytes in vitro were limited to the use of organ/explant culture (1). Using these techniques, whole pieces of skin could be kept alive in the short term, and growth was generally confined to the tissue fragment or onto the plastic immediately adjacent to the explant. The major drawback with these cultures is that they had an extremely limited lifespan and inevitably a limited application, because mixed cultures of keratinocytes and fibroblasts were obtained. Indeed, the presence of fibroblasts can often
present a major problem where keratinocyte culture is concerned. Even small amounts of contamination with fibroblasts can lead to them overgrowing the keratinocyte cultures. This is generally because of the relatively higher proliferation rates of fibroblasts compared to keratinocytes, even when under culture conditions optimized for keratinocyte proliferation.
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