Epidermal differentiation and squamous metaplasia: from stem cell to cell death

Epidermal differentiation is a multi-step process defined by a cascade of interrelated changes in the expression of growth-regulatory and differentiation-specific genes (Fig. 1). Irreversible growth arrest is an early event in epidermal differentiation which occurs when cells transit from the basal to the innermost suprabasal layer of the skin and begin to express squamous-specific genes. In culture, interferon gamma, phorbol esters, confluence and growth in suspension are effective signals to induce irreversible growth arrest and differentiation. The induction of differentiation-specific genes occurs either concomitantly with or following growth arrest and is believed to be linked to the molecular events that control irreversible growth arrest. Such a link has been demonstrated in other cell systems undergoing terminal differentiation, such as myogenesis and adipogenesis. Genes encoding proteins involved in the formation of the cross-linked envelope are one set of squamous-specific genes which are induced in the suprabasal layers and include transglutaminase I and III, involucrin, loricrin and cornifins/small proline-rich proteins. Squamous-specific genes exhibit not only different patterns of tissue-specific expression but are also induced at different stages during differentiation, suggesting that transcription of individual genes is regulated by distinct mechanisms. The latter is supported by the identification of different sets of regulatory elements controlling the transcription of these genes. The importance of understanding both the mechanisms that regulate growth arrest and the differentiation program is emphasized by the association found between specific skin diseases and genetic alterations in growth-regulatory genes as well as differentiation markers. In addition, studies into those mechanisms will provide insight into the control of squamous metaplasia and the development of squamous cell carcinomas.