ONSET OF KERATIN-17 EXPRESSION COINCIDES WITH THE DEFINITION OF MAJOREPITHELIAL LINEAGES DURING SKIN DEVELOPMENT

The type I keratin 17 (K17) shows a peculiar localization in human epi thelial appendages including hair follicles, which undergo a growth cy cle throughout adult life. Additionally K17 is induced, along with K6 and K16, early after acute injury to human skin. To gain further insig hts into its potential function(s), we cloned the mouse K17 gene and i nvestigated its expression during skin development. Synthesis of K17 p rotein first occurs in a subset of epithelial cells within the single- layered, undifferentiated ectoderm of embryonic day 10.5 mouse fetuses . In the ensuing 48 h, K17-expressing cells give rise to placodes, the precursors of ectoderm-derived appendages (hair, glands, and tooth), and to periderm. During early development, there is a spatial correspo ndence in the distribution of K17 and that of lymphoid-enhancer factor (lef-1), a DNA-bending protein involved in inductive epithelial-mesen chymal interactions. We demonstrate that ectopic lef-1 expression indu ces K17 protein in the skin of adult transgenic mice. The pattern of K 17 gene expression during development has direct implications for the morphogenesis of skin epithelia, and points to the existence of a mole cular relationship between development and wound repair.