Epidermal growth factor and the onset of epithelial epidermal wound healing.

At 10 days in ovo the embryonic chick epidermis acquires the ability to spread as a cohesive epithelial sheet when wounded. A tissue culture system has been constructed that supports epidermal cell outgrowth consistent with epidermal behaviour in vivo and permits experimental manipulation of the isolated tissue with growth factors and other hormones. This culture system consists of embryonic chick epidermis isolated at days 8, 10, and 12 of development, serum-free, chemically-defined culture medium, and the inner surface of the vitelline membrane of the hen's egg as the culture substratum. At 8 days the cellular outgrowth is mesenchymal in the absence of exogenous EGF. The 8 day tissues responds to added EGF by exhibiting precocious epithelial outgrowth. The results suggest that sensitivity to EGF or EGF-like growth factors is part of the mechanism underlying the developmental onset of epidermal wound healing in skin. The epidermal origin of the outgrowth is determined by antibody staining for specific cytokeratins. The epithelial character of the outgrowth is determined by visualizing actin microfilament distribution. The normal epithelial outgrowth shows apical/basal polarization of the sheet except at the edge. From 10 days on, the isolated epidermis exhibits epithelial outgrowth from explants in culture in the absence of exogenous EGF, suggesting endogenous production of an EGF-like factor. Glucocorticoid and mineralocorticoid hormones both produce a reduced amount of epithelial outgrowth. This retardation of the early outgrowth by glucocorticoids and mineralocorticoids could result from a reduced ability of the cut edge of the epidermis to 'disorganize' and assume the unpolarized migratory form required for rapid epidermal wound healing.