RAPID INDUCTION AND CLEARANCE OF TGF-BETA-1 IS AN EARLY RESPONSE TO WOUNDING IN THE MOUSE EMBRYO

The TGFbeta family of growth factors has been implicated as playing a significant role in many aspects of embryonic morphogenesis, and also as a mediator of adult tissue repair processes. Unlike the situation in the adult, tissue repair in the embryo does not result in scarring, and it has been suggested that this might be due, in part, to reduced levels of growth factors, particularly TGFbeta, at the wound site. We have examined the expression patterns of TGFbeta genes following wounding of limb bud lesions in cultured E11.5 mouse embryos. The timetable of wound closure was investigated by standard light and electron microscopy from the time of wounding until the lesion had re-epithelialised 24 hours later. The expression of transcripts for each of the three TGFbeta genes was examined at various time points during the healing process using radioactive in situ hybridisation to tissue sections and wholemount non-radioactive in situ hybridisation to embryo pieces. Within 1 to 3 hours of wounding, transcripts encoding TGFbeta1 were rapidly induced within the epithelial cells of the wound margin, particularly those cells at the ventral aspect of the wound. By 3 to 6 hours post-wounding, TGFbeta1 transcripts were detectable in the mesenchyme of the wound bed. No TGFbeta3 induction was observed, and possible TGFbeta2 induction was largely obscured by endogenous expression associated with pre-cartilage mesenchymal condensation. Immunocytochemical analysis of tissue sections of the wound demonstrated a rapid induction of TGFbeta1 protein within 1 hour post-wounding, but also a subsequent rapid clearance of the protein from the wound site such that, by 18 hours post-wounding, TGFbeta1 levels had returned to near background. These data are discussed in terms of the molecular mechanisms underlying embryonic wound healing and the significance of the results to an understanding of scarring following adult tissue repair.