and could show that activated c-Met is present in both epidermis and hair follicles, including hair bulge stem cells (Fig. S1, avail- able at http://www.jcb.org/cgi/content/full/jcb.200701086/DC1). Our data suggest that during wound healing, HGF/SF and c-Met may signal in an autocrine manner in the HE. Wound closure in conditional c-Met mutant mice We examined the function of the c-Met signaling system in the skin using conditional mutagenesis. For this, we crossed K14- cre mice, which express cre recombinase in the epidermis start- ing on embryonic day (E) 15 (Huelsken et al., 2001), with c-Met mutant mice, to generate animals with a K14-cre; c-Metflox/null genotype. In such animals, one allele of c-Met corresponded to the conventional mutation, c-Metnull (Bladt et al., 1995), and the other to a “floxed” allele, c-Metflox (Borowiak et al., 2004). After cre-mediated recombination, the exon encoding the essential ATP-binding site of c-Met is removed in c-Metflox, and a functional allele, which we denote as c-Met∆ is gener- ated (structures of nonrecombined and recombined alleles are shown in Fig. 2 A). K14-cre; c-Metflox/null mice will henceforth be called conditional c-Met mutant mice. Efficient recombination of c-Met was observed in the epi- dermis of conditional c-Met mutant mice. Southern blot analy- ses demonstrated that 95% of the cells in the epidermis had already recombined the c-Metflox allele at E17.5 (Fig. 2 B, c-Met∆). A similar proportion of cells containing the recombined allele were observed in the epidermis of young and adult animals, e.g., at postnatal day (P) 8 and at 12 wk (Fig. 2 B). In other epithelial tissues, such as pancreas, lung, and liver, mutation of c-Met was not observed, i.e., the nonrecombined c-Metflox allele was ob- served (Fig. 2 C). Recombination introduced by K14-cre in the skin was also assessed histologically, using the Z/AP reporter mice (Lobe et al., 1999); in such mice, yellow NBT/BCIP stain- ing measures alkaline phosphatase activity, which is detectable in recombined epidermal cells, whereas blue X-Gal staining indicates β-galactosidase activity, which is observed in nonre- combined cells. The histological analysis of skin sections of these mice demonstrated that the vast majority of the cells in the epidermis had undergone recombination, and only small groups of nonrecombined cells were detectable (Fig. 2 D; the enlarged picture in Fig. 2 E shows a group of blue nonrecombined cells, which are marked by arrow). Recombination was also observed in hair follicle cells, particularly in those cells that locate in the bulge region (Fig. 2, F and G, yellow cells); recombination was not apparent in arrector pili muscle cells (Fig. 2, F and G, blue cells). We first examined the appearance of skin and hair in con- ditional c-Met mutant mice by histology at birth and afterward. No gross morphological changes in the epidermis were detected compared with control mice. For instance, the thickness of the epidermis was comparable, and it did not display any patho- logical alterations as assessed by immunohistology using anti- bodies directed against keratin 10 and loricrin (Fig. 2 J and not Figure 3. Wound healing in control and conditional c-Met mutant mice. (A and D) Hematoxylin/eosin staining of sections of wound edges from control and mutant mice 3 (A) and 5 d (D) after wounding. Only halves of the wounds are shown (for the scheme of a complete wound see Fig. 1). (B and E) Masson trichrome staining of wound sections 3 (B) and 5 d (E) after injury. (C and F) Immunohistological analysis of wound sections from control and mutant mice 3 (C) and 5 d (F) after injury using antibodies directed against keratin 6 (red) and ﬁbronectin (green). Arrows mark the HE. Es, eschar; F, fatty tissue; G, granulation tissue; HF, hair follicle. Bar, 100 μm. 154 JCB • VOLUME 177 • NUMBER 1 • 2007