Abstract
A key initial event in hair follicle morphogenesis is the localised thickening of the skin epithelium to form a placode, partitioning future hair follicle epithelium from interfollicular epidermis. Although many developmental signalling pathways are implicated in follicle morphogenesis, the role of epidermal growth factor (EGF) and keratinocyte growth factor (KGF, also known as FGF7) receptors are not defined. EGF receptor (EGFR) ligands have previously been shown to inhibit developing hair follicles; however, the underlying mechanisms have not been characterised. Here we show that receptors for EGF and KGF undergo marked downregulation in hair follicle placodes from multiple body sites, whereas the expression of endogenous ligands persist throughout hair follicle initiation. Using embryonic skin organ culture, we show that when skin from the sites of primary pelage and whisker follicle development is exposed to increased levels of two ectopic EGFR ligands (HBEGF and amphiregulin) and the FGFR2(IIIb) receptor ligand KGF, follicle formation is inhibited in a time- and dose-dependent manner. We then used downstream molecular markers and microarray profiling to provide evidence that, in response to KGF and EGF signalling, epidermal differentiation is promoted at the expense of hair follicle fate. We propose that hair follicle initiation in placodes requires downregulation of the two pathways in question, both of which are crucial for the ongoing development of the interfollicular epidermis. We have also uncovered a previously unrecognised role for KGF signalling in the formation of hair follicles in the mouse.