Figure 7.

Figure 7. Schematic representation of signaling interactions that regulate dermal morphogenesis.

(A) Globally circulating thyroid hormone (TH) stimulates expression of transcripts encoding Pdgfα ligand in basal epidermal cells (blue). Epidermally expressed Pdgfa ligand regulates migration and differentiation of dermal mesenchyme in the stroma and pre-scale-forming cells (pre-SFCs) that accumulate just beneath the epidermis prior to scale development. (B) Epidermally expressed Fgf20a ligand stimulates differentiation of SFCs in the superficial pre-SFC population. Subsequently, SFCs express Eda ligand, which, in turn, maintains expression of Fgf20a. The mechanisms that initiate squamation and regulate hexagonal scale patterning remain unknown.