PDGF signalling in the dermis and in dermal condensates is dispensable for hair follicle induction and formation

Abstract

Embryonic hair follicle (HF) induction and formation is dependent on signalling crosstalk between the dermis and specialized dermal condensates on the mesenchymal side and epidermal cells and incipient placodes on the epithelial side, but the precise nature and succession of signals remain unclear. Platelet-derived growth factor (PDGF) signalling is involved in the development of several organs and the maintenance of adult tissues, including HF regeneration in the hair cycle. As both PDGF receptors, PDGFRα and PDGFRβ, are expressed in embryonic dermis and dermal condensates, we explored in this study the role of PDGF signalling in HF induction and formation in the developing skin mesenchyme. We conditionally ablated both PDGF receptors with Tbx18(Cre) in early dermal condensates before follicle formation, and with Prx1-Cre broadly in the ventral dermis prior to HF induction. In both PDGFR double mutants, HF induction and formation ensued normally, and the pattern of HF formation and HF numbers were unaffected. These data demonstrate that mesenchymal PDGF signalling, either in the specialized niche or broadly in the dermis, is dispensable for HF induction and formation.

Keywords: PDGF signalling; dermal papilla cells; hair follicle morphogenesis; hair follicle stem cells; stem cell niche.

Figure 1. PDGF receptors α and β are expressed in the dermis and dermal condensates of E14.5 skin and are dispensable for HF induction

(a) qRT-PCR of FACS sorted cells from E14.5 back skin shows high PDGFRα and PDGFRβ expression in the dermis. (b) Immunofluorescence staining for PDGFRα and PDGFRβ demonstrating widespread expression in back and ventral skin at E14.5. Note that both PDGFRs are also expressed in GFP⁺ dermal condensates (DCs) of Sox2^GFP+ mice. (c) Immunofluorescence staining of E14.5 dKO^Tbx18 back skin shows efficient PDGFRα and PDGFRβ ablation in Sox2^GFP+ DCs. (d) Immunofluorescence staining of E14.5 dKO^Prx1 ventral skin shows efficient ablation of PDGFRα and PDGFRβ in the entire dermis including Sox2^GFP+ DCs. (e) E14.5 WT and dKO^Tbx18 show a comparable Sox2^GFP+ DC pattern. (f) E14.5 WT and dKO^Prx1 show a similar Sox2^GFP+ DC pattern. (g–h) Quantification of HFs per field of view (FOV), assessed by staining for placode marker EDAR. HFs form in similar numbers in E14.5 dKO^Tbx18 (g) and dKO^Prx1 (h) skin compared to controls (n≥3, ≥20 FOVs for each). Dapi (blue) highlighted nuclei. Scale bar = 50μm.

Figure 2. PDGF signaling in the dermis and DCs is not required for HF formation

(a) Hematoxylin/eosin staining of E18.5 WT and dKO^Tbx18 back skin sections. (b) Quantification of total HFs per field of view (FOV; n=3, ≥50 FOVs for each). Comparable HF numbers in E18.5 WT and dKO^Tbx18 back skin. (c) Thickness measurement of E18.5 back skin (n=3, ≥30 FOVs for each). dKO^Tbx18 dermis is significantly thinner than WT. (d) Hematoxylin/eosin staining of E18.5 WT and dKO^Prx1 ventral skin sections. (e) Quantification of total HFs per field of view (n=2, ≥20 FOVs for each). WT and dKO^Prx1 show comparable HF numbers. (f) Thickness measurement of E18.5 ventral skin of WT and dKO^Prx1 (n=2, ≥30 FOVs for each). Mutant dermis is significantly thinner than WT. *p<0.05 using Student t test. Scale bar = 100μm.