Fate Before Function: Specification of the Hair Follicle Niche Occurs Prior to its Formation and Is Progenitor Dependent

Abstract

Cell fate transitions are essential for specialization of stem cells and their niches, but the precise timing and sequence of molecular events during embryonic development are largely unknown. Here, we show that dermal condensates (DC), signaling niches for epithelial progenitors in hair placodes, are specified before niche formation and function. With 3D/4D microscopy we identify unclustered DC precursors. With population-based and single-cell transcriptomics we define a molecular time-lapse of dynamic niche signatures and the developmental trajectory as the DC lineage emerges from fibroblasts. Co-expression of downregulated fibroblast and upregulated DC genes in niche precursors reveals a transitory molecular state following a proliferation shutdown. Waves of transcription factor and signaling molecule expression then consolidate DC niche formation. Finally, ablation of epidermal Wnt signaling and placode-derived FGF20 demonstrates their requirement for DC-precursor specification. These findings uncover a progenitor-dependent niche precursor fate and the transitory molecular events controlling niche formation and function. and to Judith Vivié for expert assistance with single cell RNA sequencing at the Hubrecht Institute (Netherlands). Additionally, we thank Marja Mikkola (U. Helsinki) and Peggy Myung (Yale)) for helpful discussions and valuable comments. K.W.

Mesenchyme governs hair follicle induction

“…Previous RNA-seq profiling of E14.5 back skin has indicated that Rspo1 and Rspo3, but not Rspo2 and Rspo4, are expressed in the mesenchyme (Sennett et al, 2015;Mok et al, 2019;Snippert et al, 2010). We first analysed the expression pattern of Rspo1 and Rspo3 in the back skin prior to hair follicle induction (E12.5), during hair follicle induction (E13.5), and at placode stage (E14.5) by RNA in situ hybridization.…”

“…As a result, epithelial keratinocytes migrate centripetally to form a thickening, the placode (Ahtiainen et al, 2014). Inductive signals from the placode then induce the underlying fibroblasts to migrate and aggregate underneath the placode, forming the mesenchymal component of the hair follicle, the dermal condensate (DC) (Glover et al, 2017;Biggs et al, 2018;Gupta et al, 2019;Mok et al, 2019). Subsequently, secondary inductive signals from the DC promote invagination of the epithelium to the mesenchyme, and the DC is engulfed by the epithelium and remains an integral part of the mature hair follicle, named the dermal papilla (DP) (Sennett & Rendl, 2012).…”

“…Once placode formation has initiated, signals emanating from the placode, in particular fibroblast growth factor 20 (Fgf20) and Sonic hedgehog (Shh), govern DC formation (Huh et al, 2013;Woo, Zhen & Oro, 2012). Fgf20 has been shown to induce directional migration of mesenchymal fibroblasts and to induce DC cell fate (Mok et al, 2019;Biggs et al, 2018). Fgf20 is necessary for DC formation, as demonstrated by the absence of all molecular and morphological indications of primary and secondary DCs in Fgf20 null mice (Huh et al, 2013;Biggs et al, 2018;Mok et al, 2019).…”

“…Fgf20 has been shown to induce directional migration of mesenchymal fibroblasts and to induce DC cell fate (Mok et al, 2019;Biggs et al, 2018). Fgf20 is necessary for DC formation, as demonstrated by the absence of all molecular and morphological indications of primary and secondary DCs in Fgf20 null mice (Huh et al, 2013;Biggs et al, 2018;Mok et al, 2019).…”

“…In contrast, the reverse recombination of the E12.5 hair-forming back skin epithelium with the glabrous chick skin mesenchyme produced short epithelial nodules protruding into the dermis (Dhouailly, 1973). Furthermore, studies showing that DC formation is preceded by a placodal prepattern (Huh et al, 2013;Glover et al, 2017;Biggs et al, 2018;Mok et al, 2019) and that primary and secondary hair placodes form in the absence of DCs in Fgf20 null embryos (Huh et al, 2013) have challenged the prevailing model of mesenchymal induction of hair follicle formation.…”