Hierarchical patterning modes orchestrate hair follicle morphogenesis

Abstract

Two theories address the origin of repeating patterns, such as hair follicles, limb digits, and intestinal villi, during development. The Turing reaction–diffusion system posits that interacting diffusible signals produced by static cells first define a prepattern that then induces cell rearrangements to produce an anatomical structure. The second theory, that of mesenchymal self-organisation, proposes that mobile cells can form periodic patterns of cell aggregates directly, without reference to any prepattern. Early hair follicle development is characterised by the rapid appearance of periodic arrangements of altered gene expression in the epidermis and prominent clustering of the adjacent dermal mesenchymal cells. We assess the contributions and interplay between reaction–diffusion and mesenchymal self-organisation processes in hair follicle patterning, identifying a network of fibroblast growth factor (FGF), wingless-related integration site (WNT), and bone morphogenetic protein (BMP) signalling interactions capable of spontaneously producing a periodic pattern. Using time-lapse imaging, we find that mesenchymal cell condensation at hair follicles is locally directed by an epidermal prepattern. However, imposing this prepattern’s condition of high FGF and low BMP activity across the entire skin reveals a latent dermal capacity to undergo spatially patterned self-organisation in the absence of epithelial direction. This mesenchymal self-organisation relies on restricted transforming growth factor (TGF) β signalling, which serves to drive chemotactic mesenchymal patterning when reaction–diffusion patterning is suppressed, but, in normal conditions, facilitates cell movement to locally prepatterned sources of FGF. This work illustrates a hierarchy of periodic patterning modes operating in organogenesis.

Dermal Condensate Niche Fate Specification Occurs Prior to Formation and Is Placode Progenitor Dependent

“…Previous studies have demonstrated that cell aggregation is the cellular mechanism inciting DC formation (Biggs et al, 2018; Glover et al, 2017), but when the DC niche fate is acquired and how this timing relates to placode progenitor specification remains unknown. To identify potential DC-fated precursors before cluster formation and niche establishment, we followed up on our previous work where we isolated with fluorescence-activated cell sorting (FACS) the DC as GFP + cells from embryonic Sox2 GFP knock-in reporter skin (Figure S1A) (Sennett et al, 2015).…”

“…Recent studies have further demonstrated that FGF signaling is required for cell migration to form mature DC clusters, however it is unclear whether it regulates DC fate acquisition itself (Biggs et al, 2018; Glover et al, 2017). Having discovered unclustered DC precursors prior to formation of mature DC, we next asked whether Fgf20 is required for induction of the pre-DC fate.…”

“…Contextualizing these early events in hair follicle formation is further complicated by the rapidity with which morphogenesis occurs; Wnt signaling activity appears virtually simultaneously in Pc and DC (DasGupta and Fuchs, 1999; Tsai et al, 2014; Zhang et al, 2009), rendering parsing out the precise order and timing difficult. Recent work also demonstrated direct mesenchymal self-organization ability in the absence of an epidermal pre-pattern (Glover et al, 2017) further calling into question the origin and timing of niche fate specification (Figure 1A): 1) is the niche fate specified before niche cluster formation and signaling function and 2) how is the timing of niche fate acquisition related to progenitor fate and signaling?…”

Single-Cell Analysis Reveals a Hair Follicle Dermal Niche Molecular Differentiation Trajectory that Begins Prior to Morphogenesis

“…By contrast, the events that lead to DC cell specification and DC formation remain sparsely delineated. Previous studies, including those using bulk RNA-sequencing and live imaging, have characterized some of the signals and cell behaviors of differentiated DC cells once placodes are histologically apparent (Biggs et al, 2018; Glover et al, 2017; Sennett et al, 2015; Tsai et al, 2014). While these studies uncovered mechanisms by which differentiated DC cells condense into a morphological DC, the events that precede DC cell differentiation are largely unknown.…”

“…While these studies uncovered mechanisms by which differentiated DC cells condense into a morphological DC, the events that precede DC cell differentiation are largely unknown. Two studies suggested that DC cells are specified from apparently equivalent upper dermal cells in response to nearby epithelial placode signals (Biggs et al, 2018; Glover et al, 2017). These differentiated DC cells exit the cell cycle, migrate, and undergo cell shape changes to form a dense cluster.…”