Counter-rotational cell flows drive morphological and cell fate asymmetries in mammalian hair follicles.

Organ morphogenesis is a complex process coordinated by cell specification, epithelial-mesenchymal interactions and tissue polarity. A striking example is the pattern of regularly spaced, globally aligned mammalian hair follicles, which emerges through epidermal-dermal signaling and planar polarized morphogenesis. Here, using live-imaging, we discover that developing hair follicles polarize through dramatic cell rearrangements organized in a counter-rotational pattern of cell flows. Upon hair placode induction, Shh signaling specifies a radial pattern of progenitor fates that, together with planar cell polarity, induce counter-rotational rearrangements through myosin and ROCK-dependent polarized neighbour exchanges. Importantly, these cell rearrangements also establish cell fate asymmetry by repositioning radial progenitors along the anterior-posterior axis. These movements concurrently displace associated mesenchymal cells, which then signal asymmetrically to maintain polarized cell fates. Our results demonstrate how spatial patterning and tissue polarity generate an unexpected collective cell behaviour that in turn, establishes both morphological and cell fate asymmetry.

Planar cell polarity-dependent and independent functions in the emergence of tissue-scale hair follicle patterns.

Hair follicles of the mammalian epidermis display local order and global alignment, a complex pattern instructed by the core planar cell polarity (PCP) pathway. Here we address the contributions of core PCP genes, Van Gogh-like and Frizzled, to the establishment, local refinement, and global order of embryonic and postnatal hair follicles. We find that, similar to Fz6 mutants, the disordered hair patterns of Vangl2 mutants are refined over time and eventually corrected. In both mutants, we find that tissue-level reorientation occurs through locally coordinated follicle rotation at stereotyped locations. Strikingly, Vangl2 and Fz6 mutant follicles collectively rotate with opposing directionalities, suggesting that redundant core PCP signals contribute to their directed realignment. Consistently, global follicle alignment is not restored upon conditional ablation of both Vangl1 and Vangl2 genes. Instead, spatially distinct patterns of whorls and crosses emerge and persist even after a complete cycle of hair follicle regeneration. Thus, local refinement of hair follicles into higher order patterns can occur independently of the core PCP system, however, their global alignment with the body axes requires PCP function throughout morphogenesis, growth and regeneration.