Figure 1. Niche cells undergo collective polarization to build a morphologically polarized architecture.

A. Intravitally imaged dermal papilla (DP, outlined) at the hair follicle late-growth stage (n=3 mice). DP fibroblasts are labeled with membrane-tdTomato. Gray squares illustrate adjacent multipotent progenitors, with arrows indicating their differentiation routes. (A′) 3D reconstructed DP (red outlined) based on its membrane-tdTomato signal. Fibroblast cell bodies are rendered into surfaces with different colors.

B. Percentage of fibroblasts whose cell bodies are located at DP exterior. Exterior-localized: any fibroblasts whose cell body directly contacts DP edge; interior-localized: a fibroblast cell body surrounded by other fibroblasts’ cell bodies. n=30 quiescent and 30 late-growth HFs from 3 mice.

C. Intravitally imaged single fibroblast at late-growth stage (n=3 mice). (C′-C″) Long membrane protrusions extend through other fibroblasts. (C‴) 3D reconstructed single fibroblast based on its membrane-GFP signal. Within the red-outlined DP, this fibroblast’s cell body is rendered into a pink surface and its entire membrane in a green surface.

D. Coverage percentage of DP fibroblast clones at late growth, as the length of fibroblast cell body or entire membrane relative to the entire DP length. n=103 fibroblast clones from 3 mice.

E. Longitudinal imaging of a representative DP fibroblast from early to late-growth (n=3 mice). Insets highlight membrane-GFP (gray) and fibroblast cell body (arrowheads).

F. Percentage of DP fibroblasts harboring certain direction of membrane protrusions at late growth. n=77 fibroblast clones from 3 mice.

G. Longitudinally imaging of the same DP from mid- to late-growth. The membrane protrusion compartment (red line) on top of the cell body compartment (green line) organizes a morphologically polarized niche at late-growth (Anagen IIIc-VI), in contrast to an unpolarized niche at mid-growth (late Anagen II-IIIb).

H. Length of DP compartments at mid-growth (unpolarized niche) and late-growth stage (polarized niche). Lengths of membrane protrusion and cell body compartments are significantly different between the polarized and unpolarized niche stages. n=117 unpolarized DPs and 102 polarized DPs from 3 mice.

DPs are dash-lined. In Fig.1C, E, a single fibroblast is labeled by mosaic recombined membrane-GFP (mTmG) under the fibroblast driven CreER (Pdgfrα-CreER). In Fig.1G, fibroblast nuclei are labeled in green (Pdgfrα-H2BGFP) and cell membranes are in red (membrane-tdTomato). All data are presented as mean ± S.D. Unpaired two-tailed t-test is used in Fig.1B, D, H. Tukey’s multiple comparisons test is used in Fig.1F. See also Figure S1, Data S1 (Movie).