1390 Active hair growth is fuelled by conveyor-belt like differentiation of germinative layer cells

Hair follicles (HF) undergo regular cycles of expansion (anagen), regression (catagen) and rest (telogen), with each of these stages having specific cellular and transcriptional dynamics. The mature anagen HF is characterized by distinct keratinocyte types, giving rise to transcriptionally and functionally separated concentric inner and outer HF layers. The anagen progenitor cells (also called germinative layer cells) are located juxtaposed to the dermal papilla and give rise to each of the differentiated layers in a stereotyped manner. Despite this well-known high level of organization and cell-lineage separation, it is still unclear how the germinative layer cells coordinate hair growth and commit to specific lineages. Here, we employed single-cell RNA-sequencing, single-molecule mRNA FISH and in vivo lineage-tracing experiments to show how the matrix progenitor cells commit and build the specific inner HF lineages depending on their position along the dermal papilla. We show that the matrix progenitors dynamically relocate in a conveyor-belt-like movement from bottom-to-top of the dermal papilla, switch their lineage fate and gradually restrict their potential the closer they get to the dermal papilla-top located medulla niche. This spatiotemporal cell choreography is characterized by a gradual change of expression from Lgr5 in the lower proximal cup, through Dcn, Id3 and Msx1 in the central part, to distal Lef1 expression where the medulla lineage starts. Detailed in situ mRNA stainings and clonal lineage tracing show the transcriptional changes and gradual lineage restriction of matrix progenitors in the spatial context. Taken together, we show how matrix progenitor cells progress in a conveyor-belt-like manner through several transcriptional states alongside the dermal papilla, differentiating into inner HF layer subtypes according to their location on this conveyor belt.