Mapping the Cellular and Molecular Dynamics of Mouse and Human Hair Follicles Reveals Mechanisms of Hair Growth

Abstract Introduction and aims Tissue homeostasis in continuously renewing organs, such as the skin, rely on orchestrated cellular and molecular dynamics. The hair follicle (HF) is characterized by distinct keratinocytes organized in transcriptionally and functionally distinct concentric 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. 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. Methods Here, we employed single-cell RNA sequencing, single-molecule mRNA fluorescence in situ hybridization and in vivo lineage tracing to show how the matrix progenitor cells commit and contribute to the HF lineages depending on their position. Furthermore, we developed a four-dimensional live imaging system of the human HF to construct a global map of single-cell dynamics. Results We show that the HF progenitors dynamically relocate in a conveyor-belt-like movement, switch their lineage fate and gradually restrict their potential. 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 staining and mouse clonal lineage tracing show the transcriptional changes and gradual lineage restriction of matrix progenitors in the spatial context. Combining human HF imaging with fluid dynamics theories, we uncovered a spiral-like downward movement of outer root sheath cells, feeding the germinative layer stem cells, and revealed a pulling force induced by the outer root sheath cells as a critical driver for hair growth. Conclusions Taken together, our study uncovered how HF progenitor cells progress in a conveyor-belt-like manner through several transcriptional states and discovered new cellular and molecular mechanisms regulating hair formation.