Decoding the development of human prenatal skin

Abstract

Skin, the largest organ in the human body, is composed of diverse cell types strategically organised in defined strata and specialised appendages. Its morphogenesis is underpinned by precise spatiotemporally distributed cellular and molecular changes which remain incompletely deciphered. It is seeded by immune cells, including macrophages, early during gestation but their contribution, if any, to prenatal skin and hair follicle development is uncertain. To investigate de novo hair follicle formation and the role of immune cells in skin development, a multi-omics atlas of human prenatal skin (7 to 17 post-conception weeks) was assembled, integrating single-cell and spatial transcriptomic data. Comparative assessments with adult skin and hair follicle datasets and cross-species analyses were performed to determine features specific to human prenatal skin. A hair-bearing skin organoid model, derived from human embryonic stem cells and induced pluripotent stem (iPS) cells, was benchmarked against prenatal skin to evaluate the faithfulness of skin organoids to in vivo skin and their utility to functionally interrogate skin morphogenesis. Findings were validated using immunofluorescence, in situ hybridization and tissue culture experiments. The systematic charting of prenatal skin (433,961 single cells) identified the precursor cell states, differentiation trajectories and cross-compartmental cellular interactions underpinning human hair follicle neogenesis. Re-mapping of these single cells in situ, using spatial transcriptomic data, uncovered microanatomical tissue niches where immune cells co-located and cross-talked with non-immune cells, contributing to early hair follicle development, angiogenesis, neurogenesis, and scarless healing. In particular, macrophages promoted prenatal skin vascular endothelial development. Skin organoids, which lacked immune cells, concomitantly had markedly reduced quantity and heterogeneity of endothelial cells. Addition of iPS cell-derived macrophages significantly improved vascular network formation in skin organoids. Overall, this study revealed that immune cells function as critical players in skin morphogenesis and provided a potential blueprint to enhance future skin and organoid tissue engineering.