The human skin is the largest organ of the body and comprises an outer stratified squamous epithelium, the epidermis, with an underlying connective tissue layer, the dermis, which is composed of multiple cell types. The Human Cell Atlas project has previously catalogued the cell types and subpopulations present in healthy skin derived from the trunk, however, we lack a comprehensive understanding of how these cell types are organised spatially within the tissue and how cell types differ both qualitatively and in their spatial organisation across anatomical sites. To address these questions, we have analysed single cell RNA sequencing of 120,000 cells from diverse healthy anatomical sites and 60,000 cells from basal cell carcinoma (BCC) - the most common malignancy in humans. We integrated this dataset with spatial transcriptomic data across 32 samples to map the spatial localisation of cell populations at 50um resolution. Subsequently, in situ sequencing of 12 samples using a custom skin-specific panel of 165 genes allowed us to define cell resolution maps for epithelial, immune and mesenchymal cell populations. For dermal cells of mesenchymal origin (fibroblasts and pericytes), we find that cellular identities are broadly conserved across anatomical sites, however cells from different body regions retain a transcriptional memory of developmental origin. We find 4 populations of fibroblasts and 3 populations of pericytes within the dermis in both healthy skin and BCC. In healthy skin, we assign each of these subpopulations to a specific spatial context in the healthy dermis and relate this to pathological mechanisms in BCC. Our findings provide key insights into the cellular landscape of human healthy adult skin and skin cancer.

Our understanding of how human skin cell states differ according to tissue context, body site and cancer development is limited. To address this we present a multi-scale spatial atlas of healthy adult human skin and basal cell carcinoma (BCC) incorporating single cell transcriptomics, _in vivo_ optical coherence tomography, spatial global transcriptional profiling and _in situ_ sequencing. Computational spatial projection reveals the localisation of distinct cell populations to specific tissue contexts. Despite differences in embryological origin of mesenchymal skin cells, cell populations are conserved between face and body. Nevertheless, mesenchymal cell populations retain signatures of their developmental origin. Spatial profiling supports a hair follicle origin for BCC and that cancer-associated fibroblasts are an expansion of a _POSTN_+ subpopulation associated with hair follicles in healthy skin. Our findings suggest that the identity of mesenchymal cell populations is regulated by signals emanating from adjacent structures and these signals are hijacked to promote the expansion of skin cancer stroma.

ESDR428 - A multi-scale spatial atlas of human skin across anatomical sites and cutaneous malignancy reveals novel cell states and repurposing of fibroblast subpopulations in skin cancer stroma

The scientific meeting of the European Society for Dermatological Research (ESDR) is held yearly, usually in September. With an international attendance of more than 1000 skin scientists from around the globe, the four-day event includes short oral presentations from peer-reviewed abstracts, keynote lectures from international experts, sponsored symposia, satellite programs, educational events as well as an important focus on posters, networking and building collaborations amongst scientists. For more information about this year’s meeting visit our website.