Epidermal stem cells at the point of no return

Sammanfattning: The skin is our main barrier against outside harm, and it helps to maintain internal homeostasis. These functions are mostly fulfilled by the epidermis, which is the outermost layer of the skin, formed from keratinocytes that are constantly being shed and replaced through proliferation, differentiation, and subsequent delamination of tissue resident stem cells. Keratinocytes also give rise to hair follicles which go through regular cycles of growth and rest. These processes are supported by, for example, fibroblasts, immune cells, nerves and vessels, helping to maintain the protective function of the skin. The wealth of different cell types and stem cell behaviours makes skin an attractive model system for research. The overarching aim of this thesis was to study the epidermal stem cells at their onset of differentiation – to determine their point of no return beyond which they cannot revert back to a less differentiated state. To address this question, a combination of single-cell RNA-sequencing, in situ mRNA stainings and intra vital imaging methods were used, allowing for the assessment of cells’ transcriptional states, their location in the tissue and their behaviour over time. In Paper I, we present a thorough characterization of differentiation-committed basal cells in the interfollicular epidermis. We study their behaviour in living tissue and describe their transcriptional states upon commitment and delamination. Finally, we show how mitosis of committed cells is not integral to their differentiation journey. In Paper II, we analysed full thickness adult skin in the growth and resting stages of the hair cycle to create an unbiased transcriptional atlas of all major cell types. We uncover unexpected heterogeneity in the anagen hair follicle lineages and highlight how fibroblast populations can undergo transcriptional state changes during hair cycling. In Paper III, we expanded the transcriptional atlas to embryonic development, describing the transcriptional and anatomic landmarks at the time when crucial skin structures are established. We show early heterogeneity within the fibroblast population, as well as the first signs of hair follicle development and the formation of the panniculus carnosus muscle. Taken together, this thesis investigates skin stem cells molecularly, spatially, and behaviourally when they commit to specific lineages whether in the adult or developing epidermis, in the growing hair follicle or in the embryonic fibroblast compartment. Finally, we speculate on what could be the underlying cause that pushes the stem cells over the point of no return.