Defining Sexual Dimorphism in the Skin The skin executes many functions differently based on biological sex. Nevertheless, the cellular and molecular mechanisms that give rise to these functional differences are poorly understood. Tissue-level sexual dimorphism may be rooted in the behaviors of somatic stem cells, such as hair follicle stem cells, which maintain tissues and can proliferate differently based on sex. This proposal will determine how biological sex, an organismal-level characteristic, affects stem cell behavior. I will study sex differences in hair follicle renewal during periods of both low and high sex hormones. Our preliminary data show that HFSCs, which regenerate the hair follicle in a highly synchronized manner, exhibit different proliferative behaviors in male and female mice. Before puberty, HFSCs begin to proliferate in males before females. This sexually dimorphic HFSC behavior may be due to differences in sex hormones, sex chromosomes, or both. Conversely, after puberty, when sex hormone production increases, HFSCs in females proliferate first. Distinct factors, therefore, may influence HFSC sexual dimorphism at different stages of life. Before puberty, elimination of sex hormones by gonad removal does not alter hair follicle renewal, suggesting circulating sex hormones do not establish sex differences at this stage. In adulthood, we found that sex hormone depletion by gonad removal leads to dramatically accelerated hair growth and eliminates sex differences. This accelerated hair follicle renewal is prevented in gonadectomized males by treatment with the androgen 5𝛼-dihydrotestosterone. These data suggest that androgens, a major type of sex hormone, can maintain HFSC quiescence after puberty. Cell-type specific knockouts of the androgen receptor in HFSCs and the microenvironment, or niche, reveal that androgens specifically maintain HFSC quiescence via the niche. My central hypothesis is prior to puberty, sex chromosomes primarily govern HFSC sexual dimorphism. The rise of sex hormones in puberty masks these differences, and androgens contribute to sex differences by altering the release of HFSC-modulating factors from the niche. I will interrogate the determinants of HFSC sexual dimorphism using surgical and genetic mouse models, as well as our expertise in constructing viral tools. In Aim 1, I will identify the contributions of sex chromosomes and sex hormones to sexual dimorphism in juvenile HFSC behavior and upon puberty. Aim 2 will reveal the gene products responsible for androgen-dependent HFSC quiescence using viral tools. Together, these data will identify the foundations of sex differences in HFSCs. My findings may reveal novel avenues for development of treatments for sexually dimorphic conditions.