Gonadal development is a complex process that involves sex determination followed by divergent maturation into ovaries or testes. Historically, limited tissue accessibility and lack of reliable in vitro models have impeded our understanding of human gonadogenesis, despite its importance in gonadal pathologies and infertility. Here, we generated a comprehensive map of first- and second-trimester gonadal development using a combination of single-cell and spatial transcriptomics, chromatin accessibility assays and imaging. Using this approach, we identified novel transcription factors and cell states in human germ and supporting cell lineages. We compared them with other mammalian species and found primate-specific regulatory programmes. Our data identified cell context–specific interactions shaping sex specification and development of human germ cells. We defined a novel bipotent progenitor cell (LGR5+, TSPAN8+) in late embryos that can differentiate into early Sertoli in males or pre-granulosa cells in females. In fetal ovaries, we defined two subsets of pre-granulosa cells supporting germ-cell differentiation and distributed across the cortico-medullary axis. We also found a subset of developing granulosa cells appearing during the second trimester of pregnancy that is involved in follicular assembly. In fetal testes, we defined a novel supporting population (sPAX8 cells) located at the poles of the developing testis cords. We also found two tissue-resident myeloid populations that we named microglia-like and SIGLEC15+ fetal testicular macrophages. This study provides an unprecedented spatiotemporal map of human gonadal differentiation that can be utilised as a blueprint for in vitro gametogenesis.
Sex-specific gonadal differentiation is initiated by the expression of SRY in male foetuses. This promotes a signalling pathway directing testicular development, while in female foetuses the absence of SRY and expression of pro-ovarian factors promote ovarian development. Importantly, in addition to the initiation of a sex-specific signalling cascade the opposite pathway is simultaneously inhibited. The somatic cell populations within the gonads dictates this differentiation as well as the development of secondary sex characteristics via secretion of endocrine factors and steroid hormones. Opposing pathways SOX9/FGF9 (testis) and WNT4/RSPO1 (ovary) controls the development and differentiation of the bipotential mouse gonad and even though sex-specific gonadal differentiation is largely considered to be conserved between mice and humans, recent studies have identified several differences. Hence, the signalling pathways promoting early mouse gonad differentiation cannot be directly transferred to human development thus highlighting the importance of also examining this signalling in human fetal gonads. This review focus on the current understanding of regulatory mechanisms governing human gonadal sex differentiation by combining knowledge of these processes from studies in mice, information from patients with differences of sex development and insight from manipulation of selected signalling pathways in ex vivo culture models of human fetal gonads.