Building a human pluripotent stem cell-based gonadal niche: improving in vitro systems with in vivo insights.

Background: The gonadal somatic niche is crucial for sex determination and gamete formation throughout the human life cycle. However, key steps in gonadal somatic lineage differentiation occur during embryonic and foetal development, making them difficult to study in humans. In vitro differentiation models are therefore needed to investigate gonadal development, support in vitro gametogenesis, and study infertility. A comprehensive overview of gonadal somatic niche differentiation, both in vivo and in vitro, is thus crucial.

Objective And Rationale: This review connects in vivo knowledge with in vitro differentiation systems for gonadal somatic niches, predominantly focusing on cell-cell signalling factors. It evaluates existing in vitro protocols for differentiating testicular and ovarian somatic niches, discusses them in the context of in vivo findings, and explores potential advancements in model systems.

Search Methods: A narrative review was conducted after a comprehensive search of the PubMed database through to February 2025; the review focused on search topics including: in vivo gonadal differentiation in humans and mice; in vitro differentiation of human embryonic stem cells or human-induced pluripotent stem cells into gonadal somatic cells (bipotential, Sertoli or granulosa cells); and evidence for the cell-cell signalling factors used in these protocols.

Outcomes: We investigated various strategies that aim to differentiate human pluripotent stem cells into gonadal somatic cell lineages. These include sequential growth factor differentiation recapitulating all known developmental progenitor stages, directed growth factor differentiation that omitted one or more developmental intermediates, and directed overexpression of key transcription factors. To induce differentiation, the growth factor-based protocols used various cell-cell signalling factors, with some derived from in vivo studies, while others lacked direct in vivo evidence. Despite significant advances in guiding pluripotent stem cells towards gonadal differentiation, challenges remain, such as the limited molecular and functional validation of the generated cell types. Consequently, complete human in vitro gametogenesis through co-culture techniques with pluripotent cell-derived germ cells has not yet been achieved, indicating that full functional maturation of the gonadal niche has not been attained with the current protocols.

Wider Implications: Integrating knowledge on in vivo gonadal development with enhanced differentiation protocols offers the potential to reliably generate the gonadal somatic niche in vitro. This allows for more accurate modelling of the gonad, facilitating deeper insights into the normal and pathological processes involved in gonadal development and germ cell maturation. For example, it could help to identify mechanisms linked to infertility or differences of sex development. Importantly, as many of these models are based on human pluripotent stem cells, they have the potential for personalization, enabling future patient-specific models for studying reproductive disorders and developing tailored fertility treatments.

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