Decoding the Heterogeneity of Human Undifferentiated Spermatogonia Reveals RAS-Dependent Regulation of Stem Cell Fate

Human spermatogenesis depends on a rare population of undifferentiated spermatogonia (uSPG) that maintain the lifelong capacity to self‑renew and generate differentiating germ cells. Despite their central role in male fertility, the molecular logic governing human uSPG identity and fate decisions has remained poorly defined, in part because human testicular tissue is limited and because rodent models do not fully recapitulate primate germline biology. Our study sought to address this gap by generating a high‑resolution map of human uSPG heterogeneity and identifying the signaling pathways that regulate their stemness.

Using single‑cell transcriptomics combined with functional assays, we uncovered that human uSPG are far more diverse than previously appreciated. Rather than a single homogeneous “stem cell” pool, uSPG exist along a continuum of transcriptional states, each enriched for distinct regulators of self‑renewal, quiescence, or lineage priming. This framework provides a more accurate representation of how the human germline maintains both stability and flexibility across the lifespan.

One of the most striking findings was the central role of RAS‑dependent signaling in maintaining the primitive, stem‑like compartment of uSPG. While RAS pathways have been implicated in stem cell biology in other tissues, their specific contribution to human spermatogonial fate had not been clearly defined. Our data show that RAS activity is selectively enriched in the most undifferentiated uSPG subset and that perturbing this pathway shifts cells toward differentiation. These results suggest that RAS signaling acts as a molecular gatekeeper of human SSC potential.

This work also has practical implications for fertility preservation and regenerative medicine. As more young cancer patients survive gonadotoxic therapies, there is growing interest in developing clinical protocols to expand human spermatogonial stem cells ex vivo. Our findings provide a mechanistic foundation for optimizing culture conditions by identifying pathways that sustain stemness versus those that promote differentiation. Understanding these regulatory circuits is essential for designing safe and effective SSC‑based therapies.

Finally, this study underscores the importance of studying human germline biology directly rather than relying solely on rodent models. Several pathways that are central to human uSPG identity—including the RAS‑dependent program we describe—are not equivalently represented in mice. As the field moves toward translational applications, species‑specific insights will be critical.

We hope that this work contributes to a more nuanced understanding of human spermatogonial biology and provides a roadmap for future efforts to manipulate SSC fate for therapeutic benefit.

Written by: Kun Tan, Assistant Professor, Obstetrics, Gynecology, & Reproductive Sciences, University of California, San Diego

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