The road to restore male fertility using in vitro -derived germ cells

BACKGROUND: Human-induced pluripotent stem cells (hiPSCs) offer immense potential in reproductive medicine, particularly for males who lack germ cells and cannot achieve biological parenthood through conventional ARTs. Early efforts to derive human germ cells from stem cells were hindered by low efficiency, subpar characterization, and the lack of standardized differentiation approaches. However, recent advancements have led to the development of defined protocols that mimic early embryonic development and allow the specification of transcriptomically and epigenetically validated human primordial germ cell-like cells (hPGCLCs). Current research focuses on maturing hPGCLCs in vitro, particularly within 3D culture systems that resemble their physiological microenvironment, with the aim of producing transplantable hiPSC-derived spermatogonial stem cells (SSCs) or differentiating them to sperm. At the same time, researchers are also testing whether hiPSCs generated from infertile patients can resume germline differentiation. OBJECTIVE AND RATIONALE: This narrative review aimed to summarize the key efforts and remaining challenges in differentiating male germ cells from human pluripotent stem cells (hPSCs), with a particular focus on defined and validated protocols for hPGCLC specification. In parallel, we addressed key safety and ethical considerations that must be accounted for the development of clinical applications. A deeper understanding of the approaching therapeutic use of hiPSCs in reproductive medicine is essential for developing novel regenerative fertility strategies. SEARCH METHODS: PubMed, Scopus, and Web of Science were searched for studies attempting germ cell differentiation from hPSCs using relevant keywords ('stem cells', 'human pluripotent stem cells', 'human embryonic stem cells', 'human induced pluripotent stem cells', 'somatic cell reprogramming', 'infertility', 'germline', 'spermatogenesis', 'germ cells', and 'primordial germ cells'). No time period restriction was established. Studies with an exclusive focus on female germline differentiation were excluded. To maintain a human-focused perspective, only key animal studies are presented. OUTCOMES: The literature reveals a clear segregation among protocols for deriving germ cells from hPSCs, particularly between earlier studies lacking standardized differentiation conditions and characterization, and the most recent, defined protocols having transcriptomic and epigenetic validation against in vivo hPGCs. Moreover, during the last decade, the field has seen remarkable progress, with multiple efforts aimed at maturing hPGCLCs, closely recapitulating late male embryonic germline development. Additionally, hiPSCs derived from male patients at risk of infertility, particularly those without underlying genetic syndromes, generally retain the capacity for early germline commitment. While attempts at maturating patient germ cells beyond the hPGCLC state remain limited, the rapid pace of discovery and refinement in recent years suggests that further breakthroughs, including clinically applicable fertility restoration strategies, are likely to be achieved in the near future. WIDER IMPLICATIONS: The ability to generate hiPSCs from infertile patients and to specify them into hPGCLCs supports the feasibility of obtaining hiPSC-derived SSCs for future therapeutic use. These advances raise important ethical, regulatory, and societal questions that must be actively discussed among researchers, clinicians, policymakers, and the general public to ensure responsible and equitable access to these technologies. REGISTRATION NUMBER: N/A.