Human Spermatogonial Stem Cell Culture <i>In Vitro</i>: A Review of New Materials and Recent Advances.

There is a high prevalence of infertility worldwide (17.5% of the adult population), with male factors comprising 40% of cases. Therefore, research is vital for addressing the challenges faced by infertile men. Innovative methods, diagnosis, and treatment are necessary for male infertility, and stem cell science has brought about significant breakthroughs in its treatment. Spermatogonial stem cells (SSCs) play a crucial role in maintaining reproductive function in males, and advancements in their <i>in vitro</i> culture show promising developments. An array of specific growth factors and compounds that include epidermal growth factor (EGF), leukaemia inhibitory factor (LIF), glial cell line-derived neurotrophic factor (GDNF), basic fibroblast growth factor (bFGF), stem cell factor (SCF), vitamin A/retinol, testosterone, and follicle stimulating hormone (FSH) are applied to natural or artificial surfaces to mimic the testicular niche to preserve SSCs viability and induce their differentiation. These media should maintain self-renewal ability, continuously generate daughter spermatogonia, and differentiate into spermatozoa. However, several limitations and challenges exist despite the significant progress in SSC culture techniques. One limitation is the difficulty in fully replicating the intricate <i>in vivo</i> testicular niche. This complex microenvironment that includes the dynamic interplay of somatic cells, hormones, growth factors, and external matrix components is crucial for supporting SSC self-renewal and differentiation. Currently, <i>in vitro</i> systems do not adequately replicate this microenvironment, resulting in suboptimal SSC maintenance and inconsistent differentiation outcomes. This might be a key challenge for achieving complete and functional spermatogenesis in vitro. We review the foundational concepts and recent breakthroughs in in vitro spermatogenesis, focusing particularly on studies in human systems from the past twenty years. As a conclusion from this review article, developing a defined culture system that accurately mimics the <i>in vivo</i> niche is crucial for maintaining the functional properties of spermatogonial stem cells. The establishment of such a reliable platform is an essential prerequisite for translating <i>in vitro</i> spermatogenesis into clinical applications for treating infertility.