Male infertility is attributed to a wide range of causes, including varicocele, undescended testicles, testicular cancer, and azoospermia. Stem cell therapeutic strategies for males focus on restoring spermatogenesis, particularly in cases of non-obstructive azoospermia (NOA), where sperm production is poor or absent. Spermatogonial stem cells (SSCs) are of particular interest, as they are the only cells in the testis that have the capacity to self-renew and continuously produce sperm. Studies using mesenchymal stem cells (MSCs) from various sources, such as bone marrow, adipose tissue, placenta, and amniotic fluid, have shown promising results in animal models. These cells are characterized by their ability to differentiate into germ cell-like cells, secrete growth factors that support the testicular environment, and modulate immune responses, contributing to tissue regeneration and restoration of function. For example, MSCs can differentiate into germ cell-like cells under specific stimulatory conditions in vitro and exhibit immunomodulatory and anti-inflammatory properties.

Female infertility is complex and results from various disorders affecting the ovaries, uterus, and fallopian tubes. The most prominent of these disorders are premature ovarian failure (POF), Asherman's syndrome, polycystic ovary syndrome (PCOS), and endometriosis. Stem cells, particularly mesenchymal stem cells (MSCs), show significant therapeutic potential in improving ovarian and uterine function. MSCs can contribute to the regeneration of damaged ovarian tissue, promote follicle formation, stimulate hormone production, modulate inflammatory responses, and improve the ovarian environment. Research suggests that MSCs can secrete trophic and immunomodulatory factors that contribute to the restoration of reproductive system function. For example, studies have shown that MSC transplantation can promote ovarian regeneration in animal models of ovarian failure, leading to partial restoration of fertility. In addition to MSCs, amniotic fluid-derived stem cells (AFSCs) are being studied as a promising alternative to embryonic stem cells, as they are characterized by their differentiation capacity and accessibility with fewer ethical risks. Studies indicate that AFSCs can be used in regenerative medicine to treat female infertility, through multiple mechanisms such as modulating immune and inflammatory responses, secreting growth factors, and improving the environment of damaged tissues.

Despite promising results in preclinical and early clinical studies, significant challenges remain to be overcome before stem cell therapy becomes a routine treatment for infertility. Safety and Efficacy: Future clinical studies require careful evaluation of the long-term safety and efficacy of stem cell therapy. Optimal doses, routes of administration, and the most appropriate stem cell sources for each case must be determined. Directed Differentiation and Cell Survival: Ensuring that stem cells properly differentiate into the desired germ cells and survive in the target environment after transplantation is a significant challenge. Membrane damage and apoptosis can result in the loss of a significant portion of transplanted cells. Research suggests strategies to improve cell survival, such as protecting them from reactive oxygen species (ROS) and mechanical damage using hydrogen-bonding organic frameworks, and stimulating differentiation using peptides and melatonin. Ethical and Legal Issues: The use of embryonic stem cells raises ethical and legal concerns, driving research toward alternative sources of adult stem cells and induced pluripotent stem cells (iPSCs) that avoid these issues. iPSCs offer the potential to create patient-specific cellular models, enabling the study of disease mechanisms and the development of personalized therapies. Technical and Production Challenges: Producing sufficient quantities of pure, functional stem cells for clinical use is a technical challenge. The difficulty of accurately localizing cells to target tissues after transplantation also requires the development of advanced imaging and tracking techniques. Research suggests that 3D culture can enhance stem cell differentiation and increase their yield compared to 2D cultures.

Clinical comparisons between stem cell therapy and conventional infertility treatments are crucial to determining the place of stem cells in clinical practice. Currently, most studies focus on evaluating the efficacy of stem cells as an additional or alternative treatment in cases resistant to current treatments. Advances in stem cell engineering, including genetic modification and the development of scaffolds and organoids, are expected to enhance treatment efficacy. These techniques can improve cell survival, direct cell differentiation, and promote cell integration into target tissues. Although stem cell therapy is still in its research and early clinical trials stages, growing evidence suggests its great potential to offer innovative solutions for couples struggling with infertility. Successfully translating this potential into safe, effective, and accessible clinical treatments requires further research, particularly in large-scale clinical studies that directly compare the efficacy of stem cells with conventional treatments, while addressing current technical and ethical challenges.