Insights into in vitro spermatogenesis in mammals: Past, present, future

Considering the self-renewal and differentiation ability of pluripotent stem cells, some studies have pointed out the possibility of stem cell-derived sperm production. Most studies that test this hypothesis have been conducted on rodents, with some promising results; however, studies on humans are progressing slowly, and have encountered technical and ethical hurdles. Established methods to differentiate stem cells-including embryoid bodies, co-culturing, and various feeder cells-may provide a niche that is similar to in vivo conditions and resolve epigenetic abnormalities, but a gonadal-like three-dimensional structure is still required to produce germ cells with the correct imprinting. In the last few years, sperm-like cells with fertilizing capacity were produced from mouse embryonic stem cells, and the resulting embryos from these cells yielded live offspring. Future research should move towards the use of adult stem cells, however, owing to the unavailability of embryonic cells in adults. More intensive research and techniques are required since in vitro spermatogenesis provides hope to individuals without mature sperm who cannot be treated, and may be a useful system to study the precise mechanism of spermatogenesis. In this review, we describe recent studies of in vitro spermatogenesis mechanisms and related techniques in mammals. We also discuss the possible cell surface markers and culture conditions that might improve in vitro spermatogenesis.