Spermatogonial stem cells (SSCs) enable continuous sperm production for almost the entire life of a male. Spermatogonial transplantation (SGT) was a revolutionary technique in the study of male germ-cell biology. Semen cryopreservation of young male cancer patients is becoming common in clinical practice. SSCs can be obtained from small fragmental testis tissues of pediatric cancer patients, taken by a biopsy procedure. The SSCs could be cryopreserved for later use, if patients survive the disease and grow to reproductive age. Most importantly, in vitro spermatogenesis circumvents the invasive procedure of cell transplantation, let alone the possible reintroduction of malignant cells to the patient when SSCs are cryopreserved. In vitro human spermatogenesis, just after the development of mouse in vitro spermatogenesis, might be a focus for research interest. It may not be easy to achieve. In particular, the duration of spermatogenesis is longer in humans (64 days) than in mice (35 days).