**4.3** *In vitro* **maturation of germ cells**

In theory, cryopreserved testicular tissues can also be used for *in vitro* induction of differentiated germ cells and ideally production of sperm or spermatids to be used for ICSI. If successful, this approach can circumvent the potential risk of reintroducing cancer cells into post-recovery patients. Many labs have experimented with the idea, and some have had success with maturation of later stages of human spermatogenesis (but not from SSCs), including live births (Tesarki et al., 1999). Availability of a culture system to support complete *in vitro* spermatogenesis from the SSC stage was, however, elusive until very recently when it was reported that all spermatogenic lineage cells including fertilizationcompetent sperm could be produced from neonatal mouse testes maintained exclusively in a culture system (Shinohara et al., 2011). This is a very promising step, indicating that similar results may be achievable in future using immature human testis biopsies.
