In association with in vitro culture and transplantation, isolation of spermatogonial stem cells (SSCs) is an
excellent approach for investigating spermatogonial physiology in vertebrates. However, in fish, the lack
of SSC molecular markers represents a great limitation to identify/purify these cells, rendering it difficult
to apply several valuable biotechnologies in fish-farming. Herein, we describe potential molecular markers,
which served to phenotypically characterize, cultivate and transplant Nile tilapia SSCs. Immunolocalization
revealed that Gfra1 is expressed exclusively in single type A undifferentiated spermatogonia
(Aund, presumptive SSCs). Likewise, the expression of Nanos2 protein was observed in Aund cells. However,
Nanos2-positive spermatogonia have also been identified in cysts with two to eight germ cells that
encompass type A differentiated spermatogonia (Adiff). Moreover, we also established effective primary
culture conditions that allowed the Nile tilapia spermatogonia to expand their population for at least
one month while conserving their original undifferentiated (stemness) characteristics. The maintenance
of Aund spermatogonial phenotype was demonstrated by the expression of early germ cell specific markers
and, more convincingly, by their ability to colonize and develop in the busulfan-treated adult Nile tilapia
recipient testes after germ cell transplantation. In addition to advancing our knowledge on the
identity and physiology of fish SSCs, these findings provide the first step in establishing a system that will
allow fish SSCs expansion in vitro, representing an important progress towards the development of new
biotechnologies in aquaculture, including the possibility of producing transgenic fish.