Germ cell transplantation in tilapia (Oreochromis niloticus)
Spermatogonial transplantation is a fascinating and powerful technique developed in 1994 by Brinster and colleagues. This technique has been utilized to investigate reproductive biology, mainly the aspects related to spermatogenesis and stem cell biology, offering also great potential for studies involving biotechnology, transgenic animals, and the preservation of genetic stocks of valuable animals or endangered species. Although germ cell transplantation is well characterized for mammals such as mice and rats, there is no study utilizing this approach for fish or other lower vertebrates. In view of the several advantageous characteristics offered by piscine research models, we have evaluated the suitability of adult tilapias as a recipient for syngenic or xenogenic germ cell transplantation, and all necessary approaches for spermatogonial transplantation were standardized recently in our laboratory. The results found for the tilapia-to-tilapia transplants, evaluated several weeks post-transplantation, showed the presence of PKH26 labeled (i.e. donor-derived) germ cells forming spermatogenic cysts at different sizes and stages of spermatogenic development. Regarding xenogenic transplantation, ongoing studies in our laboratory are also showing that frog (Rana catesbeiana) and tucunaré fish (Cichla monoculus) germ cells are able to colonize, and form spermatogenic cysts, in the tilapia testes. Since colonization-competent cells are usually considered to be stem cells, these findings indicate that successfully transplanted spermatogonial stem cells were able to colonize, proliferate, and eventually differentiate in the tilapia testes. Therefore, this species may represent a good experimental model to investigate the germ cell biology and the testis function in fish. Moreover, this technique could be also utilized as a potential approach for fish bioengineering, preservation of genetic stocks of endangered fish species or fish strains carrying commercially valuable traits. Regarding the frog-to-tilapia transplantation, the preliminary results found suggest that tilapia might also represent a potential model to investigate stem cell biology and spermatogenesis in lower vertebrates.