Recovery of functional oocytes from cultured premeiotic germ cells after kidney capsule transplantation

The efficiency of in vitro culture systems for a premeiotic female germ cell is still low, mostly because of our incomplete understanding of the mechanisms controlling oogenesis and the obvious difficulties in reproducing the complex in vivo environment of such a process under in vitro conditions. Here we explored the possibility of recovering the developmental potential of mouse oocytes generated in vitro from premeiotic germ cells by transplantation under a kidney capsule of adult animals. To this aim, mouse embryonic ovaries of 12.5 days postcoitum cultured in vitro in a serum-free medium for 7 or 14 days, were transplanted beneath the kidney capsule of immunodeficient mice and analyzed after 21 (7+21 group) or 14 days (14+14 group). Cultured ovaries before transplantation showed delayed oocyte meiotic progression and follicle development. Interestingly, grafted ovaries of both groups, especially those of the 7+21 group, seemed able to restore the reproductive cycle of recipients. While the almost complete absence of primordial follicles was observed in grafted ovaries, oocytes from these ovaries showed transcript levels of genes associated to oocyte maturation similar to control. Moreover, the developmental stage of follicles and oocytes of the 7+21 group ovaries were comparable to that of 21 days post partum in vivo ovaries, whereas significant developmental delay were found in the 14+14 group ovaries. Nevertheless, oocytes retrieved from transplanted ovaries of both groups matured (around 80%) and were fertilized in vitro (around 20%-45%). Two-cell embryos from the fertilized oocytes developed to hatching blastocysts (about 50%) or gave rise to healthy live offspring (from 6% to 10%) when transplanted in a host mother. In conclusion, our results indicate that premeiotic female germ cells cultured in vitro up to primordial/primary follicle stages preserve their capability to complete oogenesis and can be fertilized and generate live pups after transplantation into a suitable in vivo environment.