Research articleGrowth and fertilization of porcine fetal oocytes grafted under the renal capsules of nude mice

The fetal ovary contains a larger pool of oocytes than the adult ovary, but utilization of the fetal oocytes of large animals has hardly been examined. In this study, we investigated the developmental competence of oocytes grown in host mice harboring ovarian grafts obtained from fetal pigs. Ovarian fragments from fetuses at 55, 70, and 90 days postartificial insemination (dpi) were grafted into ovariectomized nude mice (Crlj:CD1-Foxn1nu; 55-, 70- and 90-dpi groups, respectively). For comparison, ovarian fragments from 20-day postpartum (dpp) piglets were also grafted (20-dpp group). About 60 days after detection of vaginal opening, the mice were given 62.5 U/mL porcine FSH for 13 days by infusion to enhance their follicular development. In the fetal ovaries before grafting, the percentage of germ cells in primordial follicles (termed primordial oocytes) relative to the total number of germ cells was 0.06% at 55 dpi, 2.4% at 70 dpi, and 7.2% at 90 dpi, but the majority was contained within egg nests. At 20 dpp, primordial oocytes accounted for 91.7% of the total number of germ cells and the rest were mostly in primary follicles. After FSH stimulation of host mice, formation of antral follicles was promoted in the grafts of the 70- and 90-dpi groups as well as the 20-dpp group, but a very small number of antral follicles developed in the 55-dpi group consistent with the lowest (P < 0.05) levels of circulating inhibin in that group. The mean number of full-sized oocytes with meiotic competence recovered per mouse was 6.0 in the 70-dpi, 18.0 in the 90-dpi, and 21.2 in the 20-dpp groups, whereas virtually no oocytes were recovered from mice in the 55-dpi group. Moreover, the mature oocytes in the 70- and 90-dpi groups were fertilized in vitro, as shown by formation of male and female pronuclei, but the percentage of oocytes penetrated by sperm was low in the 70- (49%) and 90-dpi (29%) groups as compared with the 20-dpp group (88%). These results clearly indicate that porcine fetal ovaries during the late pregnancy period are able to grow and produce oocytes with fertilization ability after being grafted into nude mice. The presence of primordial follicles is likely to be associated with the ability of fetal ovaries to produce antral follicles after xenografting.