253 GROWTH OF PRIMORDIAL OOCYTES FROM INFANT AND ADULT PORCINE OVARIES IN XENOGRAFTS

Abstract

Mammalian ovaries contain a lot of small oocytes in primordial follicles. A small proportion of these primordial oocytes (oocytes in primordial follicles) start to grow and reach full size, and finally they are ovulated. The mechanism regulating the activation of primordial oocytes to enter the growth phase is not understood yet. This experiment was aimed to compare the growth of primordial oocytes from infant and adult pigs. Ovarian cortical tissues containing primordial follicles collected from infant (10–20-day-old) and adult (6-month-old) pigs were xenografted under the kidney capsules of male, female (intact), and ovariectomized SCID (severe combined immune deficiency) mice. Data were subjected to one-way ANOVA, and the significance of difference among means was determined by the Tukey's multiple range test. Differences at P < 0.05 were considered statistically significant. A proportion of the primordial oocytes from the infant pigs grew to the full size at 2 months after xenografting, irrespective of the type of recipient mice. Under the same conditions, the oocytes from adult pigs did not initiate growth, although they grew after 6 months of xenografting. These results suggest that the mechanism regulating the activation of primordial oocytes in adults is different from that in infant pigs. In this regard, the involvement of FOXO3A, a member of Forkhead transcription factors, recently known as a regulator of ovarian follicular development, was studied. Western blot analysis showed stronger expression of FOXO3A in primordial oocytes from adult pigs than in those from infants. Immunolocalization was detected in the nuclei of 42 ± 7% primordial oocytes in infant pigs. In contrast, immunostaining was positive in 94 ± 2% primordial oocytes in adult porcine ovaries. Four months after xenografting ovarian tissues from adult pigs, the number of FOXO3A-positive primordial oocytes significantly declined to a proportion similar to that in the ovaries from infant pigs. These results suggest that primordial oocytes in adult pigs take a much longer time to initiate growth than those in infants and that they might be inactivated by FOXO3A.