225 GROWTH OF OOCYTES IN PIG PRIMORDIAL FOLLICLES XENOTRANSPLANTED INTO SCID MICE

Abstract

The mammalian ovary is endowed with a large number of primordial follicles that contain small oocytes. A limited number of these oocytes initiate growth, whereas others are either degenerate or remain as completely resting oocytes throughout the reproductive life of the female. The mechanism for the initiation of oocyte growth is not understood well. Small oocytes in primordial follicles of newborn rodents start to grow in the cultured ovary or ovarian tissue. For domestic animals, however, culture systems for mouse oocytes have not been valid. Xenotransplantation of ovarian tissues to immunodeficient mice can be a substitute for an effective culture system for small oocytes. Indeed, recent reports reveal the growth of small oocytes of fetal and newborn animals in xenografts (Hosoe et al., 2001;; Kaneko et al., 2003 Biol. Reprod. 53, 931–939). This experiment was conducted to study the growth of oocytes in primordial follicles of adult pig (6-month-old) in comparison to those of newborn pigs (10-day-old) in xenografts. The effect of the sex of host mice on oocyte growth in xenografts was also examined. Cortical slices containing only primordial follicles were collected under a dissection microscope from the ovaries of 6-month-old gilts (n = 8) and 10-day-old piglets (n = 6). Size of the slices was about 2 mm × 1 mm × 0.5 mm. Each slice was cut into 2 pieces;; one was fixed for histological examination and the other was transplanted. For transplantation, 6- to 8-week-old male and female SCID (severe combined immune deficiency) mice were anesthetized, their left kidneys were exteriorized, and cortical slices were inserted under the kidney capsules. After 2 months, the grafts were recovered and processed for histological examination. Histological examination confirmed that the cortical slices contained only primordial follicles before transplantation. After transplantation, 47% (171/364) of the primordial follicles of adult pig ovaries survived in the xenografts but none of those developed into primary follicles or beyond. The mean diameter of the oocytes after transplantation was 32.1 ± 0.7 μm (n = 171) which was similar to that of the oocytes before transplantation (30.7 ± 0.8 μm, n = 364). On the other hand, in the xenografts of newborn pig ovaries, 13.5, 9.7 and 0.3% of 1122 follicles developed to the primary, secondary and antral stages, respectively, in male SCID mice. In the female SCID mice, there were no antral follicles but the distribution of primordial, primary and secondary follicles was 84.5, 9.4 and 6.1%, respectively, among a total of 1094 follicles. The mean diameter of the secondary follicles in the xenografts in the male SCID mice was 263.3 ± 92.0 μm (n = 109) which was significantly higher than that of the secondary follicles in the female SCID mice (189.3 ± 44.2 μm, n = 98) (P < 0.05, t-test). The results show that primordial follicles of 6-month-old pig ovaries survive but do not develop in xenografts, whereas newborn pig primordial follicles develop to the antral stage. This suggests that the growth property of the oocytes in primordial follicles in adult pig ovaries is different from that in newborn pigs.