Bovine oocytes in secondary follicles grow and acquire meiotic competence in severe combined immunodeficient mice

Cortical tissues containing only primordial and primary follicles, or secondary follicles 140-190 μm in diameter, were collected from bovine ovaries and xenografted under the kidney capsules of female severe combined immunodeficient (SCID) mice. Histological examination revealed that all grafts were well vascularised and contained surviving follicles at 4 or 6 weeks after grafting. Primordial and primary follicles survived but did not develop beyond the one-layer stage. Secondary follicles, on the other hand, had formed antra at 4 weeks after grafting. The mean diameter of secondary follicles, which was 165.2 ± 17.0 μm (n = 42) before grafting, had developed to 442.9 ± 77.9 μm (n = 37) and 592.9 ± 116.0 μm (n = 45) in diameter at 4 and 6 weeks after grafting, respectively. The mean diameter of oocytes, which was 55.1 ± 4.9 μm (n = 42) before grafting, also increased significantly (4 weeks: 105.6 ± 6.3 μm; 6 weeks: 122.2 ± 2.6 μm; p < 0.05). Oocytes were recovered from follicles that had developed to more than 400 μm in diameter after 6 weeks, and were subjected to subsequent mature culture. Of these oocytes, 34% (11/32) resumed meiosis and 6% (2/32) matured to the second metaphase. Follicular fluid in bovine antral follicles developed in SCID mice had the 69 kDa protein, which was detected by anti-mouse albumin antibody but not by anti-bovine albumin antibody in immunoblotting analysis. These results demonstrated that bovine secondary follicles develop to the antral stage in SCID mice, and that the oocytes in the follicles acquire the meiotic competence.