We previously reported that xenografted bovine secondary follicles developed to the antral stage in severe combined immunodeficient (SCID) mice. In the present study, bovine secondary follicles 100–240 μm in diameter were xenografted under the kidney capsules of female SCID mice for 6 and 8 weeks, and we examined the oocytes' fertilization and developmental abilities. Bovine follicles developed with prolongation of grafting and became significantly larger than those before grafting. Injection of equine chorionic gonadotropin (eCG) into host mice made some surviving follicles develop larger than the other follicles. Furthermore, bovine oocytes grew in the follicles, and the mean diameter of the oocytes was 100 μm or more at 6 and 8 weeks of transplantation. Bovine oocytes that had grown in eCG-stimulated SCID mice 8 weeks after grafting were subjected to maturation culture. Some of the oocytes that had grown to 110 μm or more matured to the second metaphase (7% of oocytes 110–119 μm and 44% of those >120 μm). When the oocytes were inseminated with bovine spermatozoa, 15% (6/39) formed a female and a male pronucleus, and 2 days after insemination 24% (18/75) of oocytes cleaved and 2% (2/75) developed to the 5- to 8-cell stage. However, no embryo reached the blastocyst stage. These results indicate that bovine oocytes grown in SCID mice could be fertilized but acquired insufficient competence for embryonic development in the present conditions.

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.