Timing of Induced Ovulation in C.B-17/Icr-scid/scid and B6SJLF₁ Mice

The time of induced ovulation after superovulation was evaluated in 3- to 8-week-old C.B-17/Icr-scid/scid and B6SJLF₁ mice. Mice received intraperitoneal injections of 5 IU of equine chorionic gonadotropin (CG) followed by 5 IU of human (h) CG 48 h apart. Hormones were administered at 1700. Mice were kept on a 14-h light cycle, with lights off at 1900. Nine mice of each strain were sacrificed (3 mice x 3 replicates) at 8, 10, 12, 14, and 16 h after hCG injection. Ovaries and oviducts from each mouse were removed and placed in M-2 collection medium in one well of a 9-well Pyrex culture plate. The oviducts were flushed with M-2 medium, and the number of oocytes from each mouse was counted under a binocular Nikon dissecting microscope. Both mouse strains began ovulating at 8 h after hCG administration. Ovum counts peaked at 12 h in B6SJLF₁ mice and at 16 h in C.B-17/Icr-scid/scid mice. A statistically significant strain difference between mean oocytes collected was detected at 10 and 12 h. For both time points, differences were at the 1% level of significance. There was no difference in the mean number of oocytes collected at 14 and 16 h. The mean number of ovulated oocytes increased markedly between 10 to 12 h after hCG in B6SJLF₁ mice. The number of ovulated oocytes also increased in C.B-17/Icr- scid/scid mice, although more gradually. These results suggest that the timing of ovulation may be more tightly controlled in superovulated B6SJLF₁ than in C.B-17/Icr-scid/scid mice. Superovulated C.B-17/Icr-scid/scid mice finish ovulation 2 to 4 h later than do B6SJLF₁ mice. Therefore the best time to harvest pronuclear embryos should be 14 to 16 h after hCG administration in C.B-17/Icr-scid/scid mice and 12 h after hCG in B6SJLF₁ mice. To optimize fertilization, mating should occur within 3 h of ovulation and should be planned from 11 to 17 h after hCG for C.B-17/Icr-scid/scid mice and 9 to 15 h after hCG in B6SJLF₁ mice. Because male and female pronuclei form 4 to 7 and 6 to 9 h respectively after insemination, the most pronuclear stage embryos should be obtained by collecting them 20 to 21 h after hCG in C.B-17/Icr-scid/scid mice and 18 to 19 h in B6SJLF₁ mice.