TIMING OF INDUCED OVULATION IN C.B-17 ICR-SCID/SCID AND B6SJLF(1) MICE/

The time of induced ovulation after superovulation was evaluated in 3- to 8-week-old C.B-17/Icr-scid/scid and B6SJLF(1) mice. Mice received intraperitoneal injections of 5 IU of equine chorionic gonadotropin (C G) followed by 5 IU of human (h) CG 48 h apart. Hormones were administ ered at 1700. Mice were kept on a 14-h light cycle, with lights of fat 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 MB collection medium in on e 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 unde r a binocular Nikon dissecting microscope. Both mouse strains began ov ulating at 8 h after hCG administration Ovum counts peaked at 12 h in B6SJLF(1) mice and at 16 h in C.B-17/Icr-scid/scid mice. A statistical ly significant strain difference between mean oocytes collected was de tected 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 B6SJLF(1) mice. Th e number of ovulated oocytes also increased in C.B-17/Icr-scid/scid mi ce, although more gradually. These results suggest that the timing of ovulation may be more tightly controlled in superovulated B6SJLF(1) th an in C.B-17/Icr-scid/scid mice. Superovulated C.B-17/Icr-scid/scid mi ce finish ovulation 2 to 4 h later than do B6SJLF(1) mice. Therefore t he 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 B6SJLF(1) mice, To optimize fertilization, mating should occur within 3 h of ovulation and should be planned hom 11 to 17 h after hCG for C. B-17/Icr-scid/scid mice and 9 to 15 h after hCG in B6SJLF(1) mice. Bec ause male and female pronuclei form 4 to 7 and 6 to 9 h respectively a fter insemination, the most pronuclear stage embryos should be obtaine d by collecting them 20 to 21 h after hCG in C.B-17/Icr-scid/scid mice and 18 to 19 h in B6SJLF(1) mice.