INFLUENCE OF DAY OF ESTRUS ON EGG VIABILITY AND COMPARATIVE EFFICIENCY OF INVITRO FERTILIZATION IN DOMESTIC CATS IN NATURAL OR GONADOTROPIN-INDUCED ESTRUS

Thirty-six domestic cats received 100 iu hCG (i.m.) on day 1, 2 or 3 o f a natural, behavioural oestrus. Twenty-two anoestrous cats were inje cted with 150 iu pregnant mares' serum gonadotrophin (PMSG; i.m.) foll owed 84 h later by 100 iu hCG. Twenty-four to 26 h after hCG, all cats were examined laparoscopically to determine the number of ovarian fol licles and to recover follicular eggs. Mature eggs were cultured with conspecific spermatozoa and examined 30 h later for cleavage. Within t he natural oestrus group, cats on day 1 produced fewer (P < 0.0.5) fol licles and total eggs than females on day 2 or 3, and 88.9% of the day 1 eggs were degenerate or immature and unsuitable for in vitro fertil ization (IVF). Although only 54.5% of the cats in the PMSG/hCG group e xhibited overt oestrus, mean (+/-SEM) numbers of follicles (9.7 +/- 0. 8) and oocytes recovered (8.7 +/- 0.8) were at least twofold greater ( P < 0.001) than those measured in the natural oestrus group (3.7 +/- 0 .6; 3.4 +/- 0.6, respectively) or subgroups on day 2 (3.7 +/- 0.4; 3.3 +/- 0.4) and day 3 (5.7 +/- 0.8; 5.3 +/- 0.8). Overall, the proportio n of eggs cleaving in vitro was similar (P > 0.05) between the natural oestrus group (48.3%) and the PMSG/hCG group (50.9%), but the latter group produced more than twice the number of embryos per donor. Embryo quality was unaffected (P > 0.05) by day of hormone treatment, and mo re than 80% of all two-cell embryos were rated good-to-excellent quali ty. In summary, there is a temporal relationship between day of sexual receptivity and follicular egg viability in the domestic cat: eggs on the first day of oestrus are not optimally responsive to an LH-like s timulus. There is also no evidence that PMSG/hCG treatment compromises egg quality or subsequent fertilizability in vitro. On the contrary, use of these gonadotrophins markedly improves overall IVF efficiency b y increasing the total number of high quality embryos produced.