[Conception rate and embryo development in guinea pigs with synchronized estrus induced by progesterone implant]

Observations were made on the timing of mating and the pre-implantation development of fertilized eggs in guinea pigs synchronized by long-term progesterone treatment. Females received a subcutaneous implant of progesterone-filled silastic tubing for 14 days. Copulation was observed from the evening of day 4 to the morning of day 6 in 53 of 54 females (98%). Most of them (47/53, 89%) copulated on day 5 after removal of the tubing. Designating the day of copulation (day 5 after removal of the tubing) as day 0 of gestation, embryos collected from the genital tract were at the 4-cell, 8-cell, morula, and blastocyst stages on days 1, 3, 4 and 5 of gestation, respectively. Eggs were recovered at high incidence (85-100%) from days 1 to 5 of gestation. On day 6 gestation, no eggs were recovered from the genital tract, suggesting that implantation had occurred. The mean litter size (+/- S. D.) was 4.0 +/- 0.8 pups, which were born normally after a mean gestation period of 67 +/- 1 days in 7 synchronized females. Since the female guinea pigs synchronized by the long-term progesterone treatment had normal reproductive ability similar to that of cyclic females, this technique would make it possible to obtain animals at a scheduled time even in smaller-sized colonies. In addition, observations on the pre-implantation development of embryos in females with synchronized estrus might be a useful aid in the field of reproductive research.