In vivo embryogenesis, embryo migration, and embryonic mortality in the domestic cat

In vivo embryogenesis, embryo migration, and survival were studied in the domestic cat. Queens were naturally mated (three times daily) on the second and third days of behavioral estrus and, if ovulation occurred, ovariohysterectomized at 64 (n = 8), 76 (n = 11), 100 (n = 8), 124 (n = 7), 148 (n = 6), and 480 h (n = 8) after first copulation. Of 52 cats mated, 48 (92.3%) ovulated (as evidenced by the presence of ovarian CL), and of these, 38 (79.2%) either produced good-quality embryos or had implantation sites. From the remaining cats, only unfertilized oocytes (n = 5), degenerating embryos (n = 4), or no oocytes/ embryos (n = 1) were recovered. Embryos at 64, 76, 100, and 124 h after the first copulation typically were 1 to 4 cells (17 of 20; 85.0%), 5 to 8 cells (18 of 28; 64.3%), 9 to 16 cells (14 of 24; 58.3%), and morulae (15 of 21; 71.4%), respectively; all were within the oviducts. At 148 h, embryos primarily were compact morulae or early blastocysts (15 of 18; 83.3%), and all were within the uterus. For the preimplantation groups, the overall recovery of embryos plus oocytes per CL was 80.6%, and the mean (+/- SEM) numbers of CL and embryos were 64 h, 4.8 +/- 0.3, 3.1 +/- 0.8; 76 h, 4.7 +/- 0.3, 3.9 +/- 0.6; 100 h, 5.8 +/- 0.5, 3.3 +/- 0.8; 124 h, 4.4 +/- 0.5, 4.0 +/- 0.6; and 148 h, 6.5 +/- 1.1, 3.7 +/- 0.7, respectively. Cats in the 480-h group produced a mean of 5.6 +/- 0.5 CL and 3.9 +/- 0.5 implantation sites. In six of eight cats in this group, there was a disparity between CL number on a given ovary and number of implantation sites in the ipsilateral horn, supporting the concept of transuterine embryo migration. In summary, results indicated that 1) more than 90% of cats ovulated following this multiple mating regimen, but approximately 21% of these failed to produce any fertilized or viable embryos; 2) embryo developmental rate in vivo was biphasic, with a rapid cleavage rate to the 5- to 8-cell stage followed by a slower cleavage rate to the morula stage; 3) cat embryos entered the uterus as compact morulae or early blastocysts approximately 5.5 days after the first copulation; and 4) on the basis of implantation/CL ratio, approximately 30% of all ovulated cat oocytes underwent either fertilization failure or preimplantation embryonic mortality.