OVULATION AND EARLY EMBRYOGENESIS IN SWINE

Thirty gilts were used to examine if the sequence in which oocytes were released at ovulation contributed to differences in embryonic development and uterine secretions by Day 12 (Day 0 = onset of estrus). Oocytes of follicles destined to ovulate last were recovered 42 h after injecting proestrous gilts with hCG, incubated with a fluorescent stain, and returned to the donor's oviduct. These later-maturing oocytes subsequently became the lesser-developed (p < 0.01) embryos on Day 4. In a second experiment, lesser- vs. more-developed Day 4 embryos from additional gilts were transferred to ligated uterine horns of non-pregnant gilts. Subsequently, the lesser-developed Day 4 embryos became the smaller (p < 0.01) blastocysts within a litter on Day 12. Uterine flushings associated with lesser-developed embryos on Day 12 contained less estradiol (p < 0.01), less total protein (p < 0.10), and less acid phosphatase activity (p < 0.05), but total content of calcium was not different compared to flushings that contained more-developed embryos. Analysis of uterine flushings with two-dimensional PAGE procedures indicated advanced uteroferrin-associated glycoprotein secretion from the horn that contained more-developed embryos. Results of these experiments suggested that oocytes of later-ovulating follicles were progenitors of smaller embryos, which probably stimulated uterine secretion later than more advanced littermates on Day 12.