Antiprogesterone Reversal of Progesterone-Induced Rapid Transport of Ova in Superovulated Cattle

The ovaries of estrus synchronized and superovulated heifers were examined by transrectal ultrasonography every 4 h after the onset of estrus. After observation of the first ovulations (first ovulation scan), scanning was performed at 2-h intervals. Heifers were slaughtered at 6, 8, 10, 12, 14, 16 or 18 h after the first ovulation scan, and ova flushed from the oviducts were incubated with 3H-thymidine for 30 min, transferred to nonradioactive medium for 2 h, and finally incubated for 30 min with 14C-thymidine. Subsequently, the ova were either fixed, Eponembedded, sectioned into semi-thin sections, and prepared for autoradiography or used for aceto:orcein evaluation of pronucleus development. At the first ovulation scan 6.7±0.9 ovulations were recorded as opposed to 1.9±0.6 and 0.8±0.4 at the following 2 scannings. Consequently, the estimated time of ovulation was defined as 2 h before the first ovulation scan. At 8 h after the estimated time of ovulation equal amounts of sperm head stages and migrating pronucleus stages were found; while at 10 h, migrating pronuclei were predominant. At 12 h, the first apposed pronuclei were observed, and this developmental stage was predominant from 14 h up to the end of the experiment. The first cleavage was observed at 20 h after the estimated time of ovulation. The first incorporation of radioactive precursors was observed at 12 h after the estimated time of ovulation and the last at 19 h. Thus, the duration of the S-phase of the first cell cycle in vivo was estimated to be 7 to 9 h.