PRONUCLEUS DEVELOPMENT AND DNA-SYNTHESIS IN BOVINE ZYGOTES IN-VIVO

The ovaries of estrus synchronized and superovulated heifers were exam ined by transrectal ultrasonography every 4 h after the onset of estru s. After observation of the first ovulations (first ovulation scan), s canning 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 flu shed from the oviducts were incubated with H-3-thymidine for 30 min, t ransferred to nonradioactive medium for 2 h, and finally incubated for 30 min with C-14-thymidine. Subsequently, the ova were either fixed, Epon-embedded, sectioned into semi-thin sections, and prepared for aut oradiography or used for aceto:orcein evaluation of pronucleus develop ment. At the first ovulation scan 6.7+/-0.9 ovulations were recorded a s opposed to 1.9+/-0.6 and 0.8+/-0.4 at the following 2 scannings. Con sequently, the estimated time of ovulation was defined as 2 h before t he first ovulation scan. At 8 h after the estimated time of ovulation equal amounts of sperm head stages and migrating pronucleus stages wer e found; while at 10 h, migrating pronuclei were predominant. At 12 h, the first apposed pronuclei were observed, and this developmental sta ge was predominant from 14 h up to the end of the experiment. The firs t 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, th e duration of the S-phase of the first cell cycle in vivo was estimate d to be 7 to 9 h.