DETAILED ANALYSIS OF PRONUCLEUS DEVELOPMENT IN BOVINE ZYGOTES IN-VIVO- ULTRASTRUCTURE AND CELL-CYCLE CHRONOLOGY

The ultrastructural development of pronuclei and cytoplasm was studied in bovine zygotes developed in the oviducts. The timing of the morpho logical events was related to sonographically detected ovulation and t o the progress of the cell cycle determined by double labelling (H-3 a nd C-14-thymidine) of newly synthesized DNA combined with autoradiogra phic detection. The onset of the S-phase occurred at 11-12 hr after th e estimated time of ovulation (EO), acid this phase of the cell cycle lasted for 7-9 hr. During the G1-phase, the pronuclei contained sphere s of compact, electron-dense fibrillar material classified as nucleolu s precursor bodies. Earlyin teh S-phase (13 hr after EO) spherical fib rillogranular bodies containing larger rounded electron-dense componen ts were detected in the periphery of the pronuclei as well. At 15 hr, the latter bodies had become connected through electron-dense material with spherical multivacuolated fibrillar bodies of the same electron density as the nucleolus precursor bodies. At 17 hr, similar compact s pherical bodies, now presenting a single large vacuole, were observed on some occasions, while in other zygotes the morphology remained unch anged throughout the rest of the S and G2-phases. (C) 1996 Wiley-Liss, Inc.