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 morphological events was related to sonographically detected ovulation and to the progress of the cell cycle determined by double labelling (3H and 14C-thymidine) of newly synthesized DNA combined with autoradiographic detection. The onset of the S-phase occurred at 11-12 hr after the estimated time of ovulation (EO), and this phase of the cell cycle lasted for 7-9 hr. During the G1-phase, the pronuclei contained spheres of compact, electron-dense fibrillar material classified as nucleolus precursor bodies. Early in the S-phase (13 hr after EO) spherical fibrillogranular bodies containing larger rounded electron-dense components 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 spherical bodies, now presenting a single large vacuole, were observed on some occasions, while in other zygotes the morphology remained unchanged throughout the rest of the S and G2-phases.