Ultrastructural aspects of in vitro fertilization in sheep.

Fertilization events of the sheep oocyte have been analyzed by electron microscopy. In the fertilization cone, near the decondensing sperm nucleus, several areas composed of electron-dense filamentous material (FA) were observed. Moreover aster-like structures were present in the vicinity of the sperm neck and proximal midpiece. During swelling of the pronuclei, redistribution of cytoplasmic organelles occured near the male pronucleus in a region facing the center of the egg. In this region, containing the sperm midpiece, large aggregates of Golgi complexes and cisternae of endoplasmic reticulum were associated with FA, microtubule foci, and abundant microtubules, suggesting cytaster formation. This complex migrated with the male pronucleus and the proximal sperm midpiece toward the center of the egg. A potential role of these subcellular structures, in the process of pronuclear migration, is proposed.