Ultrastructural Observations of Boar Spermatozoa Penetrating Zona-Free Hamster Eggs

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Abstract
Boar spermatozoa preincubated in a modified KRB medium never penetrated zona-free hamster eggs, and no structural changes were observed in their acrosomes. The acrosome reaction was observed at the acrosomal cap region of spermatozoa preincubated for 5 h in the isolated uterus from a maturing gilt. Only acrosome-reacted spermatozoa fused with zona-free hamster eggs. Sperm-egg membrane fusion was essentially the same as that of homologous fertilization in other species; only the microvilli of the egg surrounding the postacrosomal region fused with sperm plasma membrane. The ultrastructural features of the sperm pronucleus in the hamster egg were dissimilar to those in the pig egg, indicating that some vitelline factors might be involved in the formation of the sperm pronucleus.