Sperm-egg interactions in the pig: monospermy, extensive polyspermy, and the formation of chromatin aggregates

The process of incorporation and metamorphosis of the sperm head within the vitellus has been examined by phase-contrast microscopy in a large series of pig eggs exhibiting either normal monospermic fertilization or extensive polyspermy. This latter condition was induced in vivo after increasing the numbers of capacitated spermatozoa in the Fallopian tubes by pre-ovulatory surgical insemination. Attention was drawn in monospermic eggs to the initial fusion between the spermatozoon and vitelline surface which produced a characteristic constriction in the equatorial region of the head. Immediately following cytoplasmic contact with the sperm nucleus, an increase in size was detectable in this structure, remnants of the inner acrosomal membrane having apparently been displaced during incorporation. In fixed preparations of activated eggs, there was some evidence that the morphologically transformed sperm nucleus underwent a brief period of shrinkage before commencing pronuclear formation. The most striking feature of the polyspermic condition was the number of spermatozoa that had entered the vitellus (2 to greater than 80), and the formation of aggregates of sperm head chromatin in eggs penetrated by more than 20-30 spermatozoa; the heads of at least 8 or 9 spermatozoa could participate in the formation of such an aggregate. Various unusual configurations were noted during breakdown of the midpiece in polyspermic eggs, and degeneration was also a general feature of the cytoplasm in situations of excessive polyspermy. Aspects of the block to polyspermy are discussed, and it is inferred that highly polyspermic pig eggs can still exhibit a zona reaction.