The Ultrastructure of the Preovulatory Human Egg Fertilized In Vitro*

Preovulatory eggs in cumulus were inseminated in vitro with washed spermatozoa which had been preincubated for 1.5 hours. After 3 hours, three eggs were processed for electron microscopy and each was sectioned serially from pole to pole. In the two eggs which had been fertilized, the expanded chromatin of the fertilizing sperm head and the chromatin of the ovum were almost completely surrounded by a developing pronuclear envelope. In one of the penetrated eggs the developing male pronucleus and associated midpiece and sperm tail were located within an incorporation cone. The surface of the cone was free of microvilli but contained a zone of microfilaments immediately beneath the plasma membrane. A similar zone of microfilaments was present beneath the advancing surface of the extruding second polar body (PB2) which was connected to the ovum by an interbody and microtubules of the meiotic spindle. Cortical granules were completely absent from the fertilized eggs but were present in the unfertilized egg. PB2 contained a nucleus at a stage of development similar to that of the early pronuclei.