Scanning Electron Microscopic Observations on The Sperm Entry Into The Eggs of Goldfish,Carassius auratus

The surface structure of chorion,sperm penetration and exocytosis of cortical alveoli in goldfish,Carassius auratus,were examined by scanning electron microscopy.Five to ten grooves and ridges could be seen in the micropylar region,and they may help the sperms reach the micropyle canal.There was a distinct cluster of microvilli on the plasma membrane,identified as the sperm entry site,beneath the micropyle.The fertilizing sperm attached to the sperm entry site within 5 seconds of the mixing of a gamete suspension.Then,the microvilli of the sperm entry site elongated,fused with plasma membrane of the sperm head and formed a fertilization cone to engulf the sperm head and midpiece.At 110 seconds postinsemination,the sperm head and midpiece were fully incorporated into the egg cytoplasm and the fertilization cone tended to disappear.Exocytosis of cortical alveoli occurred beyond 30 seconds postinsemination.At 90 seconds postinsemination,the whole egg became honeycomb-like structure because of the exocytosis of cortical alveoli and a great number of spherical bodies accumulated on the linner surface of the egg chorion.