Ultrastructural Aspects of Fertilization in Spiralian Eggs

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Abstract
Normally, the eggs of Spisula are monospermic. How polyspermy is prevented in this organism is unclear, particularly when the cortex of the fertilized ovum is examined. Using conventional microscopic procedures, little alteration of the surface of the egg is observed following insemination; the microvilli, vitelline layer and cortical granules are morphologically unchanged. Investigations employing freeze fracture replication of fertilized and unfertilized Spisula eggs demonstrate that there is a dichotomy with respect to the distribution of intra membranous particles within the plasmalemma of Spisula eggs. There is a structural reorganization of microvilli and a two-fold increase in particles on the A-face of the plasma membrane along their bases following insemination. These transformations in microvillar structure and intramembranous particle number may be involved in establishing a block to polyspermy, however, further evidence is necessary to demonstrate a cause-effect relation.

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