Fine Structural Changes in the Acrosome Reaction of the Japanese Abalone, Haliotis disus. (abalone/sperm/acrosome reaction/ultrastructure)

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Abstract
The spermatozoon of the Japanese abalone, H. discus, and its structural changes during the acrosome reaction were observed by EM. The spermatozoon has a huge acrosome in the shape of a hanging bell or a forefinger with a deep fossa at the posterior end being filled with a bundle of microfilaments. The membranes of the acrosomal apex, the trigger region, are structurally discernible from those of other acrosomal regions. Following the trigger region, a unique structure under the acrosomal membrane covers the surface of the acrosomal content in the form of a truncated cone. The acrosome reaction occurs in the jelly layer very close to the egg envelope. First, the membranes at the apex of the acrosome are vesiculated, followed by the formation of a narrow gap between the outer acrosomal membrane and the acrosomal content. Next, the bundle of microfilaments elongates, running through the center of the acrosome, reaching the trigger region and protruding out of the acrosomal top. Then release of the acrosomal content occurs in 2 steps, disclosing the membrane undercoating structure that comprises globular particles with a fuzzy material connecting them. This resembles the undercoat network found in erythrocytes.