Studies on the acrosome: IX. Course of acrosome reaction in the starfish

Abstract

When the starfish spermatozoon is exposed to dissolved egg jelly, its acrosome reacts rapidly to form a slender process which develops from the innermost part of the acrosomal depression to extend beyond the anterior face of the sperm head within 1 second. The completely elongated process (20–30 μ ) consists of a single membrane surrounding a fibrous shaft. The membrane is derived from three acrosomal components which successively form its distal, proximal, and basal portions, the last continuous with the plasma membrane around the sides of the sperm head. Polymerization of the shaft fibrils occurs as a concomitant of process elongation; a possible source of their precursor is suggested. The changes constituting this reaction begin, as in the sea urchin, with the disintegration of a thin, osmiophilic layer located immediately beneath the plasma membrane over the apex of the acrosome. The consequent local disruption of the plasma membrane opens the acrosome to the entrance of sea water; it is suggested that the ensuing changes depend upon specific interactions between the various acrosomal components and sea water constituents.