A possible explanation of the morphological changes during the acrosome reaction in Ciona intestinalis spermatozoa.
(Left) Sagittal views and (Right) frontal views.
- Left: an intact acrosome is present at the apex of the head. The acrosomal outer membrane is tightly apposed to the overlying plasma membrane. Apical substance (AS) is located at the anterior-most tip of the head.
Right: the periphery of acrosome is indicated by the dotted line.
- Left: the acrosome reaction occurs through the fusion between the acrosomal outer membrane at its peripheral margin and the overlying plasma membrane, resulting in the formation of the hybrid flat sac.
Right: solid line represents the region where membrane fusion between the acrosomal outer membrane and the overlying plasma membrane is occuring (arrow).
- Left: membrane fusion proceeds along the peripheral margin of the acrosome , resulting in the formation of small hybrid vesicle. During this process, the acrosomal contents might be released. A thin connection temporally remains between the hybrid vesicle and the plasma membrane overlying the apex.
Right: the membrane fusion along the peripheral margin of the acrosome is almost completed except for a small region where thin connection is temporally present (arrow).
- The hybrid vesicle has come off. The acrosomal inner membrane is exposed and is contiguous with the plasma membrane enclosing the apex.
for details: J. Structural Biology 111, 77-83 (1993)
