Acrosome differentiation inMolgula manhattensis (Ascidiacea, Tunicata)

The acrosome ofMolgula manhattensis spermatozoa is a moderately electron-dense slightly depressed sphere, which is enclosed by a unit membrane. It is approximately 80 × 80 × 40 nm in length, width, and height, respectively. Neither a subacrosomal substance nor a perforatrium can be identified between the acrosome and the nuclear envelopes. During early spermiogenesis, at least three or four vesicles (50–60 nm in diameter) appear in a blister at the apex of the spermatids. Later, these vesicles attach to the inner surface of the plasmalemma enclosing the blister. They then come in contact with each other along the inner surface of the plasmalemma and fuse to form a horseshoe-shaped acrosomal vesicle which rounds up during further differentiation.