(A) Longitudinal section through the head region of a spermatozoon. The acrosome is pear-shaped with an electron-dense structure(EDS)just insidethe acrosomal outer membrane(arrow)which encloses the main part of the acrosome. An indentation is observed in the posterior region of the acrosome.
The space between this indentation and the anterior region of the nucleus is filled with fine granular materials, the subacrosomal substance. The nucleus has a fossa at its posterior region. The midpiece consists of mitochondria. The acrosomal outer membrane and the overlying plasma membrane are in close contact with each other at the most anterior region of the acrosome. Bar: 0.5 um ( This is also applicable to C, D, E, F, G, H, I).
(B) Enlargement of the electron-dense structure (EDS). Fine striations are obvious (arrows). Bar: 0.1um (This is also applicable to inset in C).
(C) The acrosome reaction appears to occur by membrane fusion between the acrosomal outer membrane and the overlying plasma membrane. Membrane fusion is obvious in the inset.
(D) Acrosomal substances are released through the opening caused by membrane fusion.
(E),(F),(G),(H),(I) Longitudinal slightly oblique serial section through an acrosome which has released acrosomal substances. The acrosomal inner membrane seems to be extending as an acrosomal process (I, arrow).
