Acrosome formation during spermiogenesis in Notonecta glauca L. (Heteroptera)

The giant spermatozoon of Notonecta glauca has a 1 mm long acrosome. It has the shape of a stiff needle with round cross section and contains several different components. Morphogenesis of this simple form proceeds by very complex changes of both the internal structure and the outer shape. Surrounded by a great number of individual dictyosomes the acrosome is attached to the anterior nuclear pole where it grows to a sphere measuring about 20 microns in diameter. In its interior differentiations consisting of several components become visible which develop independently and may be characterized as basal and lateral complexes. At the end of the growing phase these two complexes come in contact with each other. After the dictyosomes have detached from the acrosome, the cell membrane surrounds the acrosome very closely so that cytoplasmic structures temporarily disappear completely from the narrow space in between. The tip of the acrosome grows out of the sphere as a slender lateral process which becomes increasingly spiralized. It shows a deep lateral groove, its contents consist of components of the basal complex. Parts of the lateral complex form a baso-lateral protuberance. Between these two structures a narrow channel extends from the subacrosomal space to the base of the lateral process. During elongation the acrosome attains spindle shape, the lateral process gradually becoming its anterior part. The distance between acrosome and cell membrane broadens again and cytoplasmic structures reappear among them about 1000 microtubules which are not surrounded by a sleeve membrane. In the posterior part cell membrane and acrosomal membrane are connected by two lines of weld spots where acrosomal membrane is eliminated as small vesicles. In the anterior part the groove becomes deeper. One leg of its initially U-shaped cross section becomes longer and is wrapped around the shorter one. The two legs subsequently fuse with the consequence that membrane and cytoplasmic components of the former groove become secondarily incorporated into the acrosome. The subacrosomal channel obliterates and the cross section of the acrosome showing acute angles in later stages finally gets round. Anterior and posterior part of the acrosome can be differentiated due to structural details even in the mature sperm.