The kinetics and possible mechanism of acrosomal process
The acrosome reaction consists of two steps--polymerization and lengthening of the acrosomal process
with subsequent exocytosis of the acrosomal vesicle, which exposes bindin; bindin then coats the acrosomal process
and aids in sperm-egg attachment and fusion.
In many invertebrates, acrosomal exocytosis of spermatozoa starts with membrane fusion at the apical point and produces an acrosomal process
(Dan, 1967; Morisawa et al., 2004).
Simultaneous with the exocytosis of lysins is the rapid extension of an acrosomal process
that projects the inner acrosomal membrane into contact with the vitelline layer overlying the egg microvilli.
In the meantime, the acrosomal process
and the sperm tail had gradually acquired a yellowish-orange fluorescence which became stronger with time over the next several minutes (top to bottom panels).
Limulus sperm shows two types of discharge of acrosomal process
-- true (Fig.
Paul and Gould-Somero (1976) found that the acrosomal process
of supernumerary sperm penetrated through the vitelline coat into the perivitelline space of the fertilized oocyte, but sperm could not fuse with the oocyte plasma membrane.
The acrosomal vesicle of protochordate spermatozoa is either ovoid, as in the urochordate Oikopleura (Holland et al., 1988), or cap-shaped, as in the cephalochordate Branchiostoma (Baccetti et al., 1972), and a distinct acrosomal process
is produced de novo following the acrosomal exocytosis at the anterior end of the sperm (Holland et al., 1988).