sperm acrosomal reaction

SPERM ACROSOME REACTION

Acrosome is large secretory vesicle that lies the nucleus in the apical region of the sperm head. Acrosomal membrane just underlying the plasma membrane is referred to as “outer” acrosomal membrane, and that overlying the nucleus is referred to as “inner” acrosomal membrane. Morphologically, the acrosome reaction is seen as multiple fusions between outer acrosomal membrane and plasma membrane at the anterior region of sperm head, extensive formation of hybrid membrane vesicles, and exposure of inner acrosomal membrane and acrosomal contents (Cardullo and Florman 1993). Again, only acrosome-reacted sperm can penetrate the ZP and fuse with egg plasma membrane.

The acrosomal serine protease called acrosin, is essential for penetration of the ZP by sperm.The absence of acrosin does cause a delay in penetration of the ZP by sperm that may be due to a delay in dispersal of acrosomal proteins during the acrosome reaction (Yamagata et al. 1998).

There are many different inducers of acrosome reaction such as progesterone but now ZP3 is natural inducer that initiates the acrosome reaction by binding acrosome-intact sperm to ZP. mZP3 purified and large glycopeptides induce sperm to acrosome-react in vitro;small mZP3 glycopeptides and purified mZP3 O-linked oligosaccharides bind to sperm and inhibit their binding to eggs, but do not induce the acrosome reaction (Wassarman 1988).

Shown are some molecular events that can lead to induction and completion of the mammalian sperm acrosome reaction upon addition of ZP3 to sperm. This figure was adapted from one kindly provided by Dr. Harvey Florman, Tufts University School of Medicine, Boston, Massachusetts.