Acrosome vesicle

Almost all animal sperm, when near to, or when contacting the outermost egg investment, undergo the exocytosis of an acrosome vesicle (AV), which releases pro-... 

Figure 3. (Upper Panel). Scanning electron micrograph of an H. rufescens spermatozoon. The sperm head, from mitochondrion (M) to tip of the acrosome vesicle (granule AV) is 7 pm. The width of the nucleus (N) is 1 pm. (Lower Panel). Transmission electron micrograph of the acrosomal vesicle showing it attached to the nucleus (NF) by the rod of actin filaments (AF). The darker material labelled 1 shows the location of the 18K protein and 2 shows the location of lysin (from Lewis et al., 1980).

Figure 6. Polyacrylamide gel electrophoresis of abalone sperm and AV contents. Lanes A and E are standard proteins of known molecular mass. Lane B, whole sperm dissolved in SDS. Lane C, the acrosome vesicle content released to seawater when exocytosis of the sperm is induced by high calcium ion concentration. Lane D, purified 16-kDa lysin. (from Lewis et al., 1982).

Way M, Sanders M, Chafel M, Tu YH, Knight A, Matsudaira P. beta-Sciuin, a homologue of the actin crosslinking protein scruin, is localized to the acrosomal vesicle of Limulus sperm. J Cell Sci. 1995 108 3155-62. 

Induction of the acrosome reaction is a key event that occurs in sea urchins, starfish, ascidians and mammals. However, it is not clear that in the amphibian Xenopus sperm undergo the same type of acrosome reaction, because thus far it has not been possible to detect a morphologically identifiable acrosome vesicle in these sperm. The acrosome reaction in echinoderms and in mammals has been well studied. During the acrosome reaction the acrosomal vesicle fuses with the plasma membrane and the acrosomal components are externalized. Several proteolytic activities thought to be involved in penetration through the extracellular matrix of the egg are deteeted in the acrosomal contents. In addition, exposure of egg coat binding molecules on the surface of the sperm occurs (see below). 

The spermatozoon consists of a head, a midpiece and a tail or flagellum. The head contains the nucleus and a vesicle known as the acrosome (Figure 19.4). It contains hydrolytic enzymes that are required during fertilisation of the ovum. 

Since the 28K-proacrosin interaction required either 8 M urea or 6 M guanidine HC1 to disrupt it (10), this protein-protein interaction may act to anchor proacrosin to the acrosomal membranes or to other proacrosin molecules. While acrosin does not fit all the criteria for an intrinsic membrane protein, acrosin was associated with and extracted from preparations of sperm membrane vesicles with 1 mM HC1 and Triton X-100 (5 , Urch, unpublished observations). Salt or combinations of salt and detergent were ineffective. The interaction of acrosin with membranes, particularly sperm acrosomal membranes, is obviously important in ZP binding and penetration. 

The acrosome is a secretory granule that is assembled in spermatids by the coalescence of Golgi apparatus-derived vesicles. This process produces a large, membrane-limited organelle with an inner acrosomal membrane apposed to the nucleus, an outer acrosomal membrane subjacent to the plasmalemma, and an equatorial segment where these two membrane domains intersect. Our inability to obtain highly purified preparations of isolated acrosomes, despite earlier claims to the contrary (Hartree, 1977), has limited our understanding of the function of this or-... 

Secretory cells often release vesicles spontaneously at a low rate whereas release rates increase when a stimulus arrives at the release site. For example, miniature synaptic potentials represent spontaneous release from the neuronal presynaptic terminal (Katz, 1966 Stevens, 1993). In contrast, sperm have only one secretory vesicle and, as discussed previously, cells that complete the acrosome reaction prior to egg contact are infertile. This problem may be circumvented, in part, by the fact that large numbers of sperm are produced by mammals, thereby minimizing the consequences of premature acrosome reactions in large populations. However,... 

This pathway is present in seminal vesicles, which synthesize fructose for the seminal fluid. Spermatozoa use fructose as a major fuel source while in the seminal fluid and then switch to glucose once in the female reproductive tract. Utilization of fructose is thought to prevent acrosomal breakdown of the plasma membrane (and consequent activation) while the spermatozoa are still in the seminal fluid. 

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