The Acrosome Reaction
The acrosome reaction is a change in the sperm that is common to many higher animals. In the sea urchin, contact with egg jelly initiates the acrosome reaction, which is a calcium-mediated process. The animations linked to this page are courtesy of Dr. David Epel at Stanford University’s Hopkins Marine Station. In response to signals presumably transduced by receptors on the surface of the sperm that bind to components of egg jelly (red), actin polymerizes from a pool of globular actin (pink) to form the acrosomal process. The acrosomal vesicle (green) fuses with the plasma membrane, releasing enzymes from the tip of the sperm that aid digestion of egg jelly. At the same time, bindin (blue) is deposited on the surface of the acrosome-reacted sperm.
During the fertilization process, sperm must first fuse to, then penetrate, the female egg in order to fertilize it. While fusing to the egg is usually little problem, penetrating through the egg’s hard shell can present a problem to the sperm. For this reason, sperm cells go through a process known as acrosome reaction.
Shortly before penetration of the egg occurs, the tip of the head of a sperm cell will rupture. As it does this, surface antigens, which had been hidden within the sperm cell, become exposed. Numerous enzymes are also released. These enzymes are responsible for breaking through the eggs tough coating and allowing fertilization to occur.
To perform the test, a sample of sperm will need to be provided. Most likely, your fertility specialist will ask you to provide this sample at the fertility clinic in order to ensure that the sample was given properly and is fresh. Once a sample has been obtained, sperm can be examined for a number of different criteria, including acrosome reaction.
In order to see how well sperm perform acrosome reaction, your fertility doctor will need to artificially induce the reaction. Once this has been done, the sperm’s ability to go through the acrosome reaction process will be evaluated.
Sperm that are unable to properly go through the acrosome reaction will not be able to fertilize an egg. However, this problem only occurs in about 5% of men that have the test done. If a lack of acrosome reaction is found to be the cause of male infertility, then the most effective infertility treatments would be ICSI, as this would by-pass the natural penetration of sperm by injecting a sperm cell directly into the egg, or sperm donation. If you decide to use a sperm donor, then fertilization will likely take place through IUI, although IVF can also be done with a sperm donor if necessary.
The acrosome reaction assay can be rather expensive and offers limited information as to a man’s fertility. Because so few men have sperm that fail to go through acrosome reaction, some fertility doctors choose not to do the test.