During fertilization, a sperm must first fuse with and then penetrate the female egg in order to fertilize it. Fusing to the egg usually causes little problem, whereas penetrating through the egg's hard shell can present more of a problem to the sperm. Therefore sperm cells go through a process known as the acrosome reaction which is the reaction that occurs in the acrosome of the sperm as it approaches the egg. The acrosome is a cap-like structure over the anterior half of the sperm's head.
As the sperm approaches the zona pellucida of the egg, which is necessary for initiating the acrosome reaction, the membrane surrounding the acrosome fuses with the plasma membrane of the sperm, exposing the contents of the acrosome. The contents include surface antigens and numerous enzymes which are responsible for breaking through the egg's tough coating and allowing fertilization to occur.
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Variations among species
There are considerable species variations in the morphology and consequences of the acrosome reaction. In several species the trigger for the acrosome reaction has been identified in a layer that surrounds the egg.
Echinoderms
In some lower animal species a protuberance (the acrosomal process) forms at the apex of the sperm head, supported by a core of actin microfilaments. The membrane at the tip of the acrosomal process fuses with the egg's plasma membrane.
In some echinoderms, including starfish and sea urchins, a major portion of the exposed acrosomal content contains a protein that temporarily holds the sperm on the egg's surface.
Mammals
In mammals the acrosome reaction releases hyaluronidase and acrosin; their role in fertilization is not yet clear. The acrosomal reaction does not begin until the sperm comes into contact with the oocyte's jelly layer. After the jelly layer is penetrated, the acrosomal enzymes begin to dissolve and the actin filament comes into contact with the zona pellucida. Once the two meet, a calcium influx occurs, causing a signaling cascade. The cortical granules inside the oocyte then fuse to the outer membrane and a transient fast block reaction occurs.
It also alters a patch of pre-existing sperm plasma membrane so that it can fuse with the egg plasma membrane.
A sperm penetration assay includes an acrosome reaction test that assesses how well a sperm is able to perform during the fertilization process. 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. This test is rather expensive and provides limited information on a man's fertility.[1]
The process
A few events precede the actual acrosome reaction. The sperm cell approaches the ovum with the help of various mechanisms, including chemotaxis. It then goes through the process of spermatozoa activation and hyperactivation and finally binds to the outer regions of the egg cell.
After reaching the Zona Pellucida the actual acrosome reaction begins. This involves releasing hyaluronidase to digest cumulus cells surrounding the oocyte and exposing acrosin attached to the inner membrane of the sperm. The cumulus cells are embedded in a gel-like substance made primarily of hyaluronic acid, and developed in the ovary with the egg and support it as it grows.
Acrosin digests the zona pellucida and membrane of the oocyte. Part of the sperm's cell membrane then fuses with the egg cell's membrane, and the contents of the head sink into the egg. ZP3, one of the proteins that make up the zona pellucida, binds to a partner molecule on the sperm. This lock-and-key type mechanism is species-specific and prevents the sperm and egg of different species from fusing. There is some evidence that this binding is what triggers the acrosome to release the enzymes that allow the sperm to fuse with the egg.
Upon penetration, the oocyte is said to have become activated. It undergoes its secondary meiotic division, and the two haploid nuclei (paternal and maternal) fuse to form a zygote. In order to prevent polyspermy and minimise the possibility of producing a triploid zygote, several changes to the egg's cell membranes renders them impenetrable shortly after the first sperm enters the egg.
The aforementioned process describes the physiologically relevant events. One should however bear in mind that a certain percentage of sperm cells will undergo a spontaneous acrosome reaction without the presence of the ovum. Those cells are not able to fertilise the egg, even if they do reach it later. Other cells will spontaneously shed their acrosome during the process of apoptosis/necrosis.
Assessment in vitro
This shedding of the acrosome or "acrosome reaction" can be stimulated in vitro by substances a sperm cell may encounter naturally such as progesterone or follicular fluid, as well as the more commonly used calcium ionophore A23187. This can be done to serve as a positive control when assessing the acrosome reaction of a sperm sample by flow cytometry [2] or fluorescence microscopy. This is usually done after staining with a fluoresceinated lectin such as FITC-PNA, FITC-PSA, FITC-ConA, or fluoresceinated antibody such as FITC-CD46.[3] The antibodies/lectins have a high specificity for different parts of the acrosomal region, and will only bind to a specific site (acrosomal content/ inner/outer membrane). If bound to a fluorescent molecule, regions where these probes have bound can be visualised.
For fluorescence microscopy a smear of washed sperm cells are made, airdried, permealized and then stained. Such a slide is then viewed under light of a wavelength that will cause the probe to fluoresce if it is bound to the acrosomal region. At least 200 cells are viewed in an arbitrary fashion and classified as either acrosome intact (fluorescing bright green) or acrosome reacted (no probe present, or only on the equatorial region). It is then expressed as a percentage of the counted ells.
For assessment with flow cytometry the washed cells are incubated with the chosen probe, (possibly washed again) and then sampled in a flow cytometer. After gating the cell population according to forward- and side-scatter the resulting data can be analysed (Eg. mean fluoresences compared). With this technique a probe for viability, like propidium iodide (PI) could also be included in order to exclude dead cells from the acrosome assessment, since many sperm cells will spontaniously lose their acrosome when they die
See also
The acrosomal reaction takes place in the fallopian tube (site of fertilization) when the sperm penetrates the primary oocyte. First stage is the penetration of corona radiata, by releasing hyaluronidase from the acrosome. Second stage is penetrating the zona pellucida, this occurs by releasing acrosin from the acrosome. After the two stages the head and tail of the sperm go into the oocyte's cytoplasm while the membrane fuses with the oocyte's membrane, meanwhile zona reaction occurs in order to prevent another spermatocytes to enter.
Some species possess spermatozoa undergo a premature acrosome reaction prior to reaching the primary oocyte. This phenomenon seems to be prevalent in organisms in which the female copulates with multiple males resulting in competition for fertilization. The purpose of the premature reaction is believed in some cases is believed to be a sacrificial act of altruism, ensuring the survival of genotypically similar cells while destroying all possibility of partaking in fertilization. In other cases, such as in the wood mouse Apodemus sylvaticus, premature acrosome reactions have been found to cause increased motility in aggregates of spermatozoa promoting fertilization [4]
References
- ^ Your path to fertility: Acrosome Reaction. 2007. doi: http://www.sharedjourney.com/define/mcp.html
- ^ Miyazaki et al. Archives of Andrology 25:243-251 (1990)
- ^ Carver-Ward et al. Journal of Assisted Reproduction and Genetics, Vol. 14, no. 2, 1997
- ^ Moore, Harry et al., Exceptional sperm cooperation in Wood Mouse.Nature 418, 174-177 (2002).
External links
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