What is the significance of the acrosome of the sperm

ZP1 is synthesized in the liver [ 55 ], transported by blood circulation and incorporated in the PVM [ 56 ]. Of all avian ZP proteins, chicken ZP2 is the only protein that exhibits specific localization, albeit at low abundance, in the germinal disc region. Given that the germinal disc region is the only place where spermatozoa penetrate the PVM [ 57 ], at least in chicken, ZP2 seems to be the likely sperm receptor of the PVM [ 58 ]. Acrosome and the AR of human spermatozoa have been studied extensively in various aspects including a acrosome biogenesis, b chemical components of the acrosome contents, c membrane dynamics before and during the AR, d relationship between capacitation and AR, e relationship between the AR and development of sperm's ability to fuse with the egg, f relationship between acrosomal dysfunction and male infertility, and g acrosome and AR as targets for contraceptive measures.

Because of ethical tissue and technical problems, we are unable to study where human spermatozoa undergo the AR and what trigger it in vivo [ 59 ]. Presumptive ARIS include follicular fluid of ovarian follicle [ 60 , 61 ], secretions from the cumulus-oocyte-complex [ 62—64 ], and oocyte's ZP [ 65—67 ]. It is known that human spermatozoa in vitro undergo the AR in response to progesterone [ 68 , 69 ] and neurotransmitters [ 70 ].

In a nonhuman primate macaque, spermatozoa in vitro first attach to the ZP loosely, followed by a tight binding to ZP before undergoing the AR [ 71 ]. Baibakov et al. They found that only the zonae with human ZP2 protein allowed human spermatozoa to bind and penetrate the ZP, suggesting that ZP2 is likely the zona component inducing human sperm AR.

Mouse IVF is routinely successful after coincubation of epididymal spermatozoa with eggs with or without cumulus oophorus in appropriate media. It has been thought that capacitated, acrosome-intact spermatozoa adhere to the ZP surface before undergoing the AR. AR-inducing activity of the purified ZP3 has been reported in many other mammalian species including humans [ 75 ].

ZP2, another zona glycoprotein of the mouse, is responsible for the binding of acrosome-reacted spermatozoa to ZP. Until recently, not much effort had been directed to see how the AR takes place in individual mouse spermatozoa during the course of fertilization. The mouse acrosome is thin and flat. Distinguishing acrosome-intact spermatozoa from acrosome-reacted ones using the ordinary light microscope is difficult. Therefore, the acrosomal status of mouse spermatozoa has been evaluated after staining of spermatozoa with a fluorescent dye, chlortetracycline which stains sperm head differently according to the status of acrosome [ 78 ].

Studies using this technique led investigators to infer that mouse spermatozoa undergo the AR after attachment to ZP [ 79 ]. In , transgenic mice with green fluorescent protein GFP in the acrosome were generated [ 80 ].

Nakanishi et al. Other researchers who used GFP spermatozoa also found that spermatozoa bound to the ZP retained their acrosomes intact for a long time [ 81 ], while those exposed to solubilized ZP quickly lost acrosomal GFP [ 82 ]. This discrepancy could be due to differences in the state e. Jin et al. They found that most fertilizing spermatozoa were acrosome-reacted before contacting the ZP. Obviously, fertilizing mouse spermatozoa do not need to have intact acrosomes when they reach the ZP surface, as it was once thought [ 84 ].

Studies with electron microscopy have clearly demonstrated that initial sperm—egg fusion event occurs between the plasma membranes overlaying the equatorial segment of sperm head and egg plasma membrane; thus, it has been puzzling for many years why acrosome-intact spermatozoa are unable to fuse with eggs even when they are brought directly in contact with the egg's plasma membrane [ 85 ].

Satouh et al. This explains clearly why acrosome-intact spermatozoa are fusion-incompetent. In vitro studies are certainly important for understanding the process and mechanism of sperm—egg interactions in mammals, but we must understand what is really going on within the oviduct where fertilization takes place under natural conditions.

Austin and Bishop [ 87 ], who first reported mammalian sperm AR, found acrosome-reacted, motile spermatozoa in the oviduct fluid of guinea pig and in the cumulus oophorus of the guinea pig and Libyan jird eggs. They found acrosome-reacted spermatozoa within the ZP and perivitelline space of guinea-pig, golden hamster, Chinese hamster, and Libyan jird eggs.

They inferred that the acrosomes are modified while spermatozoa are passing though the female genital tract and detach before spermatozoa penetrate the ZP. One should note that acrosomes of these rodent species are large and their structural changes can be detected readily under phase-contrast microscopes without fixation and staining of spermatozoa.

Cummins and Yanagimachi [ 88 ] reported that golden hamster spermatozoa collected from oviduct ampulla appear to be ready to undergo the AR and complete the AR, while they are passing through the cumulus or shortly before contacting the surface of the ZP. According to Suarez et al. While ZPs of various species are certainly able to induce or accelerate AR in the mouse [ 90 ], hamster [ 91 ], guinea pig [ 92 ], rabbit [ 93 ], bovine [ 94 ], monkey [ 71 ], and human [ 95 , 96 ], this does not mean that ZP is the sole ARIS.

As stated earlier, acrosome-reacted mouse spermatozoa are able to attach to and penetrate in ZP. In fact, mouse spermatozoa within the oviduct seem to undergo the AR before ascending from the isthmus to the ampulla [ 97—99 ]. One should note that the mouse has been the most commonly used model animal for the study of mammalian fertilization.

The AR is a widespread phenomenon among animals that use various fertilization tactics. While fertilization process can be readily examined in species with external fertilization such as sea urchin and as starfish , it is next to impossible to observe the AR of mammalian spermatozoa within the oviduct where normal fertilization takes place.

Thus, studies in marine invertebrates have offered much basic information on this biological event. In mammals, it has been a long-standing question where and what to trigger the AR in vivo. To address these questions, studies have been conducted using two different approaches, one being oriented to search and identification of substances with specific biological activities for various steps of fertilization in vitro, and the other being oriented to the identification of sites in the female tract where the AR and fertilization really occur.

The latter approach was resumed rather recently by using transgenic mice with green fluorescent acrosomes. Although the results obtained from these two approaches have many contradictions, it is hoped that two approaches eventually become complementary to each other, rather than contradictory.

In nonmammalian vertebrates such as newts and birds, egg's coats carry ARIS. At the end of capacitation process, mammalian spermatozoa may undergo the AR spontaneously, and this may be enough to render spermatozoa fertilization competent. In the mouse, spontaneous AR has been considered nonphysiological, because it renders spermatozoa fertilization incompetent [ 84 ].

We now know that fertilizing mouse spermatozoa within oviducts are acrosome-reacted before meeting eggs [ 83 , 97—99 ].

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The data indicate the need for the introduction of acrosomal markers in clinical andrology for proper diagnosis of acrosomal sperm defects.

Here, the sperm undergoes a number of physiological changes that include:. Once the sperm cells have gone through these changes in the female reproductive tract, they are ready for exocytosis. When the sperm cell comes into contact with the external coat of the female gamete the zona pellucida , some of the primary ligands lectins bind to the receptors molecules of ZP3 O-linked oligosaccharides and glycans thus binding the two gametes. This process has been linked with ensuring that the sperm binds to the egg in a species-selective manner that allows for acrosome exocytosis also referred to as acrosome reaction.

Acrosome is calcium dependent and involves acrosome exocytosis. Here, binding of the sperm to the egg is accompanied by the formation of numerous pores between the acrosome membrane and the plasma membrane of the sperm cell at the sperm head. This reaction not only results in the fusion of the two membranes, the plasma membrane of the sperm cell and the acrosome membrane, but also allows contents of the acrosome to be released.

Acrosome reaction has two main outcomes. Once the sperm cell binds to the zona pellucida surrounding the plasma membrane of the egg, pores are formed allowing acrosome components to be released. This is where secondary ligands hydrolytic enzymes come into play. One of the main components of the acrosome is a serine protease known as acrosin. In the acrosome, the protein is stored in an inactive form known as zymogen. Once the acrosome membrane is compromised, allowing the protein to be released, it's converted into its active form acrosin by the coming in contact with the glycoproteins of zona pellucida.

In its active form, the enzyme plays an important role of degrading the zona pellucida of the egg in order to create an opening through which the sperm can penetrate through. This may take about 5 to 20 minutes. Inheritance This condition is inherited in an autosomal recessive pattern , which means both copies of the gene in each cell have mutations.

Hum Reprod. Epub May Mol Hum Reprod. Epub Oct A recurrent deletion of DPY19L2 causes infertility in man by blocking sperm head elongation and acrosome formation. Am J Hum Genet. DPY19L2 deletion as a major cause of globozoospermia. Erratum in: Am J Hum Genet. Ray, Pierre [corrected to Ray, Pierre F].