Fertilization: Sperm/Egg Recognition and Contact
Close encounters of the zygotic kind
Because it has medical, sociological, agricultural and economic implications, fertilization is one of the most fascinating and intensely-studied aspects of development. You should study carefully your textbook and the links listed below to understand fertilization. Be aware of distinct mechanisms that are used by different organisms. In particular, study the activation of sperm motility and the role of chemotaxis in fertilization in your textbook.
The sea urchin acrosome reaction
Sperm-egg contact requires that the sperm penetrate surface coats that surround the egg. This is facilitated by the acrosome reaction, in which the membranes enclosing the acrosome are shed, releasing the contents of the acrosome. The acrosome reaction has been studied intensively in sea urchin sperm. In sea urchins, the acrosome reaction is stimulated by contact with the egg jelly coat. Study this process in your textbook.
Mechanisms of sperm-egg recognition and contact in mammals
Mammalian sperm must reside in the female reproductive tract before they are able to undergo the acrosome reaction. This maturation process is called capacitation.
The mammalian egg is surrounded by an extracellular envelope called the zona pellucida, to which sperm must bind and penetrate before they can make contact with the surface of the egg itself. The zona pellucida of the mouse egg contains three glycoproteins called ZP-1, ZP-2 and ZP-3 that polymerize to form a gel. The zona of newly-ovulated eggs is also surrounded by a constellation of follicle cells in a matrix of hyaluronic acid. Figure 1 shows a mouse ovum accompanied by polar body I surrounded by the zona pellucida.
Figure 1. Mouse ovum and polar body I surrounded by the zona pellucida. Image courtesy of Dr. Douglas Kline, Kent State University.
ZP-3 functions as a sperm receptor. The sperm-binding activity of ZP3 is mediated by the oligosaccharide side chains of ZP3. The role of the oligosaccharides is demonstrated by experiments in which either removal or modification of the sugars causes loss of sperm-binding activity. A peripheral membrane protein in the plasma membrane overlying the acrosome of mouse sperm called sp56 binds to the oligosaccharide moieties of ZP3. sp56 belongs to a class of carbohydrate-binding proteins called lectins. Purified sp56 binds to the zona of unfertilized eggs, but not to that of zygotes (Bookbinder et al., 1995, Fig. 1). This observation suggests that the oligosaccharides on ZP3 trap incoming sperm at the zona surface of unfertilized eggs and that this activity is lost after fertilization.
Bookbinder et al. (1995) have also reported a correlation between the presence of sp56 and species specificity of sperm-egg recognition (Table 1). Mouse and hamster sperm contain sp56 and bind to the mouse egg. Guinea pig and human sperm, however, lack sp56 and do not bind. Different lectins may be involved in human and rabbit sperm-egg binding. In humans, the oligosaccharides on ZP3 differ from those in mice (Aitken, 1995). The carbohydrates present on ZP3 may provide a code that facilitates species specificity of sperm-egg binding. Investigators may be able to exploit this mechanism in developing new contraceptives or in diagnosis and treatment of male infertility.
Binding of the sperm to the zona triggers the acrosome reaction (see "Overview of the mammalian acrosome reaction" from Zygote), which allows the sperm to penetrate the zona. The sperm form a slit in the zona that is approximately the width and height of their head. The slit is apparently formed through a combination of force and digestion by the enzymes released by the acrosome reaction (Allen and Green, 1997).
After penetrating the zona, incoming sperm enter the perivitelline space surrounding the egg and land on the egg plasma membrane, where the equatorial segment of the sperm head initiates sperm-egg adhesion. A sperm protein called fertilin is thought to be involved in mediating adhesion in mammals. Fertilin has a molecular domain (called the disintegrin domain) that is known to interact with membrane proteins called integrins. This led to the expectation that integrins on the egg surface serve as the sperm receptor. Various observations supported this hypothesis, including:
An integrin that is necessary for fertilization has recently been identified on the surface of mouse eggs (Almeida et al., 1995) and is a putative sperm receptor on the egg surface. Almeida et al. used antibodies to integrins to demonstrate the presence of specific integrins on the surface of mouse eggs. Significant amounts of the alpha6 subunit were detected (Fig. 2). This subunit is known to form heterodimers with a ß subunit known as ß1.
To test whether sperm bind to alpha6ß1, Almeida et al. used approaches that allowed them to block alpha6- and ß1-integrin binding, respectively. A monoclonal antibody (GoH3) that reacts with alpha6 inhibited sperm binding to zona-free mouse eggs in a dose-dependent fashion (Figs. 3C and 4).
They also employed a mouse embryonal carcinoma cell line, F9, which expresses several surface integrins, including alpha6ß1, as a test system for sperm binding. They knocked out the gene expressing ß1, resulting in a cell line called F9 TKO. Sperm bound avidly to normal F9 cells but poorly to the F9 TKO cells (Fig. 5A). This result indicated that a ß1-containing integrin is involved in sperm binding. They also used the anti-alpha6 antibody GoH3 to show that it inhibited sperm binding to F9 cells (Fig. 5B).
To examine sperm binding in cells that either do or do not express alpha6, Almeida et al. utilized cultured mouse macrophages. These cells do not express alpha6 and do not bind sperm. However, when they were transfected with an alpha6 gene, sperm binding was seen (compare Fig. 5E and F). The alpha6 subunits were presumably binding with endogenous ß1 subunits to produce functional alpha6ß1. Together, these studies with cultured cells indicate that simultaneous expression of alpha6 and ß1 is necessary for sperm binding.
Next, Almeida et al. examined the role of fertilin in alpha6ß1-mediated sperm binding by using a synthetic peptide corresponding to the predicted integrin-binding portion of the disintegrin domain. As shown in Figure 6A, this peptide inhibited sperm binding to eggs, indicating that fertilin mediates sperm binding to alpha6ß1. This possibility was further examined by testing the effects of this peptide on the binding of anti-alpha6 antibody to the egg surface. As shown in Figure 7, the peptide significantly reduced antibody binding to eggs, indicating that the peptide and the antibody competed for binding to the same molecule.
These data suggest that binding between disintegrins in sperm and integrins in the egg plasma membrane is responsible for gamete interaction in mammals.
The final step in fertilization is sperm/egg fusion. Although fusion is known to be facilitated by the acrosome reaction, the actual mechanism of fusion is unclear. Presumably, some change occurs on the sperm surface during the acrosome reaction to facilitate fusion. The initial site of fusion on the sperm is the equatorial segment (see your text). The antigenic properties of the equatorial segment change after the acrosome reaction. Furthermore, neutralizing antibodies against the equatorial segment of acrosome-reacted sperm block sperm/egg fusion. These results suggest that alterations to the equatorial segment facilitate sperm/egg fusion (Allen and Green, 1997).
Links to Related Material
"Overview of the mammalian acrosome reaction" from Zygote
Evans, J.P., Kopf, G.S. and Schultz, R.M. 1997. Characterization of the binding of recombinant mouse sperm fertilin b subunit to mouse eggs: evidence for adhesive activity via an egg b1integrin-mediated interaction. Develop. Biol. 187: 79-93.
Evans, J.P., Schultz, R.M. and Kopf, G.S. 1997. Characterization of the binding of recombinant mouse sperm fertilin a subunit to mouse eggs: evidence for function as a cell adhesion molecule in sperm-egg binding. Develop. Biol. 187: 94-106.
Giusti, A.F, Hoang, K.M. and Foltz, K.R. 1997. Surface localization of the sea urchin egg receptor for sperm. Develop. Biol. 184: 10-24.
Just, M.L. and Lennarz, W.J. 1997. Reexamination of the sequence of the sea urchin egg receptor for sperm: implications with respect to its properties. Develop. Biol. 184: 25-30.
Stears, R.L. and Lennarz, W.J. 1997. Mapping sperm binding domains on the sea urchin egg receptor for sperm. Develop. Biol. 187: 200-208.
Sutovsky, P., Oko, R., Hewitson, L. and Schatten, G. 1997. The removal of the sperm perinuclear theca and its association with the bovine oocyte surface during fertilization. Develop. Biol. 188: 75-84.
Tian, J., Gong, H., Thomsen, G.H. and Lennarz, W.J. 1997. Xenopus laevis sperm-egg adhesion is regulated by modifications in the sperm receptor and the egg vitelline envelope. Develop. Biol. 187: 143-153.
Allen, C.A. and Green, D.P.L. 1997. The mammalian acrosome reaction: gateway to sperm fusion with the oocyte? BioEssays 19: 241-247.
Almeida, E.A.C., Huovila A.-P.J., Sutherland A.E., Stephens L.E., Calarco P.G., Shaw L.M., Mercurio A.M., Sonnenberg A., Primakoff P., Myles D.G., and White J.M. 1995. Mouse integrin alpha6ß1 functions as a sperm receptor. Cell 81: 1095-1104.
Bookbinder, L.H., Cheng, A. and Bleil, J.D. 1995. Tissue- and species-specific expression of sp56, a mouse sperm fertilization protein. Science 269: 86-89.
Browder, L.W., Erickson, C.A. and Jeffery, W.R. 1991. Developmental Biology. Third edition. Saunders College Pub. Philadelphia.
Myles, D.G., Kimmel, L.H., Blobel, C.P., White, J.M. and Primakoff, P. 1994. Identification of a binding site in the disintegrin domain of fertilin required for sperm-egg fusion. Proc. Natl. Acad. Sci. USA 91: 4195-4198.
Palermo, G., Joris, H., Devroey, P. and Van Steirteghem, A.C. 1992. Pregnancies after intracytoplasmic injection of single spermatozoon into an oocyte. Lancet 340: 17-18.
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Leon Browder & Laurie Iten (Ed.) Dynamic Development
Last revised Tuesday, June 30, 1998