Fertilization is the process whereby two sex cells (gametes) fuse together to create a new individual with genetic potentials derived from both parents. Fertilization, then, accomplishes two separate activities: sex (the combining of genes derived from the two parents) and reproduction (the creation of new organisms). Thus, the first function of fertilization involves the transmission of genes from parent to offspring, and the second function of fertilization is to initiate in the egg cytoplasm those reactions that permit development to proceed. Although the actual details of fertilization vary enormously from species to species, the events of conception generally consist of four major activities:
STRUCTURE OF THE GAMETES
As we shall soon see, a complex dialogue exists between egg and sperm. The egg is able to activate the sperm metabolism that is essential for fertilization, and the sperm reciprocates by activating the egg metabolism needed for the onset of development. But before proceeding to investigate each of these features of fertilization, we must first discuss the structures of the sperm and egg, the two cell types specialized for fertilization.
Sperm
Each sperm is known to consist of a nucleus with one set of chromosomes, a propulsion system to move the nucleus, and a sac of enzymes that enable the nucleus to enter the egg. Most of the cellular material of the sperm has been eliminated, the haploid nucleus made very streamlined, and its DNA tightly compressed during the process of sperm maturation. In front of this compressed haploid nucleus lies the acrosomal vesicle that contains enzymes that digest proteins and sugars. These stored enzymes are used to digest the outer coverings of the egg. In many species, such as sea urchins, a region of globular actin molecules lies between the nucleus and the acrosomal vesicle. These proteins are used to extend a fingerlike process during the early stages of fertilization. In these species, recognition between sperm and egg involves molecules on this acrosomal process. Together, the acrosome and nucleus constitute the head of the sperm.
The means by which sperm are propelled varies according to how the species has adapted to environmental conditions. In some species the sperm travel by amoeboid motion. In most species, however, each sperm is able to travel long distances by whipping its flagellum or sperm tail. The mitochondria (which are the energy producing organelles of cells) collect about the flagellum near the base of the nucleus and become incorporated into the midpiece of the sperm. (The mitochondria of the midpiece can then act as the motor to propel the sperm.) In many mammals, a layer of dense fibers has interposed itself between the mitochondrial sheath and body of the flagella. This fiber layer stiffens the sperm tail. Because the thickness of this layer decreases toward the tip, the fibers probably serve to prevent the sperm head from being whipped around too suddenly. Thus, the sperm has undergone extensive modification for the transmission of its nucleus to the egg.
Egg.
All the material necessary for the beginning of growth and development must be stored in the mature egg (ovum). Therefore, whereas the sperm has eliminated most of its cellular material, the developing egg not only conserves its material, but also is actively involved in accumulating more. It either synthesizes or absorbs proteins, such as yolk, that act as food reservoirs for the developing embryo. Thus, birds' eggs are enormous single cells that have become swollen with their accumulated yolk. Even eggs with relatively sparse yolk are comparatively large. The volume of a sea urchin egg is about 2 x 105 cubic micrometers, more than 10,000 times the volume of the sperm. So, while sperm and egg have equal haploid nuclear components, the egg also has an enormous cytoplasmic storehouse that it has accumulated during its maturation. This cytoplasmic trove includes proteins, ribosomes, and transfer RNA (tRNA), messenger RNA, and morphogenic factors.
Enclosing the cytoplasm is the egg plasma membrane. This membrane must regulate the flow of certain ions during fertilization and must be capable of fusing with the sperm membrane. Above the plasma membrane is the vitelline envelope. This is essential for the species-specific binding of sperm. In mammals, the vitelline envelope is very thick and is called the zona pellucida. The mammalian egg is also surrounded by a layer of cells, the cumulus cells. These represent ovarian follicular cells, which were nurturing the egg at the time of its release from the ovary. Sperm have to get past these cells, also, to fertilize the egg.
Lying immediately beneath the plasma membrane of the sea urchin egg is the cortex. The cytoplasm in this region contains high concentrations of globular actin molecules, which during fertilization, form long cables of known as microfilaments. Microfilaments are necessary for cell division, and they also are used to extend the egg surface into the microvilli, which aid sperm entry into the cell). Within this cortex are the cortical granules. These membrane structures are homologous to the acrosomal vesicle of the sperm being organelles containing proteolytic enzymes. However, whereas each sperm contains one acrosomal vesicle, each sea urchin egg contains approximately 15,000 cortical granules. Moreover, in addition to containing the digestive enzymes, the cortical granules also contain enzymes and polysaccharides that prevent other sperm from entering the egg after the first sperm has entered. Many types of eggs also secrete an egg jelly outside their vitelline envelope. This sugar and protein meshwork can have numerous functions. Most often, though, it is used to either attract or activate sperm. The egg, then, is a cell specialized for receiving sperm and initiating development.
RECOGNITION OF SPERM AND EGG: ACTION AT A DISTANCE
Many marine organisms release their gametes into the environment. This environment may be as small as a tidepool or as large as the ocean. Moreover, this environment is shared with other species that may shed their sex cells at the same time. These organisms are faced with two problems: (1) How can sperm and egg meet in such a dilute concentration? (2) What mechanism prevents starfish sperm from trying to fertilize sea urchin eggs? Two major mechanisms have evolved to solve these difficulties: species-specific attraction of sperm and species-specific sperm activation.
Sperm attraction
Species-specific sperm attraction (a type of chemotaxis) has been documented in numerous species. (That is, to reach the site of fertilization, sperm of a particular species follow a gradient of some chemical released by an egg of the same species.) In addition, the egg controls not only the type of sperm it attracts but also the time sperm are attracted.
The acrosome reaction
A second interaction between sperm and egg involves the activation of sperm by egg jelly. In most marine invertebrates, this acrosome reaction his two components: the fusion of the acrosomal vesicle with the sperm plasma membrane (which releases the contents of the acrosomal vesicle) and the extension of the acrosomal process. The acrosome reaction can be initiated by soluble egg jelly, by the egg jelly surrounding the egg, or even by contact with the egg itself in certain species. It can be activated artificially by increasing the calcium concentration of seawater.
RECOGNITION OF SPERM AND EGG: CONTACT
Species-specific recognition in sea urchins.
Once the sea urchin sperm has traversed the egg jelly, the acrosomal process of the sperm contacts the outer layer of the egg's vitelline envelope. A major species-specific recognition step occurs at this point when the acrosomal protein, bindin, interacts with specific proteins on the egg's vitelline envelope. The proteins on the sperm and the egg interact and bind the sperm to the egg. Thus, species-specific recognition of sea urchin gametes occurs at the levels of acrosome activation and sperm adhesion to the vitelline envelope.
Gamete recognition and binding in mammals
So far we have been focusing our discussion on those organisms for which fertilization takes place externally. In mammals, fertilization is internal, and the fertilization process has been adapted to this environment. Indeed, investigators have concluded that the reproductive tract of females plays a very active role in the fertilization process. Mammalian fertilization occurs in the oviduct, requiring the successful movement of the millions of sperm past a series of anatomical and physical barriers. Newly ejaculated mammalian sperm are unable to undergo the acrosomal reaction without residing for some amount of time in the female reproductive tract. This requirement for capacitation varies from species to species and can be mimicked in vitro by incubating sperm in tissue culture media or in fluid from the oviducts. The molecular changes that account for capacitation are still unknown. Once at the site of fertilization, sperm must pass through the cumulus cells, bind to and move through the zona pellucida, and then fuse with the plasma membrane.
Several investigators have demonstrated that sperm, both normal and abnormal, are prevented from fertilizing eggs at a variety of levels. Only a small fraction of the sperm deposited in the vagina later enter the oviduct; the majority remain in the uterus where they are eaten by white blood cells. The second barrier to fertilization appears to be at the uterotubual junction. There may also be a selective reduction in the number of sperm entering the oviduct. Morphologically abnormal sperm are less capable of entering the oviduct due to some mechanism at the uterotubal junction and that this region serves as a partial barrier to fertilization.
To reach the egg, the mammalian sperm must first pass through the extracellular matrices that surround the egg. The innermost layer, adjacent to the egg, is the zona pellucida. This shell is made during egg production and is present on all mammalian eggs at the time of fertilization. The outer layer is composed of cumulus cells, which are shed along with the egg during ovulation. In some species, the cumulus cells are not present at the time of fertilization and do not constitute a barrier to sperm even when they are present. Capacitated sperm pass freely through the cumulus layer to the zona, while noncapacitated and prematurely capacitated sperm are held up in the matrix. In the best-studied mammals-mice and hamsters-the acrosome reaction occurs after the sperm has bound to the zona pellucida.
The zona pellucida in mammals plays a role analogous to that of the vitelline envelope. The binding of sperm to the zona is relatively, but not absolutely, species-specific (species specificity should not be a major problem when fertilization occurs internally). The mouse sperm can concentrate its proteolylic enzymes directly at the point of attachment at the zona pellucida.
Fusion between egg and sperm cell membranes
Recognition of sperm by the vitelline envelope or zona is followed by the lysis of that portion of the envelope in the region of the sperm head. This lysis is followed by the fusion of the sperm cell membrane with the cell membrane of the egg. The first step in fusion is the formation of small extensions of the egg surface called microvilli. Sperm-egg fusion appears to cause the polymerization of actin and the extension of several microvilli to form the fertilization cone. This cytoplasmic protoberance averages 7 m m in length and 2m m in width. Homology between the egg and the sperm is again demonstrated, because the fertilization cone, like the acrosomal process, appears to be extended the polymerization of actin. The sperm and egg membranes join together and material from the sperm cell membrane can later be found on the egg membrane. The sperm nucleus and tail pass through the cytoplasmic bridge, which is widened by the actin polymerization.
Fusion is an active process, often mediated by specific "fusogenic" proteins. Proteins such as the HA protein of influenza virus and the F protein of Sendai virus are known to promote cell fusion, and it is possible that bindin is also such a protein. It appears, then, that one of the proteins on the sperm head is capable of stimulating the fusion of the sperm and egg cell membranes.