Molecular Biology of the Cell - Volume 3 - Alberts B., Bray D., Lewis J., Raff M., Roberts K., Watson J. 1994

From Cells to Multicellular Organisms
Germ Cells and Fertilization
Fertilization

Once released from the gonad, both egg and sperm are destined to perish within hours unless they find each other and fuse during fertilization. Fertilization rescues these Cells from death: the egg is activated and initiates its developmental program, while the nuclei of the two Gametes merge to form The Genome of a new Organism. Much of what we know about fertilization stems from studies on marine invertebrates, particularly sea urchins (Fig. 15-40). In such organisms, fertilization takes place in seawater, into which vast numbers of sperm and eggs are released. This process of external fertilization is far more amenable to study than the internal fertilization of mammals, which occurs within the female reproductive tract. Consequently, our Structure/133.html">Discussion of fertilization will focus primarily on sea urchins. Despite the vast evolutionary distance separating mammals from sea urchins, the CELLULAR AND MOLECULAR mechanisms underlying their fertilization appear remarkably similar.

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Fig. 15-40. Photographs of two sea urchin species commonly used in fertilization studies. Top: Strongylocentrotus purpuratus; bottom: Strongylocentrotus franciscanus. Shown at nearly life size. (Courtesy of Victor Vacquier.)

15.4.1. Contact with the Egg Jelly Coat Triggers the Acrosome Reaction in Sea Urchin Sperm [19]

A typical female sea urchin contains about 10⁷ eggs, whereas a male produces roughly 10¹² sperm. This Abundance makes it possible to obtain sea urchin gametes in massive quantities as pure populations, with all cells at the exact same stage of development. When the gametes are mixed, the various STAGES OF SPERM-egg interaction proceed synchronously to within a fraction of a second. Fertilization begins the moment the sperm HEAD makes contact with the jelly coat (Fig. 15-24) of the egg. This contact triggers the acrosome reaction in the sperm, during which the Contents of the acrosome are discharged into the surrounding medium. In sea urchins and many other marine invertebrates, the release of acrosomal contents is accompanied by The formation of a long, Actin-rich acrosomal process that extends from the anterior tip of the sperm. As shown in Fig. 15-41, the tip of this process is coated with Components of the former acrosomal membrane as well as acrosomal contents, which include: (1) hydrolytic Enzymes that enable the sperm to digest its way through the egg jelly coat and reach the vitelline layer; (2) specific Proteins that mediate the binding of the tip of the process to the vitelline layer (see below); and (3) hydrolytic enzymes that allow the acrosomal process to clear a path through this layer to the egg Cell/33.html">Plasma Membrane. Upon contact, the membrane at the tip of the acrosomal process fuses with the egg plasma membrane, allowing the sperm nucleus to enter the egg (Fig. 15-41).

In sea urchin sperm, the acrosome reaction is induced by a polysaccharide component (a sulfated fucose polymer) of the egg jelly coat: if this substance is isolated from eggs and added to sperm, it triggers a normal acrosome reaction within seconds. The jelly coat polysaccharide binds to a glycoprotein receptor on the sperm plasma membrane, causing the membrane to depolarize; this depolarization presumably opens Ca²⁺ channels in the membrane, allowing Ca²⁺ ions to flow into the sperm. At the same time, the jelly polysaccharide activates plasma membrane proton pumps in the sperm, which drive the efflux of H⁺ ions in exchange for Na⁺. The resulting rise in pH within the sperm head and the increase in cytosolic Ca²⁺ concentration trigger the acrosome reaction. It has been proposed that the elevation of intracellular pH causes unpolymerized actin in the sperm Cytoplasm to dissociate from specialized proteins that normally bind actin and block its polymerization (see Section 11.2.12); this leads to the rapid Formation of the acrosomal process via explosive actin polymerization.

Fig. 15-41. This diagram illustrates the details of the acrosome reaction in the sea urchin. Upon contact of the sperm with the egg jelly coat, the acrosomal contents are released via exocytosis (1), followed by rapid actin polymerization, which forms a long acrosomal process that penetrates the jelly coat (2). Proteins released from the acrosome (shown as black dots) adhere to The surface of the acrosomal process and serve both to bind the sperm to the vitelline layer and to degrade this layer (3). When the former acrosome (whose membrane now forms the tip of the acrosomal process) comes into contact with the egg plasma membrane (3), the two membranes fuse, the actin filaments depolymerize, and the sperm enters the egg (4). How do sperm locate the egg for fertilization after the gametes are released into seawater? Sea urchin eggs secrete a peptide called resact, which acts as a species-specific chemoattractant for sea urchin sperm. Resact binds to a transmembrane receptor on the sperm surface, which has been identified as the enzyme guanylyl cyclase. This enzyme catalyzes the synthesis of cyclic GMP within the sperm.

However, the elongation of the acrosomal process involves more than just actin polymerization. An influx of various ions (Ca²⁺, Na⁺, and Cl^-) increases the number of osmotically active molecules in the sperm head, driving Water into The Cell. The resulting sharp rise in hydrostatic pressure likely contributes to the extension of the acrosomal process.

15.4.2. Sperm-Egg Binding Is Mediated by Species-Specific Macromolecules [20]

Species Specificity in fertilization is especially critical for aquatic animals whose eggs and sperm are released into the water, as this environment creates a constant risk of cross-fertilization with other species. In sea urchins, such specificity is manifested at the level of sperm binding to the vitelline layer underlying the jelly coat: although sperm may occasionally undergo the acrosome reaction upon contacting an egg of a different species, they fail to bind to these foreign eggs and therefore cannot fertilize them.

A substance responsible for the species-specific binding of sperm to the vitelline layer has been isolated from sea urchin sperm. This protein, termed bindin, is localized within the acrosome. Following its release during the acrosome reaction, bindin coats the surface of the acrosomal process and facilitates the attachment of the sperm to the egg. Each sea urchin species produces a distinct type of bindin, and these bindin molecules bind exclusively to the vitelline layer of eggs from the same species. Evidence indicates that the vitelline layer of a given sea urchin species contains a species-specific glycoprotein that acts as the bindin receptor during this binding process. Furthermore, because bindin can induce the fusion of artificial lipid vesicles in vitro, it is thought to act as a catalyst for the fusion of the Plasma Membranes of the acrosomal process and the egg once direct contact has been established (Section 6.5.16).

Fig. 15-42. Bindin molecules coating the surface of a sea urchin sperm acrosomal process (schematic). These molecules are believed to bind to a specific carbohydrate chain on a receptor molecule located within the egg vitelline layer.

15.4.3. Egg Activation Is Mediated by Changes in Intracellular Ion Concentrations [21]

As soon as an activated sea urchin sperm attaches to the egg, the acrosomal process rapidly clears a path through the vitelline layer. The membrane at the tip of the process fuses with The Plasma Membrane at the apex of an egg microvillus (Fig. 15-43). Neighboring microvilli rapidly elongate and cluster around the sperm; subsequently, as the microvilli gradually retract, the sperm is pulled head-first into the egg.

The sperm triggers the developmental program already established within the egg. Prior to fertilization, the egg is metabolically quiescent: it does not synthesize DNA, and RNA and Protein Synthesis proceed at very low rates. An egg that has left the Ovary and lost the support of its surrounding nurse cells will perish within hours unless rescued by a sperm. The binding of a sperm to the egg surface induces a dramatic increase in metabolic activity, DNA Synthesis, and subsequent Cleavage divisions. Yet the sperm itself serves merely as a trigger for a program already encoded within the egg; it is not strictly essential for the process itself, as an egg can be activated by a variety of nonspecific chemical or physical stimuli. For instance, the egg of a frog can be effectively stimulated by a mere prick with a needle. (The Development of an egg activated in the absence of sperm is termed parthenogenesis; numerous organisms, including certain vertebrates, reproduce routinely via parthenogenesis.) The Initial Stages of egg activation cannot depend on the synthesis of new proteins, as they proceed flawlessly in the presence of poisons that inhibit protein synthesis.

In sea urchins, all early events of egg activation are triggered by shifts in intracellular ion concentrations. Within the first few seconds or minutes after sperm are added to an egg suspension, three distinct ionic shifts occur: (1) an increase in plasma membrane permeability to Na⁺ causes membrane depolarization within seconds; (2) a massive release of Ca²⁺ ions from internal calcium stores (Section 11.6.3) leads to a dramatic rise in cytosolic Ca²⁺ concentration over the course of about 10 s; and (3) within 60 s, an H⁺ efflux coupled to Na⁺ uptake begins, resulting in a substantial elevation of intracellular pH (Fig. 15-44). As described below, these ionic shifts serve two vital physiological Functions: first, they render the egg refractory to penetration by additional sperm (polyspermy block), and second, they initiate the early steps of the developmental program.

Fig. 15-43. An electron micrograph capturing a moment during the fertilization of a sea urchin egg by a sperm. The membrane at the tip of the acrosomal process has fused with the egg plasma membrane at the apex of one of its surface microvilli. An unfertilized sea urchin egg is covered by more than 100,000 microvilli. (Courtesy of Frank Collins.)

Fig. 15-44. The two ionic shifts responsible for activating a sea urchin egg following fertilization. Approximately 10 s post-fertilization, Ca²⁺ ions are released from intracellular stores into the Cytosol; their concentration returns to a level slightly above that of the unfertilized egg within 2.5 minutes. Roughly 60 s later, an H⁺ extrusion mechanism coupled to Na⁺ influx is activated, leading to a sustained elevation of intracellular pH.

15.4.4. Depolarization of the Egg Plasma Membrane Provides a Fast Block to Polyspermy [22]

Although numerous sperm may bind to an egg, normally only a single sperm fuses with the egg plasma membrane and delivers its nucleus into the cytoplasm. If two or more sperm were to fuse with the egg (a condition known as polyspermy), extra mitotic spindles would form, resulting in aberrant chromosome segregation during cleavage divisions. Such events generate non-diploid cells, and embryonic development quickly arrests. Consequently, eggs surrounded by large numbers of sperm must rapidly erect a barrier against The entry of extra sperm once fertilization has begun. The Mechanism of this rapid block to polyspermy varies among species.

In fish, the egg coats feature a narrow channel called the micropyle through which sperm must pass single file; the passage of the first sperm stimulates the egg to release cortical granule contents that plug the opening, blocking the entry of subsequent sperm. However, the eggs of most organisms lack a micropyle and can fuse with a sperm anywhere across their surface. In the eggs of certain animals (such as sea urchins and amphibians), polyspermy is prevented by a rapid depolarization of the plasma membrane following fusion with the first sperm. The resting Membrane Potential of an unfertilized sea urchin egg is about -60 mV. Within seconds of sperm contact, the membrane potential sharply plummets and reverses sign, reaching approximately +20 mV before gradually returning toward baseline after about a minute (Fig. 15-45). If this depolarization—driven primarily by Na⁺ influx into the egg upon sperm contact—is prevented by performing fertilization in a low-Na⁺ medium, the incidence of polyspermy increases dramatically. Furthermore, if an unfertilized egg is artificially depolarized by passing a current through microelectrodes, sperm can still bind to the egg surface but are unable to fuse; if the membrane is subsequently repolarized, the bound sperm will rapidly fuse with the egg and penetrate it. Although the molecular mechanism remains unclear, it is likely that the fertilization potential (depolarization) alters the conformation of a critical egg plasma membrane protein, rendering it incompetent to support sperm-membrane fusion.

Fig. 15-45. Changes in the sea urchin egg membrane potential following fertilization. The rapid depolarization somehow prevents other sperm from fusing with the egg, thus providing a fast block to polyspermy.

The egg plasma membrane potential returns to normal within a few minutes after fertilization; therefore, another mechanism is required to prevent polyspermy over a longer period. In most eggs (including mammalian eggs), this barrier is created by substances released from cortical granules located immediately beneath the egg plasma membrane.

15.4.5. The cortical reaction is responsible for the late block to polyspermy [23]

Sea urchin egg cortical granules fuse with the plasma membrane and release their contents 10–50 s after the sperm contacts the egg. The cortical reaction is triggered by a sharp increase in the concentration of free Ca²⁺ ions in the cytosol. In the activated sea urchin egg, the Ca²⁺ concentration increases approximately one hundredfold less than a minute after sperm attachment, and then declines back to baseline levels within one to two minutes (Fig. 15-46). The Role of Ca²⁺ ions in triggering the cortical reaction can be directly demonstrated in experiments with isolated sea urchin egg plasma membranes; cortical granules remain attached to the inner surface of such membranes, and when a small amount of Ca²⁺ is added to this preparation, exocytosis occurs within a few seconds.

In sea urchin eggs, the cortical reaction leads to at least two independent consequences: 1) Proteolytic Enzymes released from the cortical granules rapidly destroy the bindin receptors of the vitelline layer responsible for sperm binding, and 2) the released contents of the cortical granules cause the vitelline layer, previously adjacent to the plasma membrane, to separate from the egg surface, while enzymes cross-link the proteins of this layer, making it more rigid. As a result of these events, a fertilization envelope is formed, to which sperm cannot bind and through which they cannot penetrate (Fig. 15-47).

Fig. 15-46. Cortical granules attached to the isolated membrane of an unfertilized sea urchin egg (scanning electron micrograph). Upon addition of Ca²⁺ ions to this preparation, the cortical granules fuse with the plasma membrane and release their contents via exocytosis. Since each cell contains about 15,000 cortical granules, the cortical reaction more than doubles the egg surface area in less than a minute; some of the additional membrane material goes toward elongating microvilli across the entire egg surface, while the rest enters coated pits and vesicles (V. D. Vacquier, Dev. Biol., 43, 62-74, 1975.)

Fig. 15-47. This diagram illustrates how the sea urchin egg cortical reaction prevents the entry of additional sperm. The released contents of the cortical granules cause the space beneath the vitelline layer to expand and modify this layer so that bindin receptors disappear, turning it into a "fertilization envelope" that sperm cannot penetrate. This "hardening" of the vitelline layer occurs primarily through the formation of covalent cross-links between Tyrosine side chains of proteins, resulting in an extensive insoluble protein network.

15.4.6. Egg activation is mediated by the Inositol phospholipid signaling pathway [24]

Membrane depolarization is the first detectable change following fertilization, but it appears to be required solely for the Prevention of polyspermy. Artificial membrane depolarization does not lead to egg activation; conversely, preventing membrane depolarization during fertilization does not inhibit activation.

There is compelling evidence that the egg development program is triggered by a transient rise in cytosolic Ca²⁺ concentration, which propagates as a circular wave from the site of sperm entry across the entire egg (see Fig. 4-35). Cytosolic Ca²⁺ concentration can be raised artificially either by direct microinjection of these ions into the egg or by using Ca²⁺-carrying ionophores such as A23187 (see Section 6.4.19). This approach successfully activates the eggs of all animals studied to date, including mammals. Conversely, if the rise in Ca²⁺ concentration is prevented by introducing the calcium-chelating agent EGTA, fertilization-induced activation fails to occur. Ca²⁺ ions exert their intracellular effects through at least one key mechanism: they bind to the Ca²⁺-binding protein calmodulin, which in turn activates A wide variety of cellular proteins (Section 12.4.3). Calmodulin has been found in large amounts in all egg cells examined.

How does fertilization lead to an increase in cytosolic Ca²⁺ ion concentration within the egg? As discussed in Chapter 12, extracellular ligands binding to cell-surface receptor proteins stimulate the Hydrolysis of phosphatidylinositol bisphosphate (PIP₂) in the plasma membrane, yielding inositol trisphosphate (InsP₃) and diacylglycerol. InsP₃, in turn, triggers the release of Ca²⁺ from intracellular stores (Section 12.3.9) into the cytosol, whereas diacylglycerol activates protein kinase C (Section 12.3.10). Experiments on sea urchins support the hypothesis that this same mechanism is responsible for the elevation of cytosolic Ca²⁺ levels during fertilization. The concentration of InsP₃ rises within seconds of fertilization, immediately followed by an increase in cytosolic Ca²⁺; moreover, microinjection of InsP₃ into an unfertilized egg raises cytosolic Ca²⁺ and activates the egg. As expected, sperm activation is mediated by a G protein that stimulates a specific phospholipase C, which catalyzes this hydrolysis (see Section 12.3.9). However, it remains unclear whether the sperm binds to a plasma membrane receptor on the egg

that is functionally coupled to phospholipase C via a G protein, or whether it delivers a G protein activator directly into the egg upon membrane fusion.

Because the rise in cytosolic Ca²⁺ concentration following fertilization is short-lived (lasting roughly 1 minute), it clearly cannot directly drive events occurring at later stages of egg activation. In sea urchins, these later events include an acceleration of protein synthesis starting about 8 minutes post-fertilization and the initiation of DNA synthesis roughly 30 minutes later. A wealth of data indicates that the activation of protein kinase C plays a crucial role in these later stages, with an increase in intracellular pH being particularly essential during this period.

15.4.7. In some organisms, the late biosynthetic processes associated with egg activation are induced by an increase in intracellular pH [25]

In sea urchins, the diacylglycerol-mediated activation of protein kinase C leads to the activation (primarily via phosphorylation) of the Na⁺–H⁺ exchanger in the egg plasma membrane. This membrane transport protein utilizes the energy stored in the transmembrane Na⁺ ion gradient to pump H⁺ ions out of the cell (see Section 6.4.10). This H⁺ efflux causes the intracellular pH to rise from 6.6 to 7.2, a level that is maintained throughout subsequent zygote development (see Fig. 15-47). The unusually low intracellular pH of unfertilized sea urchin eggs is thought to be the primary factor maintaining the egg in a metabolically quiescent state; furthermore, strong evidence indicates that the post-fertilization rise in pH is precisely what triggers late biosynthetic activity in fertilized sea urchin eggs. First, if unfertilized eggs are artificially alkalinized by incubation in an ammonia-containing medium (Fig. 15-48), both PROTEIN SYNTHESIS AND METABOLISM/36.html">DNA Replication are markedly stimulated even in the absence of an elevated free cytosolic Ca²⁺ concentration. Second, if eggs are placed in Ca²⁺-free seawater immediately after fertilization (thereby abolishing the Na⁺ gradient required to pump out H⁺ ions), the intracellular pH fails to rise, and subsequent late activation events do not occur. Such eggs can be rescued by adding ammonia to the medium, which increases intracellular pH and induces protein and DNA synthesis even in the absence of extracellular Na⁺.

Fig. 15-48. Elevation of intracellular pH upon incubation of cells (e.g., eggs) in an ammonia-containing medium. Ammonia diffuses across the plasma membrane and reacts with intracellular H⁺ ions to form NH⁺₄, resulting in a drop in intracellular H⁺ concentration and a corresponding rise in pH.

De novo RNA Synthesis is not required for the marked upregulation of Protein synthesis in fertilized eggs, as this phenomenon still occurs in the presence of actinomycin D, an inhibitor of RNA Transcription. It is generally believed that Protein synthesis is normally enhanced through at least two independent mechanisms: 1) previously stored maternal mRNA molecules become accessible for Translation; and 2) Ribosomes are activated, allowing them to translate mRNA more rapidly. In contrast, the acceleration of protein synthesis in unfertilized eggs treated with ammonia results solely from the enhanced mobilization of existing mRNA transcripts. While mRNA mobilization is likely a consequence of the elevated intracellular pH, ribosome translocation along mRNA templates appears to be accelerated by a different, yet unidentified factor. The detailed mechanisms underlying these Two Types of activation remain to be elucidated.

Although fertilization is a uniquely specialized phenomenon, it employs the same Intracellular Signaling pathways that regulate cellular processes in somatic cells (see Chapter 12). The temporal sequence of several key events following the fertilization of sea urchin eggs is summarized in Table 15-1.

Table 15-1. Sequence of Events Following Sea Urchin Egg Fertilization

15.4.8. The fusion of sperm and sea urchin egg pronuclei involves centrioles introduced by the sperm [26]

A fertilized egg cell is called a zygote. In most species, including sea urchins, fertilization is only complete once the haploid nuclei (known as pronuclei) of the two gametes fuse. Because egg cells are relatively large, the sperm and egg pronuclei must travel significant distances before they can unite. Unsurprisingly, this directed movement relies heavily on the Cytoskeleton.

A sea urchin sperm contributes more than just its DNA to the zygote—it also delivers two centrioles. These paternal centrioles play a critically important role during fertilization because the egg loses its own centrioles during the final meiotic division. The sperm centrioles establish a center from which microtubule arrays radiate (a structure known as the sperm aster), which appears to guide the male pronucleus toward the female pronucleus; treating the cells with colchicine to depolymerize microtubules halts pronuclear migration entirely. Once the two approaching pronuclei make contact, their membranes fuse to form the diploid zygote nucleus. The two sperm centrioles, accompanied by their asters, then duplicate to form the two poles of the mitotic spindle for the first cleavage division.

15.4.9. Mammalian eggs can be fertilized in vitro [27]

Studying mammalian eggs is immeasurably more difficult than studying sea urchin eggs. While researchers have millions of sea urchin eggs at their disposal, they must make do with mere dozens or hundreds when working with mammalian ova. Nevertheless, it is now possible to fertilize these eggs in vitro. (Although we will continue to use the term "egg", it should be noted that in mammals we are actually dealing with a fertilized secondary oocyte—see Section 15.3.7.) This approach yields profound practical benefits: mammalian zygotes transferred to the Uterus can develop into normal offspring, a technique that has allowed many infertile women to bear healthy children. In vitro fertilization of mammalian eggs also provides a powerful system for investigating the underlying mechanisms of fertilization.

Such studies have revealed that while the overall sequence of events in mammalian and sea urchin fertilization is broadly similar, individual steps can differ significantly.

Some differences involve the sperm itself. Mammalian spermatozoa are incapable of fertilizing an egg until they undergo a process called capacitation, which is induced by secretions within the female reproductive tract. The exact mechanism of capacitation remains unclear; it likely involves changes in the lipid and glycoprotein COMPOSITION OF THE sperm plasma membrane, as well as an increase in metabolic rate and motility. A capacitated mouse sperm penetrates the thick layer of follicular cells and binds specifically to one of the major Glycoproteins of the zona pellucida—a protective coat functionally equivalent to the sea urchin vitelline layer (see Section 15.3.2). In at least several species, this same egg glycoprotein appears to trigger the acrosome reaction in the sperm. In mice, for instance, the zona pellucida consists exclusively of glycoproteins synthesized by growing oocytes, which self-assemble into an interconnected, three-dimensional fibrous meshwork (see Fig. 15-23). One of these glycoproteins, designated ZP3, is responsible for sperm binding and for initiating the acrosome reaction. It is believed that upon binding to the zona pellucida, the spermatozoon recognizes a specific carbohydrate sequence on the ZP3 glycoprotein. The molecules mediating this recognition are localized within the sperm plasma membrane, whereas in sea urchins they reside in the acrosomal membrane. Like the vitelline layer of sea urchin eggs, the zona pellucida acts as a barrier to interspecies fertilization; removing it often abolishes this barrier. For example, hamster eggs stripped of their zona pellucida using specific Enzymes can be fertilized by human spermatozoa. Unsurprisingly, such hybrid embryos (often called "humsters") fail to develop.

Fig. 15-49. The course of the acrosome reaction during mammalian fertilization. In mice, the same zona pellucida glycoprotein is believed to be responsible for both sperm binding and the Induction of the acrosome reaction. Note that the mammalian sperm approaches the egg plasma membrane tangentially, meaning that membrane fusion occurs along the lateral surface of the sperm head rather than at its apex. In mice, the zona pellucida is approximately 7 μm thick, and the sperm traverses this barrier at a speed of about 1 μM/min.

During the acrosome reaction, mammalian sperm release proteases and hyaluronidase, which play a crucial role in helping the sperm penetrate the zona pellucida. However, unlike sea urchins, mammalian sperm do not form an elongated acrosomal process; in most mammalian species, the equatorial region of the plasma membrane (located just behind the acrosome) fuses with the egg membrane first, rather than the acrosomal membrane itself (Fig. 15-49). Because relatively few mammalian sperm successfully reach the ovulated egg (fewer than 200 out of 3 ∙ 10⁸ human spermatozoa released during coitus reach the site of fertilization), a rapid block to polyspermy is not strictly necessary: mammals lack the swift plasma membrane depolarization that accompanies fertilization and prevents polyspermy in sea urchins and amphibians. On the other hand, enzymes released during the cortical reaction of mammalian eggs modify the extracellular coat to establish a slow block to polyspermy. In mouse eggs, for instance, an enzyme alters the ZP3 glycoprotein such that it can no longer bind sperm or induce the acrosome reaction. In the eggs of certain other mammals, the cortical reaction alters the plasma membrane rather than the zona pellucida, thereby preventing the fusion of additional spermatozoa.

Another distinctive feature of mammalian fertilization is that centrioles are contributed by the egg rather than the sperm. Furthermore, in fertilized mammalian eggs, the two pronuclei do not fuse directly; instead, they migrate toward each other, but their Chromosomes do not comingle until the nuclear envelope of each pronucleus breaks down in preparation for the first cleavage division. Most of the other early events of sea urchin fertilization listed in Table 15-1 (such as phosphatidylinositol bisphosphate hydrolysis and the elevation of cytosolic Ca²⁺ concentrations) also occur in mammalian eggs. The subsequent changes mark the onset of Embryogenesis, during which the zygote develops into a complete organism. Embryogenesis is arguably the most astonishing phenomenon in all of biology—a process that forms the subject of the next chapter.

Summary

Fertilization begins the moment a sperm head makes contact with the protective extracellular coat surrounding the egg. This contact triggers the acrosome reaction in the sperm, releasing its enzymatic contents; several of these liberated proteins facilitate sperm passage through the egg coats, ultimately enabling the fusion of the sperm and egg plasma membranes. Fertilization triggers dramatic changes within the egg itself, initiated by the hydrolysis of phosphatidylinositol bisphosphate located in the egg's plasma membrane. Egg activation alters the cell surface to prevent the entry of additional sperm: one component of this polyspermy block results from the cortical reaction, in which the contents of cortical granules are released to restructure the extracellular coat. In addition, internal changes prepare the zygote, once the parental pronuclei have united, to embark upon its developmental program.

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