The cortical granule reaction in living eggs of the toad, Xenopus laevis

Ion Channel Function During Oocyte Maturation and Fertilization

The proper maturation of both male and female gametes is essential for supporting fertilization and the early embryonic divisions. In the ovary, immature fully-grown oocytes that are arrested in prophase I of meiosis I are not able to support fertilization. Acquiring fertilization competence requires resumption of meiosis which encompasses the remodeling of multiple signaling pathways and the reorganization of cellular organelles. Collectively, this differentiation endows the egg with the ability to activate at fertilization and to promote the egg-to-embryo transition. Oocyte maturation is associated with changes in the electrical properties of the plasma membrane and alterations in the function and distribution of ion channels. Therefore, variations on the pattern of expression, distribution, and function of ion channels and transporters during oocyte maturation are fundamental to reproductive success. Ion channels and transporters are important in regulating membrane potential, but also in the case of calcium (Ca2+), they play a critical role in modulating intracellular signaling pathways. In the context of fertilization, Ca2+ has been shown to be the universal activator of development at fertilization, playing a central role in early events associated with egg activation and the egg-to-embryo transition. These early events include the block of polyspermy, the completion of meiosis and the transition to the embryonic mitotic divisions. In this review, we discuss the role of ion channels during oocyte maturation, fertilization and early embryonic development. We will describe how ion channel studies in Xenopus oocytes, an extensively studied model of oocyte maturation, translate into a greater understanding of the role of ion channels in mammalian oocyte physiology.

The Ca2+-activated Cl- channel Ano1 controls microvilli length and membrane surface area in the oocyte

Ca(2+)-activated Cl(-) channels (CaCCs) play important physiological functions in epithelia and other tissues. In frog oocytes the CaCC Ano1 regulates resting membrane potential and the block to polyspermy. Here, we show that Ano1 expression increases the oocyte surface, revealing a novel function for Ano1 in regulating cell morphology. Confocal imaging shows that Ano1 increases microvilli length, which requires ERM-protein-dependent linkage to the cytoskeleton. A dominant-negative form of the ERM protein moesin precludes the Ano1-dependent increase in membrane area. Furthermore, both full-length and the truncated dominant-negative forms of moesin co-localize with Ano1 to the microvilli, and the two proteins co-immunoprecipitate. The Ano1-moesin interaction limits Ano1 lateral membrane mobility and contributes to microvilli scaffolding, therefore stabilizing larger membrane structures. Collectively, these results reveal a newly identified role for Ano1 in shaping the plasma membrane during oogenesis, with broad implications for the regulation of microvilli in epithelia.

Behaviour of cytoplasmic organelles and cytoskeleton during oocyte maturation

Assisted reproduction technology (ART) has become an attractive option for infertility treatment and holds tremendous promise. However, at present, there is still room for improvement in its success rates. Oocyte maturation is a process by which the oocyte becomes competent for fertilization and subsequent embryo development. To better understand the mechanism underlying oocyte maturation and for the future improvement of assisted reproduction technology, this review focuses on the complex processes of cytoplasmic organelles and the dynamic alterations of the cytoskeleton that occur during oocyte maturation. Ovarian stimulation and in-vitro maturation are the major techniques used in assisted reproduction technology and their influence on the organelles of oocytes is also discussed. Since the first birth by assisted reproduction treatment was achieved in 1978, numerous techniques involved in assisted reproduction have been developed and have become attractive options for infertility treatment. However, the unsatisfactory success rate remains as a main challenge. Oocyte maturation is a process by which the oocyte becomes competent for fertilization and subsequent embryo development. Oocyte maturation includes both nuclear and cytoplasmic maturation. Nuclear maturation primarily involves chromosomal segregation, which has been well studied, whereas cytoplasmic maturation involves a series of complicated processes, and there are still many parts of this process that remain controversial. Ovarian stimulation and in-vitro maturation (IVM) are the major techniques of assisted reproduction. The effect of ovarian stimulation or IVM on the behaviour of cell organelles of the oocyte has been postulated as the reason for the reduced developmental potential of in-vitro-produced embryos. To further understanding of the mechanism of oocyte maturation and future improvement of assisted reproduction treatment, the complex events of cytoplasmic organelles and the cytoskeleton that occur during oocyte maturation and the influence of ovarian stimulation and IVM on these organelles are described in this review.

How to make a good egg!: The need for remodeling of oocyte Ca(2+) signaling to mediate the egg-to-embryo transition

The egg-to-embryo transition marks the initiation of multicellular organismal development and is mediated by a specialized Ca(2+) transient at fertilization. This explosive Ca(2+) signal has captured the interest and imagination of scientists for many decades, given its cataclysmic nature and necessity for the egg-to-embryo transition. Learning how the egg acquires the competency to generate this Ca(2+) transient at fertilization is essential to our understanding of the mechanisms controlling egg and the transition to embryogenesis. In this review we discuss our current knowledge of how Ca(2+) signaling pathways remodel during oocyte maturation in preparation for fertilization with a special emphasis on the frog oocyte as additional reviews in this issue will touch on this in other species.

Myosin-1c couples assembling actin to membranes to drive compensatory endocytosis

Compensatory endocytosis follows regulated exocytosis in cells ranging from eggs to neurons, but the means by which it is accomplished are unclear. In Xenopus eggs, compensatory endocytosis is driven by dynamic coats of assembling actin that surround and compress exocytosing cortical granules (CGs). We have identified Xenopus laevis myosin-1c (XlMyo1c) as a myosin that is upregulated by polyadenylation during meiotic maturation, the developmental interval that prepares eggs for fertilization and regulated CG exocytosis. Upon calcium-induced exocytosis, XlMyo1c is recruited to exocytosing CG membranes where actin coats then assemble. When XlMyo1c function is disrupted, actin coats assemble, but dynamic actin filaments are uncoupled from the exocytosing CG membranes such that coats do not compress, and compensatory endocytosis fails. Remarkably, there is also an increase in polymerized actin at membranes throughout the cell. We conclude that XlMyo1c couples polymerizing actin to membranes and so mediates force production during compensatory endocytosis.

Ca2+ signaling differentiation during oocyte maturation

Oocyte maturation is an essential cellular differentiation pathway that prepares the egg for activation at fertilization leading to the initiation of embryogenesis. An integral attribute of oocyte maturation is the remodeling of Ca2+ signaling pathways endowing the egg with the capacity to produce a specialized Ca2+ transient at fertilization that is necessary and sufficient for egg activation. Consequently, mechanistic elucidation of Ca2+ signaling differentiation during oocyte maturation is fundamental to our understanding of egg activation, and offers a glimpse into Ca2+ signaling regulation during the cell cycle.

Mammalian membrane block to polyspermy: new insights into how mammalian eggs prevent fertilisation by multiple sperm

To inhibit fertilisation by more than one sperm (a condition known as polyspermy), eggs have developed preventative mechanisms known as blocks to polyspermy. The block at the level of the egg extracellular coat (the zona pellucida in mammals, the vitelline envelope in non-mammals) has been well characterised in many different animal species and the block at the level of the egg plasma membrane is understood in some non-mammalian species. However, virtually nothing is known about the membrane block to polyspermy in mammalian eggs, despite data dating back 50–90 years that provide evidence for its existence. In the present review, we will discuss the background on blocks to polyspermy used by animal eggs and then focus on the membrane block to polyspermy in mammalian eggs. This will include a summary of classical studies that provide evidence for this block in mammalian eggs, assays used to study the mammalian membrane block and what has been elucidated from recent experimental studies about the cellular signalling events that lead to membrane block establishment and the mechanism of how the membrane block may prevent additional fertilisation.

Calcium signaling differentiation during Xenopus oocyte maturation

Ca(2+) is the universal signal for egg activation at fertilization in all sexually reproducing species. The Ca(2+) signal at fertilization is necessary for egg activation and exhibits specialized spatial and temporal dynamics. Eggs acquire the ability to produce the fertilization-specific Ca(2+) signal during oocyte maturation. However, the mechanisms regulating Ca(2+) signaling differentiation during oocyte maturation remain largely unknown. At fertilization, Xenopus eggs produce a cytoplasmic Ca(2+) (Ca(2+)(cyt)) rise that lasts for several minutes, and is required for egg activation. Here, we show that during oocyte maturation Ca(2+) transport effectors are tightly modulated. The plasma membrane Ca(2+) ATPase (PMCA) is completely internalized during maturation, and is therefore unable to extrude Ca(2+) out of the cell. Furthermore, IP(3)-dependent Ca(2+) release is required for the sustained Ca(2+)(cyt) rise in eggs, showing that Ca(2+) that is pumped into the ER leaks back out through IP(3) receptors. This apparent futile cycle allows eggs to maintain elevated cytoplasmic Ca(2+) despite the limited available Ca(2+) in intracellular stores. Therefore, Ca(2+) signaling differentiates in a highly orchestrated fashion during Xenopus oocyte maturation endowing the egg with the capacity to produce a sustained Ca(2+)(cyt) transient at fertilization, which defines the egg's competence to activate and initiate embryonic development.

The biology of cortical granules

An egg-that took weeks to months to make in the adult-can be extraordinarily transformed within minutes during its fertilization. This review will focus on the molecular biology of the specialized secretory vesicles of fertilization, the cortical granules. We will discuss their role in the fertilization process, their contents, how they are made, and the molecular mechanisms that regulate their secretion at fertilization. This population of secretory vesicles has inherent interest for our understanding of the fertilization process. In addition, they have import because they enhance our understanding of the basic processes of secretory vesicle construction and regulation, since oocytes across species utilize this vesicle type. Here, we examine diverse animals in a comparative approach to help us understand how these vesicles function throughout phylogeny and to establish conserved themes of function.

Induction of maturation-promoting factor during Xenopus oocyte maturation uncouples Ca(2+) store depletion from store-operated Ca(2+) entry

During oocyte maturation, eggs acquire the ability to generate specialized Ca(2+) signals in response to sperm entry. Such Ca(2+) signals are crucial for egg activation and the initiation of embryonic development. We examined the regulation during Xenopus oocyte maturation of store-operated Ca(2+) entry (SOCE), an important Ca(2+) influx pathway in oocytes and other nonexcitable cells. We have previously shown that SOCE inactivates during Xenopus oocyte meiosis. SOCE inactivation may be important in preventing premature egg activation. In this study, we investigated the correlation between SOCE inactivation and the Mos-mitogen-activated protein kinase (MAPK)-maturation-promoting factor (MPF) kinase cascade, which drives Xenopus oocyte maturation. SOCE inactivation at germinal vesicle breakdown coincides with an increase in the levels of MAPK and MPF. By differentially inducing Mos, MAPK, and MPF, we demonstrate that the activation of MPF is necessary for SOCE inactivation during oocyte maturation. In contrast, sustained high levels of Mos kinase and the MAPK cascade have no effect on SOCE activation. We further show that preactivated SOCE is not inactivated by MPF, suggesting that MPF does not block Ca(2+) influx through SOCE channels, but rather inhibits coupling between store depletion and SOCE activation.

Separase is required for chromosome segregation during meiosis I in Caenorhabditis elegans

BACKGROUND

Chromosome segregation during mitosis and meiosis is triggered by dissolution of sister chromatid cohesion, which is mediated by the cohesin complex. Mitotic sister chromatid disjunction requires that cohesion be lost along the entire length of chromosomes, whereas homolog segregation at meiosis I only requires loss of cohesion along chromosome arms. During animal cell mitosis, cohesin is lost in two steps. A nonproteolytic mechanism removes cohesin along chromosome arms during prophase, while the proteolytic cleavage of cohesin's Scc1 subunit by separase removes centromeric cohesin at anaphase. In Saccharomyces cerevisiae and Caenorhabditis elegans, meiotic sister chromatid cohesion is mediated by Rec8, a meiosis-specific variant of cohesin's Scc1 subunit. Homolog segregation in S. cerevisiae is triggered by separase-mediated cleavage of Rec8 along chromosome arms. In principle, chiasmata could be resolved proteolytically by separase or nonproteolytically using a mechanism similar to the mitotic "prophase pathway."

RESULTS

Inactivation of separase in C. elegans has little or no effect on homolog alignment on the meiosis I spindle but prevents their timely disjunction. It also interferes with chromatid separation during subsequent embryonic mitotic divisions but does not directly affect cytokinesis. Surprisingly, separase inactivation also causes osmosensitive embryos, possibly due to a defect in the extraembryonic structures, referred to as the "eggshell."

CONCLUSIONS

Separase is essential for homologous chromosome disjunction during meiosis I. Proteolytic cleavage, presumably of Rec8, might be a common trigger for the first meiotic division in eukaryotic cells. Cleavage of proteins other than REC-8 might be necessary to render the eggshell impermeable to solutes.

Glycoproteins from Bufo arenarum vitelline envelope with fertility-impairing effect on homologous spermatozoa

When spermatozoa from Bufo arenarum are incubated with molecules extracted from the vitelline envelopes of homologous oocytes, they lose their fertilizing capacity. Those molecules are glycoproteins, and the elimination of mannoside residues from them results in activity loss, while digestion of the proteic moiety did not alter their biological effect. Sepharose-concanavalin A columns were used to purify the glycoproteins, since the active fraction binds to the column. The fertility-impairing effect observed does not seem to be mediated by an acrosome reaction-inducing effect.

Fertilization-induced changes in the vitelline envelope of echinoderm and amphibian eggs: self-assembly of an extracellular matrix

The surface of the unfertilized sea urchin egg is covered by the vitelline layer (VL), a fibrous extracellular matrix that contains receptors for sperm. At fertilization, cortical granule exocytosis releases enzymes and structural proteins that cause the VL to elevate and become remodelled into the mechanically and chemically tough fertilization envelope. This envelope prevents further penetration of sperm and protects the embryo during early development. A thicker, more complex vitelline envelope surrounds the Xenopus laevis egg. This fibrous coat is also restructured at fertilization to produce an impenetrable barrier to sperm. The biochemical steps that occur during self-assembly of these fertilization envelopes are reviewed, and the ultrastructural changes that occur, as seen in platinum replicas of quick-frozen, deep-etched, and rotary-shadowed eggs, are illustrated.

Protein kinase C acts downstream of calcium at entry into the first mitotic interphase of Xenopus laevis

Transit into interphase of the first mitotic cell cycle in amphibian eggs is a process referred to as activation and is accompanied by an increase in intracellular free calcium [( Ca2+]i), which may be transduced into cytoplasmic events characteristic of interphase by protein kinase C (PKC). To investigate the respective roles of [Ca2+]i and PKC in Xenopus laevis egg activation, the calcium signal was blocked by microinjection of the calcium chelator BAPTA, or the activity of PKC was blocked by PKC inhibitors sphingosine or H7. Eggs were then challenged for activation by treatment with either calcium ionophore A23187 or the PKC activator PMA. BAPTA prevented cortical contraction, cortical granule exocytosis, and cleavage furrow formation in eggs challenged with A23187 but not with PMA. In contrast, sphingosine and H7 inhibited cortical granule exocytosis, cortical contraction, and cleavage furrow formation in eggs challenged with either A23187 or PMA. Measurement of egg [Ca2+]i with calcium-sensitive electrodes demonstrated that PMA treatment does not increase egg [Ca2+]i in BAPTA-injected eggs. Further, PMA does not increase [Ca2+]i in eggs that have not been injected with BAPTA. These results show that PKC acts downstream of the [Ca2+]i increase to induce cytoplasmic events of the first Xenopus mitotic cell cycle.

Quick-freeze, deep-etch, rotary-shadow views of the extracellular matrix and cortical cytoskeleton of Xenopus laevis eggs

The quick-freeze, deep-etch, rotary-shadow technique provides a powerful tool to study the structural dynamics of extracellular matrices. Using this technique, we show that the extracellular investments of the Xenopus laevis egg are multilayered and securely anchored to the egg surface. The cortical cytoskeleton within the egg contains embedded cortical granules with surrounding endoplasmic reticulum and is capped by a thin reticular sheet that contacts the inner surface of the plasma membrane. The extracellular matrix undergoes three distinct changes at fertilization: a) formation of a "smooth" layer below the vitelline envelope (VE), b) transformation of the VE itself to an altered VE composed of concentric fibrous sheets, and c) formation of a dense, "briar-patch"-like fertilization layer at the upper surface of the VE.

Activators of protein kinase C trigger cortical granule exocytosis, cortical contraction, and cleavage furrow formation in Xenopus laevis oocytes and eggs

Prophase I oocytes, free of follicle cells, and metaphase II eggs of the amphibian Xenopus laevis were subjected to transient treatments with the protein kinase C activators, phorbol 12-myristate 13-acetate (PMA), phorbol 12,13-didecanoate, and 1-olyeoyl-2-acetyl-sn-glycerol. In both oocytes and eggs, these treatments triggered early events of amphibian development: cortical granule exocytosis, cortical contraction, and cleavage furrow formation. Surprisingly, activation of oocytes occurred in the absence of meiotic resumption, resulting in cells with an oocytelike nucleus and interior cytoplasm, but with a zygotelike cortex. PMA-induced activation of oocytes and eggs did not require external calcium, a prerequisite for normal activation of eggs. PMA-induced activation of eggs was inhibited by retinoic acid, a known inhibitor of protein kinase C. In addition, pretreatment of eggs with retinoic acid prevented activation by mechanical stimulation and inhibited activation by calcium ionophore A23187. The results suggest that protein kinase C activation is an integral component of the Xenopus fertilization pathway.

The cortical reaction in the egg of Discoglossus pictus: a study of the changes in the endoplasmic reticulum at activation

In Discoglossus pictus previous ultrastructural observations have shown that at the animal dimple, where sperm fuse with the egg, cortical granules (CG), vacuoles, and tightly packed clusters of small cisternae are present. At fertilization the clusters open (i.e., become loose) and give rise to longer cisternae arranged in whorls and chains which migrate toward the plasma membrane. The vacuoles fuse to form cisternae and exocytose along with the CG. In the rest of the egg periphery, while exocytosis occurs, the clusters do not open as a result of activation (C. Campanella, R. Talevi, U. Atripaldi, and L. Quaglia (1986). In "Molecular and Cellular Biology of Fertilization" (J.L. Hedrick, Ed.). Plenum, New York). We have recently conducted electrophysiological studies which have detected inward currents at the dimple center, outward current at the rest of the egg surface, and an eightfold increase in [Ca2+]i which propagates from the site of activation throughout the egg (R. Nuccitelli, D. Kline, W. Busa, R. Talevi, and C. Campanella (1988). Dev. Biol. 130, 120-132). In this paper we have asked whether the anionic current and the Ca2+ increase could be causally related to the changes of the smooth endoplasmic reticulum (SER) at activation. The results obtained by activating the eggs in ion-substituted Ringers indicate that (1) the migration of cisternae is not dependent on the polarity of the activation current crossing the dimple, but is strongly impaired, together with CG exocytosis, by 5 x Cl- Ringer; (2) TMB-8, a drug which partially blocks calcium release (C. Y. Choiu and M. J. Malagodi (1975). Brit. J. Pharmacol. 53, 279-288), partially inhibits opening of cisternae clusters and the formation of an SER network in the dimple. This suggests a causal relationship between the Ca2+ rise and the cluster transformation at activation.

In vitro formation of the "S" layer, a unique component of the fertilization envelope in Xenopus laevis eggs

The extracellular matrix (ECM) of unfertilized Xenopus laevis eggs consists of an elaborate filamentous network in the perivitelline space (PS) and a thick fibrillar vitelline envelope (VE), with a thin layer of horizontal filaments (HF) separating the two. At fertilization this ECM is converted into the fertilization envelope (comprised of the fertilization (F) layer and altered VE), and a third layer, the smooth (S) layer, is formed at the upper boundary of the PS (Larabell and Chandler, 1988). In this report, we use quick-freeze, deep-etch, rotary-shadow electron microscopy to show that an intact S layer can be formed in vitro by incubation of unfertilized eggs in an exudate obtained from cortical granules. Within 5 min numerous 36-nm-diameter particles assemble in a highly ordered array at the microvillar tips. These particles appear to "melt" and to form patches of smooth material and within 10 min one continuous sheet has formed. The presence of the VE is required for formation of the S layer, and we suggest that the HF layer is the site of assembly.

The extracellular matrix of Xenopus laevis eggs: a quick-freeze, deep-etch analysis of its modification at fertilization

Eggs of the amphibian, Xenopus laevis, were quick-frozen, deep-etched, and rotary-shadowed. The structure of the extracellular matrix surrounding these eggs, including the perivitelline space and the vitelline envelope (VE), was visualized in platinum replicas by electron microscopy. The perivitelline space contains an elaborate filamentous glycocalyx which connects microvillar tips to the plasma membrane, to adjacent microvilli, and to the overlying VE. The VE is comprised of two layers, the innermost of which is a thin network of horizontal fibrils lying on the tips of the microvilli. The outermost is a thicker layer of large, cable-like fibers which twist and turn throughout the envelope. Upon fertilization, three dramatic modifications of the matrix occur. A thin sheet of smooth material, termed the smooth layer, is deposited on the tips of the microvilli and separates the egg from the overlying envelopes. The VE above is transformed from a thick band of cable-like fibers to concentric fibrous sheets, the altered VE. Finally, an ornate band of particles, corresponding to the fertilization layer in previous studies, is deposited at the altered VE/jelly interface. The altered VE and the fertilization layer comprise the fertilization envelope, which effects the structural block to polyspermy.

Lectin binding sites in the vitelline envelope of Bufo arenarum oocytes: role in fertilization

Incubation of dejellied spawned oocytes from Bufo arenarum with different lectins results in a decrease of oocyte fertility. Concanavalin A was the most effective lectin; phytohemagglutinin P and wheat germ lectin were less effective. Agglutinin from soybean was scarcely active. These lectin effects could be ascribed to a hindering of specific sites for some proteases, since the same treatment renders the oocyte vitelline envelope insensitive to spermatolysin (an essential requisite for fertilization) and to trypsin. Also in this case concanavalin A was the most effective lectin. Univalent concanavalin A was also effective in blocking the fertility of dejellied oocytes. These results indicate that the residues of alpha-D-glucose and alpha-D-mannose present in the vitelline envelope are involved in gamete interactions in Bufo arenarum. This idea is also supported by the finding that dejellied oocytes (fertilizables) have a number of binding sites for concanavalin A that is three or four orders of magnitude higher than coelomic or fertilized oocytes (both not penetrable by spermatozoa).

The wave of activation current in the Xenopus egg

A ring-shaped wave of inward current, the activation current, propagates across the Xenopus egg from the site of activation during the positive phase of the activation or fertilization potential. This activation current wave is due to an increased chloride conductance and reflects the propagated of the ionic channels responsible for the fertilization potential. These channels are present in the animal and vegetal hemispheres; however, the magnitude of the activation current is 6-7 times greater in the animal hemisphere. Outward current of a smaller magnitude and spread out over a larger area precedes and follows the inward current except at the point of activation where the current is first inward. The inward current wave is detected in all eggs activated by sperm and in eggs activated by pricking with a sharp needle, by application of the Ca2+ ionophore, A23187, and by intracellular iontophoresis of Ca2+ or inositol 1,4,5-trisphosphate. Reduction of the inward current by TMB-8, which blocks intracellular calcium release in some cells, suggests that the activation current channels are calcium sensitive and that the current wave is concomitant with a wave of increased intracellular calcium initiated by sperm-egg interaction. The wave of cortical granule exocytosis and two or more contraction waves follow the current wave.

Isolation of actin-binding protein and villin from toad oocytes

Two actin-modulating proteins have been purified from toad oocytes. A high-molecular weight protein, similar in structure and function to macrophage actin-binding protein, accounts for the isotropic actin-crosslinking activity in oocyte homogenates. A calcium-dependent activity in toad oocyte homogenates which shortens actin filaments is accounted for by a 95,000-dalton protein which resembles villin, an actin-severing and -bundling protein of avian epithelial brush borders. In the presence of high (greater than or equal to microM) calcium, this protein shortens actin filaments in a concentration-dependent fashion and stimulates filament assembly when added to monomeric actin. In the absence of calcium the protein promotes the formation of actin filament bundles. Therefore, in the toad oocyte actin can be crosslinked into a network by actin-binding protein. Calcium regulation of the actin network may be mediated by villin. These results are different from those reported in echinoderm eggs.

Cortical vesicle breakdown in fertilized eggs of Fundulus heteroclitus

A scanning and transmission electron microscope study has been made of the cortical alveoli of the egg of Fundulus heteroclitus. The study includes both unactivated eggs and fertilized eggs fixed at intervals of 1 second to 10 minutes after insemination. The alveoli appear to vary considerably in size, in contents, and in morphological aspects of their breakdown. As it undergoes dehiscence, each vesicle may form one or several openings in the egg surface; dense granules and particulate, fibrous, or membranous material, apparently in any combination, are liberated to the nascent perivitelline space. It appears that much of the excess membrane externalized during the reaction is strung out in threads and probably lost to the perivitelline space. The evidence does not suggest that the excess membrane either "dissolves" or is retrieved by the egg cytoplasm. That part of the cortical vesicle membrane which remains continuous with the oolemma gradually becomes microvillous and loses it morphological identity. Granules and particulate matter, presumably liberated from the cortical alveoli, are seen adhering to the inner surface of chorions removed from activated eggs. The micropyle appears to be sealed with similar material. Supernumerary sperm are observed inside the chorion in some instances. The cortical reaction appears to play secondary role in the prevention of polyspermy and to be somehow related to the subsequent formation of a normal embryonic blastodisc.

The jelly envelopes and fertilization of eggs of the newt, Notophthalmus viridescens

Fertilization in Notophthalmus viridescens is internal and involves passage of the sperm through five layers of egg jelly (J5-J1, from outermost to innermost), each of which is secreted by a discrete region of the oviduct. Polyspermy is normal. Passage of the sperm through the jelly and into the egg was studied by a technique of artificial insemination similar to natural insemination, in that undiluted fluid from the vas deferens was applied directly to eggs with various layers of jelly present, followed by flooding with water three to five minutes later. In general, successful fertilization increased as the number of jelly layers increased; jellyless coelomic eggs were not fertilizable. Sperm passage through the jelly and into the egg usually occurs within one to three minutes. Upon hydration of the jelly, barriers to sperm penetration develop in layers J5 and J3. Changes in the egg jelly thus seem to be involved in the restriction of polyspermy to a low level.

Induced formation of asters and cleavage furrows in oocytes of Xenopus laevis during in vitro maturation

We have previously reported that injection of purified basal bodies or sperm into unfertilized eggs of Xenopus laevis induced the formation of asters and irregular cleavage furrows. Fully grown oocytes were found to be unable to form asters or cleavage furrows. In this paper we show that the oocyte acquires the ability to form asters upon basal body injection at the time of germinal vesicle breakdown during in vitro maturation. Our evidence indicates that aster formation requires progesterone-stimulated changes in the oocyte and mixing of cytoplasm and germinal vesicle plasm. The ability of the oocyte to form cleavage furrows arises six to eight hours after germinal vesicle breakdown. We infer that some maturational change in the cell cortex occurs to enable the egg surface to furrow. Experiments on the relationship of aster formation to furrow initiation indicates that asters stimulate furrow formation. However, some furrowing could be induced without aster formation in mature oocytes and unfertilized eggs by an activation stimulus, showing that asters are not essential for cleavage initiation. The significance of these observations are discussed in the light of our current understanding of meiotic maturation, cell cleavage and aster growth.

A free calcium wave traverses the activating egg of the medaka, Oryzias latipes

Aequorin-injected eggs of the medaka (a fresh water fish) show an explosive rise in free calcium during fertilization, which is followed by a slow return to the resting level. Image intensification techniques now show a spreading wave of high free calcium during fertilization. The wave starts at the animal pole (where the sperm enters) and then traverses the egg as a shallow, roughly 20 degrees-wide band which vanishes at the antipode some minutes later. The peak free calcium concentration within this moving band is estimated to be about 30 microM (perhaps 100-1,000 times the resting level). Eggs activated by ionophore A23187 may show multiple initiation sites. The resulting multiple waves never spread through each other; rather, they fuse upon meeting so as to form spreading waves of compound origin. The fertilization wave is nearly independent of extracellular calcium because it is only slightly slowed (by perhaps 15%) in a medium containing 5 mM ethylene glycol-bis[beta-aminoethyl ether]N,N'-tetraacetic acid (EGTA) and no deliberately added calcium. It is also independent of the large cortical vesicles, which may be centrifugally displaced. Normally, however, it distinctly precedes the well-known wave of cortical vesicle exocytosis. We conclude that the fertilization wave in the medaka egg is propagated by calcium-stimulated calcium release, primarily from some internal sources other than the large cortical vesicles. A comparison of the characteristics of the exocytotic wave in the medaka with that in other eggs, particularly in echinoderm eggs, suggests that such a propagated calcium wave is a general feature of egg activation.

Isolation, physicochemical properties, and the macromolecular composition of the vitelline and fertilization envelopes from Xenopus laevis eggs

As a step toward defining in molecular terms the sperm-triggered block to polyspermy reaction established by the egg at fertilization, vitelline (VE) and fertilization (FE) envelopes were isolated from eggs of the Sounth African clawed toad Xenopus laevis and some of their physicochemical properties determined. Envelopes were isolated after lysis of the fertilized or unfertilized eggs by sieving techniques; isolated envelopes retained their in situ morphology as determined by electron microscopy. The isolated envelopes had different solubility properties and, in general, VE was more readily dissolved by aqueous solvents than FE, although both could be completely dissolved by detergents or chaotropic agents. Changes in envelope solubility correlated with the progression of the cortical reaction implicating a role for cortical granule material in modifying the solubility properties of the envelope. The VE and FE were composed of protein and carbohydrate with no lipid components detected. As determined by immunodiffusion experiments, the FE contained the same antigens as the VE plus components derived from the cortical granules and the innermost jelly layer, J. The macromolecular composition of the envelopes was determined by sodium dodecyl sulfate gel electrophoresis. The VE contained at least 11 glycoproteins with molecular weights ranging from 125 000 to less than 16 000 with two components (40 000 and 33 000) accounting for almost two-thirds of the total stainable material. The FE contained ten glycoproteins that had the same molecular weights as those in the VE. One glycoprotein component underwent a reduction in molecular weight from 77 000 to 67 500 when the VE was converted to the FE. This molecular weight change was interpreted as the probable result of limited proteolysis. In addition, the FE gel electrophoresis patterns contained macromolecular components derived from the cortical granules and jelly layer, J, consistent with the immunodiffusion experiments. These components were absent when the FE was prepared in the absence of Ca2+, suggesting a role for Ca2+ in binding the VE, cortical granules, and J components together. We concluded that the conversion of the glycoproteinaceous VE to FE at fertilization is caused by interaction of the VE with components from the cortical granules and jelly layer J. These interactions are of both a chemical and physical nature.