The cortical cytoskeleton and its role in sperm penetration of the mammalian egg

A specific flagellum beating mode for inducing fusion in mammalian fertilization and kinetics of sperm internalization

The salient phases of fertilization are gamete adhesion, membrane fusion, and internalization of the spermatozoon into the oocyte but the precise timeline and the molecular, membrane and cell mechanisms underlying these highly dynamical events are far from being established. The high motility of the spermatozoa and the unpredictable location of sperm/egg fusion dramatically hinder the use of real time imaging optical techniques that should directly provide the dynamics of cell events. Using an approach based on microfluidics technology, the sperm/egg interaction zone was imaged with the best front view, and the timeline of the fertilization events was established with an unparalleled temporal accuracy from the onset of gamete contact to full sperm DNA decondensation. It reveals that a key element of the adhesion phase to initiate fusion is the oscillatory motion of the sperm head on the oocyte plasma membrane generated by a specific flagellum-beating mode. It also shows that the incorporation of the spermatozoon head is a two steps process that includes simultaneous diving, tilt, and plasma membrane degradation of the sperm head into the oocyte and subsequent DNA decondensation.

What the sperm says and the egg hears - a tale of two proteins and more

While considerable information exists regarding the early interactions of spermatozoon and egg that lead to successful fertilization, the molecular biology of events that result in the incorporation of the spermatozoon within the cortical ooplasm is largely undefined. There is circumstantial evidence suggesting that this process involves the interactions of specific oolemmal receptors and their ligands on sperm that bear similarities to mechanisms used in phagocytosis by macrophages. We have postulated that the egg may act as a 'non-professional phagocyte' during its association with the spermatozoon. This review surveys those events, provides an historical context, and creates a paradigm for further investigation.

Cumulus cells affect distribution and function of the cytoskeleton and organelles in porcine oocytes

Mammalian oocytes grow and undergo meiosis within ovarian follicles. Oocytes are arrested at the first meiotic prophase, being controlled or influenced by follicular somatic cells. Under the influence of gonadotropins, immature oocytes resume meiosis. During meiotic progression, some cytoplasmic changes occur, so-called cytoplasmic maturation. However, porcine follicular oocytes vary greatly in developmental competence. The present review summarizes recent studies highlighting the importance of cumulus cells in maintaining the developmental ability and in reorganizing the cytoskeleton and organelles of porcine oocytes. Factors affecting wide variation of the nuclear and cytoplasmic maturation observed in the porcine oocytes are discussed. (Reprod Med Biol 2006; 5: 183-194).

Dynamics and unexpected localization of the plakin binding protein, kazrin, in mouse eggs and early embryos

The cell uses the cytoskeleton in virtually every aspect of cell survival and function. One primary function of the cytoskeleton is to connect to and stabilize intercellular junctions. To accomplish this task, microtubules, actin filaments, and intermediate filaments utilize cytolinker proteins, which physically bind the cytoskeletal filament to the core proteins of the adhesion junction. The plakin family of linker proteins have been in the spotlight recently as critical components for embryo survival and, when mutated, the cause of diseases such as muscular dystrophy and cardiomyopathies. Here, we reveal the dynamics of a recently discovered plakin binding protein, kazrin (kaz), during early mouse development. Kaz was originally found in adult tissues, primarily epidermis, linking periplakin to the plasma membrane and colocalizing with desmoplakin in desmosomes. Using reverse transcriptase-polymerase chain reaction, Western blots, and confocal microscopy, we found kaz in unfertilized eggs associated with the spindle apparatus and cytoskeletal sheets. As quickly as 5 min after egg activation, kaz relocates to a diffuse peri-spindle position, followed 20-30 min later by clear localization to the presumptive cytokinetic ring. Before the blastocyst stage of development, kaz associates with the nuclear matrix in a cell cycle-dependent manner, and also associates with the cytoplasmic actin cytoskeleton. After blastocyst formation, kaz localization and potential function(s) become highly complex as it is found associating with cell-cell junctions, the cytoskeleton, and nucleus. Postimplantation stages of development reveal that kaz retains a multifunctional, tissue-specific role as it is detected at diverse locations in various embryonic tissue types.

Membrane and cortical abnormalities in post-ovulatory aged eggs: analysis of fertilizability and establishment of the membrane block to polyspermy

Fertilization at increased times after ovulation is associated with poor reproductive outcomes. This study examines the effects of post-ovulatory ageing on egg membrane function through analyses of mouse eggs collected at 13 and 22 h post-HCG ('young' and 'aged' eggs, respectively). Experiments in which fertilized zona pellucida-free young and aged eggs are challenged with additional sperm reveal that aged eggs are less able to establish a membrane block to prevent polyspermy, since sperm penetrate 24% of fertilized aged eggs but are unable to penetrate fertilized young eggs. This is not due to a failure of aged eggs to respond to fertilization, as the extent of sperm-induced cortical granule exocytosis is similar in aged and young eggs. Post-ovulatory ageing also affects egg membrane receptivity to sperm as a subset of zona pellucida-free aged eggs are slow to fertilize or resistant to fertilization. Sperm binding to young and aged eggs is similar, but aged eggs develop cytoskeletal abnormalities that may affect membrane/cortical function, such as the ability of the egg membrane to support sperm-egg fusion. These data demonstrate that the poor reproductive outcomes associated with post-ovulatory ageing could be a result of reduced fertilization, due to reduced egg membrane receptivity to sperm, or a result of increased incidence of polyspermy, due to the reduced ability to establish a membrane block to polyspermy. This analysis of egg membrane function deficiencies provides insights into post-ovulatory ageing and has implications for assisted reproductive technologies.

Cytochalasin-D retards sperm incorporation deep into the egg cytoplasm but not membrane fusion with the egg plasma membrane

The fertilization process is impaired when spermatozoa are previously incubated with Cytochalasin-D (Cyt-D). Although this fact reveals the participation of polymerized actin in fertilization, the specific event obstructed by Cyt-D treatment has not been determined. To identify this event, we capacitated guinea pig spermatozoa in minimal capacitating medium with pyruvate and lactate (MCM-PL) with Cyt-D, to inseminate hamster zona pellucida (ZP)-free eggs. Cyt-D (70 microM) decreased F-actin relative concentration in capacitated spermatozoa to a larger extent than in spermatozoa incubated under control conditions. Cyt-D also cancelled the F-actin increase normally observed in acrosome-reacted cells, and decreased the number of these cells with normal F-actin localization at the equatorial zone. Insemination of eggs with Cyt-D treated spermatozoa did not change early fertilization events such as the egg cortical reaction (CR), membranes fusion, and egg F-actin new localization, but clearly retarded, by 16 hr, spermatozoa incorporation deep into the egg cytoplasm, and decondensation of egg metaphase II chromosomes. These results show that actin polymerization is necessary for spermatozoa incorporation deep into the egg cytoplasm, but not for plasma membrane fusion nor egg activation early steps.

Involvement of Calcium Signaling and the Actin Cytoskeleton in the Membrane Block to Polyspermy in Mouse Eggs1

This study examines the effects of actin microfilament-disrupting drugs on events of fertilization, with emphasis on gamete membrane interactions. Mouse eggs, freed of their zonae pellucidae, were treated with drugs that perturb the actin cytoskeleton by different mechanisms (cytochalasin B, cytochalasin D, jasplakinolide, latrunculin B) and then inseminated. Cytochalasin B, jasplakinolide, and latrunculin B treatments resulted in a decrease in the percentage of eggs fertilized and the average number of sperm fused per egg. However, cytochalasin D treatment resulted in an increase in the average number of sperm fused per egg and the percentage of polyspermic eggs. This increase in polyspermy occurred despite the observation that cytochalasin D treatment caused a decrease in sperm-egg binding and did not affect spontaneous acrosome reactions or sperm motility. This suggested that cytochalasin D-treated eggs had an impaired ability to establish a block to polyspermy at the level of the plasma membrane. The effect of cytochalasin D on the block to polyspermy was not due to a general disruption of egg activation because sperm-induced calcium oscillations and cortical granule exocytosis were similar in cytochalasin D-treated and control eggs. However, buffering of intracellular calcium levels with the calcium chelator BAPTA-AM resulted in an increase in polyspermy. Together, these data suggest that a postfertilization decrease in egg membrane receptivity to sperm requires functions of the egg actin cytoskeleton that are disrupted by cytochalasin D. Furthermore, egg activation-associated increased intracellular calcium levels are necessary but not sufficient to affect postfertilization membrane dynamics that contribute to a membrane block to polyspermy.

Molecular and biochemical regulation of early mammalian development

Fertilization initiates a rapid series of changes that restructures the egg into the zygote and initiates the program of early development. These changes in the cell occur while the genetic complement of the egg and sperm are in a highly condensed state and unable to participate in transcription. The egg cytoplasm, formed by the maternal genome, contains the necessary components that mediate the early restructuring of egg into zygote. These changes are mediated by a series of cytoplasmic signal transduction events initiated by the rise in [Ca2+]i caused when the sperm penetrates the egg. The structural changes that the egg undergoes are rapid and result in the extensive remodeling of this specialized cell. Protein kinase C (PKC) and calcium/calmodulin-dependent protein kinase II (CaM KII) are two pivotal signaling agents that mediate several of these rapid modifications in cell structure. Studies indicate the meiotic spindle serves as an architectural element in the egg that acts to colocalize elements from several of the key signaling pathways and may provide a means for these pathways to interact. In mammals, transcription begins earlier than in zygotes from other classes of organisms, starting several hours after fertilization in the male and female pronuclei and continuing in the embryonic nuclei. Studies indicate that nuclei undergo an initial state that is permissive for transcription, and then in Gap 2 of the two-cell embryo, enter a transcriptionally repressive state. These changes have been linked to the times during the cell cycle when the DNA is replicated, and also have been proposed as a requirement for proper initiation of the program of early development.

Evidence for the involvement of a Src-related tyrosine kinase in Xenopus egg activation

Recently, we have purified a Src-related tyrosine kinase, named Xenopus tyrosine kinase (Xyk), from oocytes of Xenopus laevis and found that the enzyme is activated within 1 min following fertilization [Sato et al. (1996) J. Biol. Chem. 271, 13250-13257]. A concomitant translocation of a part of the activated enzyme from the membrane fraction to the cytosolic fraction was also observed. In the present study, we show that parthenogenetic egg activation by a synthetic RGDS peptide [Y. Iwao and T. Fujimura, T. (1996) Dev. Biol. 177, 558-567], an integrin-interacting peptide, but not by electrical shock or the calcium ionophore A23187 causes the kinase activation, tyrosine phosphorylation, and translocation of Xyk. A synthetic tyrosine kinase-specific inhibitor peptide was employed to analyze the importance of the Xyk activity in egg activation. We found that the peptide inhibits the kinase activity of purified Xyk at IC50 of 8 microM. Further, egg activation induced by sperm or RGDS peptide but not by A23187 was inhibited by microinjection of the peptide. In the peptide-microinjected eggs, penetration of the sperm nucleus into the egg cytoplasm and meiotic resumption in the egg were blocked. Indirect immunofluorescence study demonstrates that Xyk is exclusively localized to the cortex of Xenopus eggs, indicating that Xyk can function in close proximity to the sperm-egg or RGDS peptide-egg interaction site. Taken together, these data suggest that the tyrosine kinase Xyk plays an important role in the early events of Xenopus egg activation in a manner independent or upstream of calcium signaling.

Cortical granule exocytosis in hamster eggs requires microfilaments

Earlier work has demonstrated that hamster eggs that do not release a second polar body after fertilization in vitro lack a block to polyspermy (Stewart-Savage and Bavister, 1987: Gamete Res 18:333-338). Since polar body release requires microfilaments, the involvement of microfilaments in cortical granule exocytosis was examined. When hamster eggs were treated with cytochalsin B (CB) for 1 hr and then coincubated with sperm for 90 min, there was a dose-dependent increase in both the percentage of eggs with more than one sperm penetrating the zona pellucida and the mean number of sperm that penetrated the zona, with a maximum effect at 20 micrograms CB/ml (100% polypenetration, 3.0 +/- 0.3 sperm/egg). Cytochalasin-treated eggs retained 85% of their cortical granules 55 min after insemination, as compared to unfertilized eggs. Longer time periods did not result in any further reduction. As seen with the scanning confocal microscope, an extensive microfilament network was present in the cortex of untreated eggs, with the cortical granules located within the cortical network. The cortical microfilament network was highly reduced in CB-treated eggs. When viewed with the electron microscope, the same number of cortical granules were located next to the plasma membrane in both cytochalasin-treated and untreated, unfertilized eggs. These data indicate that intact microfilaments are required for normal cortical granule exocytosis in the hamster egg, but the role of the microfilaments in exocytosis is unresolved.

Activation of protein kinase C after fertilization is required for remodeling the mouse egg into the zygote

Fertilization of the mammalian egg initiates numerous biochemical and structural changes which remodel the egg into a single-celled zygote. To date, the most extensively studied phenomenon of fertilization in virtually all species has been the relationship between sperm penetration and the induction of the initial rise in intracellular-free calcium ([Ca2+]i) concentration within the egg. In contrast, relatively few studies have focused on the biochemical events following this rise in calcium, and even fewer studies have directly linked the biochemical events to the structural changes which must ensue for proper development of the embryo. In this study, we exploited recently developed technologies to investigate the action of protein kinase C (PKC), a presumed downstream transducer of the initial rise in [Ca2+]i, during fertilization and artificial activation with calcium ionophore or phorbol 12-myristate 13-acetate (PMA). The newly developed myristoylated PKC pseudosubstrate (myrPKC psi) was used to specifically inhibit PKC, thereby averting the trauma of injecting the egg with nonmyristoylated PKC psi. Following fertilization, eggs which were pretreated with myr-PKC psi were not capable of forming a second polar body and pronuclear formation was significantly inhibited. Spatial and temporal localization of PKC using confocal microscopy to visualize the PKC reporter dye, Rim-1, demonstrated localization of PKC to the lateral aspects of the forming second polar body after fertilization, or after artificial activation with calcium ionophore or PMA. In vivo biochemical analysis of eggs which were fertilized or artificially activated demonstrated that PKC activity rose at the same time (40 min) as the second polar body formed and then subsided over the next 5 hr post activation. From these data, we conclude that PKC plays an integral role in directing the transformation from egg to embryo.

Remodeling of the specialized intermediate filament network in mammalian eggs and embryos during development: regulation by protein kinase C and protein kinase M

The sheets serve as an maternal supply of assembled, cytokeratin, intermediate filaments. They are remodeled at each major developmental transition in mammalian early development, that is fertilization, embryonic compaction, blastocyst formation, and formation of the primitive ectoderm and primitive endoderm during implantation into the uterine wall. Our results indicate that the sheets exist as specialization for placental development as they have a major role in the maintenance of epithelial integrity at the time the embryo is implanting into the uterine wall. They also contribute intermediate filaments to the junctional complexes required for embryonic compaction. Our analyses demonstrate the they are regulated at the time of fertilization by the action of PKC/PKM, a kinase that acts as a cellular chronometer with both temporal and spatial precision that remodels the egg into the zygote.

A role for intermediate filaments in the establishment of the primitive epithelia during mammalian embryogenesis

Investigations of the cytoskeleton in mammalian eggs and embryos have revealed the existence of an unusual array of crosslinked intermediate filaments composed of cytokeratins 5, 6, 16, and 'Z' that are referred to as cytoskeletal sheets. We have been investigating the function of these cytoskeletal sheets during embryogenesis. In this investigation we report the rapid appearance of extensive arrays of tonofilaments extending across blastomeres and in association with intercellular desmosomal junctions appearing at the time the embryo hatches from its zona pellucida, through the time of implantation of the embryo into the uterine wall. Just prior to the time of gastrulation these tonofilaments disappear. Electron microscopy and immunoconfocal microscopy demonstrate that the tonofilaments are composed of cytokeratins characteristic of the type found earlier in development, that is types 5 and 6; whereas, cytokeratin type 8 which has been shown to be synthesized in blastocysts is localized primarily at perinuclear regions. Cytokeratins 8 and 18 are synthesized to about the same extent as actin at the time the tonofilaments appear whereas the synthesis of cytokeratins 5 and 6 is greatly reduced. Our results suggest that cytokeratins 5 and 6 in the tonofilaments may arise from the stored form of cytokeratins in the cytoskeletal sheets. Consequently, our results suggest that the sheets may serve as a maternal reserve of cytokeratin employed by the embryo at the time of implantation to form extensive arrays of tonofilaments in the embryo that likely provide structural integrity to the embryo as it is subjected to mechanical stress during invasion and implantation into the uterine wall.

Localization of microfilaments during oocyte maturation of golden hamster

The localization and changes in microfilaments (MF) during golden hamster oocyte maturation were examined by an immunofluorescein method and confocal laser scanning microscopy (CLSM). We also studied the relationship between the changes in MF and oocyte nuclear and cytoplasmic maturation. During in vivo maturation, generalized submembranous MF were found initially which gradually became more prominent at the site of the first polar body extrusion. However, 43.7% of the in vitro matured metaphase 2 stage oocytes lacked the submembranous MF structure. This fact may partly account for the low fertilization rate of in vitro matured oocytes. MF were not found in the folicular oocytes cultured in cytochalasin D-containing medium, and metaphase-like chromosomes were located at the center of the oocyte and first polar body extrusion did not occur. Twenty-five percent of the oocytes, which were arrested at meiosis by hypoxanthine, synthesized submembranous MF structure although the nuclear stage of these oocytes was germinal vesicle. These facts suggest that MF plays a role in nuclear behavior but there are some differences in the changes taking place within the nucleus and MF. MF may play a role in oocyte cytoplasmic maturation although the details of this have yet to be established.

Ovulated oocytes collected from PMSG/hCG-treated and cycling Djungarian or Siberian hamsters (Phodopus sungorus) are structurally similar with no evidence of polar body formation, indicating arrest in meiosis I

This study was undertaken (1) to devise a method of inducing multiple follicular development and subsequent ovulation in the Djungarian or Siberian hamster (Phodopus sungorus) and (2) to assess the quality of ovulated oocytes collected from PMSG/hCG treated animals in comparison to naturally ovulating animals. Hamsters (4-5 weeks; n = 70) received 5 IU PMSG followed 50-52 hr later by 10 IU hCG. Ovulated oocytes were collected 14-20 hr after hCG injection. Ovulated oocytes were flushed from oviducts of cycling animals (7-12 weeks; n = 30) exhibiting two consecutive estrous cycles. Oocytes were fixed and subjected to triple fluorescence immunostaining using anti-tubulin antibodies, fluorescein phalloidin, and Hoechst 33258. The mean number of ovulated oocytes collected from cycling animals was 4.8 +/- 0.4 (range 1-7). Ovulation occurred in 73% of the PMSG/hCG-stimulated animals. The mean number of oocytes ovulated from stimulated animals was 9.2 +/- 0.8 (range 0-22). The ovaries of animals that did not ovulate or that ovulated few oocytes did respond to PMSG, as indicated by the presence of multiple follicular development and pre-ovulatory stigmata. There was no evidence of a polar body in ovulated oocytes collected from PMSG/hCG-treated or cycling animals, indicating that oocytes were arrested in meiosis I. In the majority (80%) of ovulated oocytes from PMSG/hCG-treated and cycling animals, cortically placed chromosomes were aligned on a metaphase plate equidistant from a bipolar spindle. Sparse f-actin staining was observed in the region of the ooplasm surrounding the chromosomes.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasma membrane block to sperm entry occurs in mouse eggs upon parthenogenetic activation

The ability of parthenogenetically activated mouse eggs to establish a plasma membrane (PM) block to sperm penetration was studied. Zona-free eggs preloaded with Hoechst 33342 were activated by exposure to ethanol or OAG (1-oleoyl-2-acetyl-sn-glycerol) and inseminated after different periods. Eggs challenged with sperm at 30- or 60-min postactivation displayed a fertilization frequency significantly lower than that of control eggs. Conversely, when insemination was carried out at 120-min postactivation, the proportion of fertilized eggs was equivalent to that observed in the control group. Moreover, we report that when the eggs were induced to resume meiosis without any notable loss of CGs (egg exposure to OAG at 100 microM external Ca2+ or to heat shock), a normal ability to be penetrated was recorded at 30-min postactivation. Similar behaviour was exhibited by eggs that underwent a CG exocytosis close to that triggered by sperm in absence of nuclear activation (microinjection of inositol 1,4,5-trisphosphate into the egg at 1 microM cytosolic concentration). Present data support the conclusion that parthenogenetically activated mouse eggs are capable of a transitory PM block response that requires both CG exocytosis and meiosis resumption to occur.

Calmodulin during human sperm incorporation into hamster oocyte: an immunogold electron microscope study

In the present study, immunogold labeling of ultrathin sections of human sperm, before and after incorporation into hamster oocyte, was used to obtain insight into the ultrastructural localization and possible function of calmodulin during fertilization. In heads of ejaculated, capacitated, and acrosome-reacted fixed human sperm, calmodulin was mainly found in two compartments, the subacrosomal layer and the postacrosome. After sperm-egg fusion, the subacrosomal calmodulin was unaltered and surrounded by the fertilization cone in which actin was abundant. There was no co-localization of calmodulin and actin. In contrast, postacrosomal calmodulin disappeared as soon as the sperm head was incorporated into egg cytoplasm. These unique localizations and redistributions are in agreement with the concept of a calmodulin targeting from acrosome toward postacrosome through the subacrosomal layer during spermatogenesis (Weinman et al., 1986b: J Histochem Cytochem 34:118). Moreover, they strongly suggest a role for calmodulin both in sperm-egg fusion and in the initial pulse of Ca2+ occurring during fertilization.

Ontogeny of the cytoskeleton during mammalian oogenesis

Mammalian oogenesis is a process which requires a variety of changes in the structure and function of the specialized female germ cell. Evidence suggests that the cytoskeleton may mediate several of these structural and functional changes. In this review we evaluate what is known of cytoskeletal function during oogenesis, with emphasis on specialized cytoskeletal features in mammals. Existing investigations suggest that the oocyte, as a highly specialized cell, contains unique cytoskeletal elements which exhibit functions restricted to the process of early development.

Cell surface localisation and stability of uvomorulin during early mouse development

We have examined immunocytochemically the subcellular distribution of the cell adhesion molecule uvomorulin in cleavage stage mouse embryos using conventional and confocal microscopy, under a range of detergent extraction and fixation regimes. Only traces of uvomorulin were detectable on the surface of unfertilised oocytes, whereas between 6 and 11 h after activation detergent-resistant surface expression was evident. This shift correlates with previously demonstrated changes in the pattern of synthesis and accumulation of uvomorulin from precursor state in unfertilised oocytes to mature protein after fertilisation. Embryos at subsequent stages up to the 8-cell stage exhibited a uniform distribution of uvomorulin on free surfaces and its concentration in regions of contact between blastomeres. At the 8-cell stage, during compaction, there was increased intercellular adhesion with concomitant accumulation of uvomorulin at intercellular contacts, whilst free surface uvomorulin was reduced and became relatively more susceptible to detergent extraction. When compact 8-cell embryos were decompacted in calcium-free medium, uvomorulin at contacts decreased while free surface and cytoplasmic staining increased. Blastomeres disaggregated from 4- and 8-cell embryos showed traces or 'footprints' of anti-uvomorulin staining in regions previously in apposition. These footprints disappeared over 45-60 min, during which time uvomorulin distribution became uniform. Possible mechanisms underlying the rearrangements which take place both at fertilisation and during compaction and experimental decompaction are discussed.

Cytoskeletal sheets of mammalian eggs and embryos: a lattice-like network of intermediate filaments

Mammalian eggs and embryos possess a major cytoskeletal network composed of large planar "sheets" distributed throughout the cytoplasm. Cytoskeletal sheets are found neither in mammalian somatic cells nor in eggs or embryos of non-mammals. In this study, we have investigated the structural composition of the sheets in eggs and embryos of the golden Syrian hamster by (1) analysis of replicas from quick-frozen, deep-etched specimens, (2) analysis of thick, resin-embedded specimens using an intermediate voltage electron microscope (IVEM), (3) laser diffraction of EM images, (4) differential extraction with detergents, and (5) immunocytochemistry. Our results indicate that each sheet is composed of two closely apposed arrays of 10-nm filaments. Each filament within an array is held in register with its neighbor by lateral cross-bridges and the two parallel arrays of filaments are interconnected by periodic cross-bridges about 20 nm in length. Laser diffraction of negatives from IVEM images indicates that each array is composed of fibers that form a square lattice, and the two arrays are positioned in register by cross-bridges forming a single sheet. This lattice forms the skeleton of the sheets which is covered with a tightly packed layer of particulate material. By incubation in media containing different ratios of mixed-micelle detergents, it is possible to remove components sequentially from the sheets and to extract the particulate material. Immunocytochemical localization demonstrates that the sheets bind antibodies to keratin, and to a small extent actin, but do not bind antibodies to vimentin or tubulin. Examination of sheets within embryos at the time of embryonic compaction demonstrates that the sheets begin to fragment and disassemble in regions of blastomeres where desmosomes form, but undergo no structural alterations in interior and basal surfaces of the blastomeres. In regions of blastomere-blastomere contact the sheets fragment and associate with granules resembling keratohyalin granules found in keratinocytes.

Cytoskeletal sheets appear as universal components of mammalian eggs

The eggs of two mammalian species have been shown to contain novel cytoskeletal elements, referred to as cytoskeletal sheets, which undergo stage-specific changes in spatial organization at three key developmental transitions, fertilization, compaction, and blastocyst formation. If cytoskeletal sheets have an integral role in these developmental transitions, the sheets should be present in the eggs of other mammals as well. We examined the eggs of four additional species to determine if sheets were present. Our results indicate that sheets were present and they can be categorized into two classes based on their surface appearance. Cytoskeletal sheets in eggs of hamsters and rats have a smooth surface appearance, while eggs from humans, cows, pigs, and mice have a fibrous surface appearance. In addition, we observed that species-specific variations exist in the width of the sheets and in the density of the sheets (i.e., number per micron 2) in the eggs. These species-specific variations may relate to the role of the sheets during early development.

The sperm entry site during fertilization of the zebrafish egg: localization of actin

The sperm entry site (SES) of zebrafish (Brachydanio rerio) eggs was studied before and during fertilization by fluorescence, scanning, and transmission electron microscopy. Rhodamine phalloidin (RhPh), used to detect polymerized filamentous actin, was localized to microvilli of the SES and to cytoplasm subjacent to the plasma membrane in the unfertilized egg. The distribution of RhPh staining at the SES correlated with the ultrastructural localization of a submembranous electrondense layer of cortical cytoplasm approximately 500 nm thick and containing 5- to 6-nm filaments. Actin, therefore, was organized at the SES as a tightly knit meshwork of filaments prior to fertilization. Contact between the fertilizing sperm and the filamentous actin network was observed by 15-20 sec postinsemination or just before the onset of fertilization cone formation. Growing fertilization cones of either artificially activated or inseminated eggs exhibited intense RhPh staining and substantial increase in thickness of the actin meshwork. Collectively, TEM and RhPh fluorescence images of inseminated eggs demonstrated that the submembranous actin became rearranged in fertilization cones to form a thickened meshwork around the sperm nucleus during incorporation. The results reported here suggest that activation of the egg triggers a dramatic polymerization of actin beneath the plasma membrane of the fertilization cone. Furthermore, the actin involved in sperm incorporation is sensitive to the action of cytochalasins.

Phospholipid turnover and ultrastructural correlates during spontaneous germinal vesicle breakdown of the bovine oocyte: effects of a cyclic AMP phosphodiesterase inhibitor

The turnover of [32P]orthophosphate in bovine oocyte phospholipids was studied during the early stages of spontaneous meiotic maturation, and during inhibition of this process by the cAMP phosphodiesterase inhibitor 3-isobutyl-1-methyl-xanthine (IBMX). Radioactive lipids were separated by TLC and the meiotic stage was determined cytogenetically. Ultrastructure of the nuclear membrane was examined using transmission EM. During the commitment period to meiotic resumption, which precedes germinal vesicle breakdown (GVBD), small localized convolutions appeared in the intact nuclear membrane. This was accompanied by a decrease in [32P]phosphatidic acid (PA) and an increase in [32P]-phosphatidylcholine (PC). This was followed by extensive convolutions, and subsequent dissociation, of the nuclear membrane, concomitant with a tremendous surge in [32P]PC and [32P]phosphatidylethanolamine (PE). The cAMP-mediated maintenance of meiotic arrest involved retention of entire nuclear envelope integrity and total inhibition of the surge in [32P]PC and [32P]PE which accompanied GVBD. The increase in [32P]phosphatidylinositol (PI) associated with all stages of early meiotic resumption was unaffected by IBMX. Microinjection of heparin inhibited GVBD, and injection of inositol 1,4,5-trisphosphate (IP3) overrode IBMX-maintained meiotic arrest in almost 40% of the oocytes. The results suggest that there may be several functions for phospholipid turnover in the regulation of spontaneous meiotic resumption in the bovine oocyte. The first precedes the commitment period, and involves IP3 generation to serve as the primary signal for meiotic resumption. The second occurs concomitant with the commitment period, is unaffected by the level of intracellular cAMP, and is associated with the general turnover of phospholipid. The third is associated with GVBD, and is cAMP-sensitive, and may represent stimulation of de novo synthesis of phospholipid, thereby permitting disruption of the nuclear membrane.

Cytoskeleton of the mouse egg and embryo: reorganization of planar elements

Examination of detergent-extracted mouse eggs and embryos reveals the existence of two cytoskeletal networks. One network is the typical thin filament network observed in somatic cells while the other is composed of large planar elements. These latter cytoskeletal structures, with individual widths of 60.0 +/- 6.8 nm, alter their spatial organization in a developmental stage-specific manner. The planar elements are composed of filaments with a diameter of 10 nm aligned side-by-side with these filaments exhibiting a linear periodicity of 20.0 +/- 1.6 nm. A biochemical fraction containing components of the planar elements has been prepared from different stages of development and disappearance of prominent polypeptides from this fraction correlates with the altered spatial organization of the planar elements. Ultrastructure and biochemistry of cytoskeletal planar elements in eggs and embryos of the mouse are comparable with cytoskeletal sheets of Syrian hamster eggs and embryos, suggesting these cytoskeletal components may have a functional role in mammalian embryogenesis. Because such structures have not been identified in eggs or embryos of species other than mammals, their function may be unique to mammalian embryogenesis.