Changes in the distribution of vinculin during preimplantation mouse development

Soft culture substrates favor stem-like cellular phenotype and facilitate reprogramming of human mesenchymal stem/stromal cells (hMSCs) through mechanotransduction

Biophysical cues influence many aspects of cell behavior. Stiffness of the extracellular matrix is probed by cells and transduced into biochemical signals through mechanotransduction protein networks, strongly influencing stem cell behavior. Cellular stemness is intimately related with mechanical properties of the cell, like intracellular contractility and stiffness, which in turn are influenced by the microenvironment. Pluripotency is associated with soft and low-contractility cells. Hence, we postulated that soft cell culture substrates, presumably inducing low cellular contractility and stiffness, increase the reprogramming efficiency of mesenchymal stem/stromal cells (MSCs) into induced pluripotent stem cells (iPSCs). We demonstrate that soft substrates (1.5 or 15 kPa polydimethylsiloxane – PDMS) caused modulation of several cellular features of MSCs into a phenotype closer to pluripotent stem cells (PSCs). MSCs cultured on soft substrates presented more relaxed nuclei, lower maturation of focal adhesions and F-actin assembling, more euchromatic and less heterochromatic nuclear DNA regions, and increased expression of pluripotency-related genes. These changes correlate with the reprogramming of MSCs, with a positive impact on the kinetics, robustness of colony formation and reprogramming efficiency. Additionally, substrate stiffness influences several phenotypic features of iPS cells and colonies, and data indicates that soft substrates favor full iPSC reprogramming.

Polarity in Cell-Fate Acquisition in the Early Mouse Embryo

Establishing polarity is a fundamental part of embryogenesis and can be traced back to the earliest developmental stages. It can be achieved in one of two ways: through the preexisting polarization of germ cells before fertilization or via symmetry breaking after fertilization. In mammals, it seems to be the latter, and we will discuss the various cytological and molecular events that lead up to this event, its mechanisms and the consequences. In mammals, the first polarization event occurs in the preimplantation period, when the embryo is but a cluster of cells, free-floating in the oviduct. This provides a unique, autonomous system to study the de novo polarization that is essential to life. In this review, we will cover modern and past studies on the polarization of the early embryo, using the mouse as a model system, as well as hypothesizing the potential implications and functions of the biological events involved.

CD2-associated protein is widely expressed and differentially regulated during embryonic development

CD2-associated protein (CD2AP) is an adapter protein that is involved in various signaling and vesicular trafficking processes and also functions as a linker between plasma membrane proteins and the actin cytoskeleton. The protein is known to have important functions in T cells and glomerular podocytes, but it is also expressed by many other adult-type tissues and cells. Here we analyzed the expression of the protein during early embryonic development and organogenesis of the mouse. The results showed differential tissue-specific regulation of CD2AP in developing and maturing organs. In oocytes and pre-implantation embryos, CD2AP was located diffusely in the cytoplasm, whereas in late blastocysts it was concentrated to the intercellular contacts. During organogenesis, CD2AP was distinctly upregulated upon, e.g., the pretubular aggregation of metanephric mesenchyme cells and the appearance of the osteoblastic rim around cartilages during endochondral ossification. High CD2AP expression was also observed during epithelial-like conversion of some highly specialized secretory cell types such as the odontoblasts, the cells of the choroid plexus and the decidualized cells of the endometrial stroma. In other instances, such as the development of the proximal tubuli of the kidney and the flat alveolar epithelium of the lung, the protein was downregulated upon differentiation and maturation of the cells. Finally, certain cells, e.g., glomerular podocytes, those forming the collecting ducts of the kidney, and the urothelium of the kidney pelvis, expressed CD2AP throughout their differentiation and maturation. Multiple molecules and complex pathways regulate embryogenesis, and scaffolding proteins apparently have pivotal roles in targeting and finetuning, e.g., growth factor- or hormone-induced processes. The cell-type specific spatio-temporal regulation of CD2AP during development suggests that this adapter protein is a key regulatory partner in many signaling pathways and cellular processes governing differentiation and morphogenesis.

Of mice, frogs and flies: generation of membrane asymmetries in early development

Embryonic development begins with cleavage of the fertilized egg. Cleavage comprises two major processes: cytokinesis and formation of a polarized epithelial cell layer. The focus of this review is comparison of the generation of membrane polarity during embryonic cleavage in three different developmental model systems. In mammalian embryos, as exemplified by analysis of the mouse, generation of distinct membrane domains is uncoupled from cleavage divisions and is initiated in a specific developmental phase, called compaction. In Xenopus laevis embryos, generation of polarized blastomeres occurs simultaneously with cytokinesis. The origin of specific membrane domains of X. laevis polar blastomeres, however, can be traced back to oogenesis. Finally, in Drosophila melanogaster, generation of polarized cells occurs at cellularization. The relevance of cell adhesion, cell junctions and cytocortical scaffolds will be discussed for each of the model systems. Despite enormous morphologic differences, the three models share many common features; in particular, many important molecular interactions are conserved.

The initial phase of embryonic patterning in mammals

Although specification of the antero-posterior axis is a critical intial step in development of the fetus, it is not known either how, or at what stage in development, this process begins. Such information is vital for understanding not only normal development in mammals but also monozygotic twinning, which, at least in man, is associated with a significantly increased incidence of birth defects. According to recent studies in the mouse, specification of the fetal anteroposterior axis begins well before gastrulation, and probably even before the conceptus implants. Moreover, evidence is accruing that the origin of relevant asymmetries depends on information that is already present in the zygote before it embarks on cleavage. Hence, early development in mammals does not differ as markedly from that in other animals as has generally been assumed. Consequently, at present, the possibility of adverse effects of techniques used to assist human reproduction cannot be disregarded.

The role of actin filaments and microtubules in hepatocyte spheroid self-assembly

Cultured rat hepatocytes self-assemble into three-dimensional structures or spheroids that exhibit ultrastructural characteristics of native hepatic tissue and enhanced liver-specific functions. The spheroid formation process involves cell translocation and changes in cell shape, indicative of the reorganization of the cytoskeletal elements. To elucidate the function of the cytoskeleton, hepatocytes undergoing spheroid formation were treated with drugs that disrupt the different cytoskeletal components. Cytochalasin D, which targets the actin filaments, caused inhibition of spheroid formation. The role of microtubules in this process was assessed by incubating the cells with taxol or nocodazole. Perturbation of microtubules had minimal effects on spheroid assembly. Scanning electron micrographs showed no morphological differences between spheroids formed in control cultures and those formed in the presence of taxol or nocodazole. In addition, the effects of those agents on hepatocyte functions were investigated. Albumin secretion and cytochrome P450 2B1/2 activities of hepatocytes were comparable in spheroids formed in the presence of taxol or nocodazole to those formed in control cultures. The levels of these liver-specific activities were lower in cytochalasin D--treated cultures where only dispersed cells or cell clumps were found but spheroids had not found. Thus, hepatocytes require an intact actin network to self-assemble efficiently into functional tissue-like structures. Perturbation of the microtubule lattice does not impair the formation process. Events that transpire during hepatocyte spheroid self-assembly exhibit striking similarities to processes commonly observed in tissue morphogenesis. The results provide insight into the mechanisms that cells employ to organize into tissues and can contribute to our understanding of how to control the cellular assembly in tissue engineering and clinical applications.

Polarity in early mammalian development

Although overall polarity is discernable morphologically in both the growing and mature oocyte in mammals, it is typically relatively inconspicuous in the zygote. Furthermore, the conceptus exhibits an essentially radial organization during cleavage which was long held to persist until the primitive streak forms at the onset of gastrulation. This view has been challenged by various recent studies which clearly show that asymmetries are evident both morphologically and at the molecular level from very early in development. Collectively, these new findings argue that specification of the anterior-posterior axis of the fetus depends on information that is localized extra-embryonically in cells which begin to differentiate before the conceptus implants in the uterus.

Tropomyosin in preimplantation mouse development: identification, expression, and organization during cell division and polarization

Tropomyosin is an actin-binding cytoskeletal protein which has been extensively characterized in a variety of cell types and tissues, with the exception of very early developmental stages during which cellular polarization first occurs. We have identified five polypeptides in mouse preimplantation conceptuses which show many of the characteristics of tropomyosin. They form the major portion of the heat-stable cytoskeletal protein fraction of blastomeres and have the characteristic isoelectric and SDS-PAGE migration characteristics on 1-D and 2-D gels. All five polypeptides were synthesized in late 2- and 4-cell, and all 8-cell stages, with three of the five polypeptides showing lower synthetic levels in fertilized eggs and early 2-cell conceptuses. These heat-stable proteins showed specific differences from proteins isolated from mouse 3T3 fibroblasts by the same method, namely higher Mr isoforms were not represented, also some of the isoforms can be labeled by incorporation of [14C]proline. The cellular distribution of tropomyosin in early stage conceptuses was examined using monoclonal and affinity-purified polyclonal antibodies. Tropomyosin becomes associated both with the blastomere cortex postfertilization and with the cleavage furrow during cytokinesis. The interphase cortical association is uniform until the 8-cell stage, when tropomyosin becomes associated with the developing apical pole and is excluded from the basolateral cortex. This polar localization is inherited along with the pole at the 8- to 16-cell division, but experiments in which cell division is artificially prolonged show that tropomyosin localization does not represent a permanent marking of the pole. We conclude that the early mouse conceptus contains a unique and specific set of tropomyosins which respond to polarizing signals.

Vinculin and cell-cell adhesion

Vinculin, a 117-kDa protein, is a constituent of adhesion plaques and adherence junctions in non-muscle cells. We investigated the role of vinculin on the physical strength of cell-cell adhesion by conducting disaggregation assays on aggregates of parental wild-type F9 mouse embryonal carcinoma cells (clone BIM), two vinculin-depleted F9 cell lines, gamma 227 and gamma 229, and a reconstituted gamma 229 cell line (R3) that re-express vinculin. Immunoblotting demonstrated that the four cell lines used in the study had similar expressions of the cell-cell adhesion molecule E-cadherin and associated membrane proteins alpha- and beta-catenin. Double immunofluorescence analysis showed that, in contrast to the vinculin-null cell lines. BIM and R3 cells expressed abundant vinculin at the cell margins in adhesion plaques and in cell-cell margins that also contained actin. Laminar flow assays showed that both the vinculin-positive and vinculin-negative cell aggregates that were formed in culture in the course of 24 to 48 hours largely remained intact despite the imposition of shear flow at high shear rates. Since laminar flow imposed on cell aggregates act to separate cells from each other, our data indicate that F9 cells that were adherent to a substrate formed strong cell-cell adhesion bonds independent of vinculin expression. On the other hand, aggregates of vinculin-depleted gamma 229 and gamma 227 cells that were formed in suspension during a two-hour static incubation at 37 degrees C were desegregated more easily with the imposition of shear flow than the BIM and R3 cell aggregates formed under identical conditions. Loss of vinculin was associated with a reduction in cell-cell adhesion strength only among those cells lacking contact to a substrate. Overall, the results indicate that vinculin is not needed for forming strong cell-cell adhesion bonds between neighboring carcinoma cells which are adherent to the basal lamina.

Developmentally regulated markers of in vitro-produced preimplantation bovine embryos

Expression of various developmentally regulated markers was screened throughout the preimplantation stages of in vitro-derived bovine embryos. This was done by investigating the distribution of several nuclear, cytoplasmic and extracellular proteins by means of immunofluorescence microscopy. While lamin B appeared as a constitutive component of nuclei of all preimplantation stages, lamins A/C had a stage-related distribution. The early cleavage stage nuclei contained lamins A/C which generally disappeared in the following stages, with the possible exception of a few positive nuclei in the morula and early blastocyst stage. In the expanded blastocyst stage the nuclei of trophectoderm cells became positive while no positivity was observed in the inner cell mass cells. Starting from day 6, the appearance and/or polarised distribution of various cytoskeletal and cytoskeleton-related components such as F-actin, alpha-catenin and E-cadherin gave an insight into the timing of events related to compaction of bovine embryos. Compaction was correlated with the first differentiation event, i.e. the formation of trophectoderm; this is the first embryonic epithelium, characterised by cytokeratins and desmoplakin. Extracellular fibronectin was first detected in the early blastocyst stage shortly before the morphological differentiation of primitive endoderm, and in the later stages it was localised at the interface between trophectoderm and extraembryonic endoderm. Laminin and collagen IV were expressed by the endoderm cells and contributed to the extracellular matrix underlying the trophectoderm. This study is a first attempt to characterise the cells of in vitro-derived bovine embryos valid for cell line derivation.

Characterisation and role of integrins during gametic interaction and egg activation

It has recently been proposed that some of the processes induced by fertilisation in mammals may be mediated by integrins. By performing immunofluorescence labelling and Western blots with antibodies directed against some of the alpha and beta subunits of integrins, we show here the presence of some of these proteins in human and hamster oocytes. Among them, alpha 2 and alpha 5 were also present on in vitro preparations of sea urchin egg cortices. In addition, antibodies raised against these two proteins were the most effective at inhibiting attachment and fusion of human spermatozoa with hamster oocytes. We suggest that alpha 2 and alpha 5 integrin chains may be common mediators in adhesion-fusion mechanisms triggered by fertilisation. Using similar techniques, we show that eggs are rich in three cytoskeletal proteins known to be linked to the beta chain of integrins: talin, vinculin and alpha-actinin. Moreover, we found that talin and alpha-actinin were associated with proteins phosphorylated on tyrosine after fertilisation in sea urchin eggs. We suggest that integrins might be involved during fertilisation and trigger egg activation through cytoskeletal structures.

Targeted disruption of vinculin genes in F9 and embryonic stem cells changes cell morphology, adhesion, and locomotion

Vinculin, a major constituent of focal adhesions and zonula adherens junctions, is thought to be involved in linking the microfilaments to areas of cell-substrate and cell-cell contacts. To test the role of vinculin in cell adhesion and motility, we used homologous recombination to generate F9 embryonal carcinoma and embryonic stem cell clones homozygous for a disrupted vinculin gene. When compared to wild-type cells, vinculin-mutant cells displayed a rounder morphology and a reduced ability to adhere and spread on plastic or fibronectin. Decreased adhesion of the mutant cells was associated with a reduction in lamellipodial extensions, as observed by time-lapse video microscopy. The locomotive activities of control F9 and the vinculin-null cells were compared in two assays. Loss of vinculin resulted in a 2.4-fold increase in cell motility. These results demonstrate an important role for vinculin in determining cell shape, adhesion, surface protrusive activity, and cell locomotion.

Identification and localization of integrin subunits in oocytes and eggs of the mouse

Results of a recent study have implicated egg integrins in sperm binding to the egg plasma membrane (Blobel et al., 1991: Nature 356:248-252). In this report, immunoprecipitation was used to identify, and confocal immunofluorescence microscopy was used to localize, several different integrin subunits in mouse eggs. Antibodies to alpha 2, alpha 5, alpha v, and beta 1 subunits, as well as antibodies to the fibronectin receptor (FNR; alpha 5 beta 1 and/or alpha 3 beta 1) and vitronectin receptor (VNR; alpha v beta 3 and/or alpha v beta 5), detect polypeptides of the appropriate molecular weights following immunoprecipitation. beta 1 is localized preferentially to either the microvillar or amicrovillar membrane/cortical regions of eggs, and these asymmetric localizations depend on the antibody used. Proteins recognized by anti-FNR antibodies are localized preferentially to the amicrovillar membrane/cortical region. Germinal vesicle-intact oocytes display a symmetric plasma membrane distribution using beta 1 and FNR antibodies, and the asymmetric distribution develops as a consequence of oocyte maturation and is clearly observed by metaphase I. In contrast to the membrane localization of these integrin subunits, alpha 2, alpha 5, and VNR are predominantly localized in the cytoplasm of both oocytes and eggs. In the oocyte, each of these integrin subunits is uniformly distributed throughout the cytoplasm. Oocyte maturation is associated with a redistribution of alpha 5 and VNR, leading to an asymmetric cytoplasmic distribution with an increased localization towards the spindle. alpha v, which is localized in the plasma membrane/cortex of both oocytes and eggs, does not show such a change during oocyte maturation. Results of these experiments are discussed in the context of a role for integrins in mediating sperm plasma membrane-egg plasma membrane interactions leading to egg activation.

Molecular maturation of cell adhesion systems during mouse early development

During cleavage, the mouse embryo expresses a variety of cell adhesion systems on its cell surfaces. We have reviewed biogenetic and assembly criteria for the formation of the uvomorulin/catenin, tight junction and desmosome adhesion systems as the trophectoderm differentiates. Each system reveals different mechanisms regulating molecular maturation. Adhesion processes contribute to the generation of distinct tissues in the blastocyst by modifying the expression pattern of blastomeres entering the non-epithelial inner cell mass lineage. Cell adhesion also influences the spatial organisation, but rarely the timing of expression, of proteins involved in trophectoderm differentiation.

The cytoskeleton and associated proteins during cleavage, compaction and blastocyst differentiation in the pig

The organization of the cytoskeleton during early pig embryogenesis was investigated by using fluorescence and electron microscopy. The early morphogenesis of the pig embryo differed from that of the mouse, the standard model of the early mammalian development. In the pig, both compaction and polarization were gradual, and definitive polarization of cell surface microville occurred first shortly before blastocyst formation; the compaction and polarization of the mouse embryo are completed as early as at the 8 cell stage. Furthermore, the pig morula undergoes cycles of compaction and decompaction throughout its development. Distinct changes in the distribution of actin and the actin-associated proteins alpha-fodrin, vinculin and E-cadherin coincided with these events. In the pig, all these molecules were evenly distributed at all aspects of the blastomeres during early cleavage and then gradually accumulated in regions of intercellular contacts toward the blastocyst stage; microfilaments in trophectoderm cells formed a cortical meshwork associated with apical microvilli and adherent junctions (zonula adherens). In the mouse, the corresponding changes occur earlier, at the 8 cell stage. Microtubules formed a network-like cortical layer beneath the microvilli at the free outer surfaces of pig blastomeres. Cytokeratin bundles were not observed until the early blastocyst, where they characteristically associated with newly formed desmosomes. In both species a close correlation between morphologically defined developmental stages and the organization of the cytoskeleton: actin and actin-associated proteins are involved in polarization and compaction, whereas the appearance of intermediate filament bundles coincides with the building of the first epithelium, the trophectoderm; it is in the timing of events that a contrast between species is observed.

Localisation of tight junction protein cingulin is temporally and spatially regulated during early mouse development

The molecular maturation of the tight junction in the mouse early embryo has been investigated by monitoring the distribution of cingulin, a 140 × 10(3) M(r) peripheral (cytoplasmic) membrane constituent of the junction, at different stages of development and in different experimental situations. Although tight junction formation does not begin until compaction at the 8-cell stage, cingulin is detectable in oocytes and all stages of cleavage, a factor consistent with our biochemical analysis of cingulin expression (Javed et al., 1992, Development 117, 1145–1151). Using synchronised egg and embryo stages and isolated cell clusters, we have identified three sites where cingulin is localised, the cytocortex, punctate cytoplasmic foci and tight junctions themselves. Cytocortical cingulin is present at the cumulus-oocyte contact site (both cell types), in unfertilised and fertilised eggs and in cleavage stages up to 16-cell morulae, particularly at microvillous domains on the embryo outer surface (eg. apical poles at compaction). Embryo manipulation experiments indicate that cortical cingulin is labile and dependent upon cell interactions and therefore is not merely an inheritance from the egg. Cingulin cytoplasmic foci are evident only in outer cells (prospective trophectoderm) from the 32-cell stage, just prior to cavitation, and decline from approx. 8 hours after cavitation has initiated. The appearance of these foci is insensitive to cycloheximide treatment and they colocalise with apically derived endocytic vesicles visualised by FITC-dextran, indicating that the foci represent the degradation of cytocortical cingulin by endocytic turnover. Cingulin is detectable at the tight junction site between blastomeres usually from the 16-cell stage, although earlier assembly occurs in a minority (up to 20%) of specimens. Cingulin assembly at the tight junction is sensitive to cycloheximide and is identifiable approx. 10 hours after cell adhesion is initiated and ZO-1 protein assembles. Collectively, our results indicate that (i) cingulin from nonjunctional sites does not contribute to tight junction assembly and (ii) the molecular maturation of the junction appears to occur progressively over at least two cell cycles.

Role of ultrastructural studies in the analysis of cell lineage in the mammalian pre-implantation embryo

Ultrastructural studies have contributed significantly to our understanding of cell lineage differentiation in the mammalian pre-implantation embryo. Such studies have documented, and continue to document, morphological, biochemical, and physiological characteristics of the cell lineages established during the pre-implantation period in eutherian embryos, principally that of the mouse. This review evaluates these contributions and identifies areas of study in which ultrastructural analysis is most likely to have an important role in the future.

Spatio-temporal distribution of the adherens junction-associated molecules vinculin and talin in the early avian embryo

To gain an insight into the possible involvement of the cytoskeletal components and cellular junctions in morphogenetic processes during development, we have studied the spatio-temporal distribution of two major adherens-junction-associated molecules, vinculin and talin, during avian embryogenesis, using immunofluorescence microscopy and immunoblotting. Both molecules were detected at very early stages during morphogenesis and were found in a wide variety of tissues deriving from the three primary germ layers. A number of tissues, including smooth and striated muscles, endothelia, and some hemopoietic precursors, expressed vinculin and talin at especially high levels either transiently or permanently. Conversely, only a few cell types, e.g., circulating erythrocytes and neurones in the central nervous system lacked or expressed them at very low levels. In addition, expression of vinculin and talin was in some cases modulated in connection with morphological rearrangements of tissues. In particular, they were transiently enhanced in restricted areas of the ectoderm and endoderm undergoing extensive foldings. However, other morphogenetic events such as local disruptions of epithelia were not accompanied by extensive modifications in their expression. Finally, it appeared that, in most cases, vinculin and talin overlapped in their distribution, and the level of their expression was regulated coincidently with the notable exceptions of the primordium of the central nervous system, the nephron, and the liver where each molecule followed independent regulatory patterns. It appears from this study that the spatio-temporal distribution of vinculin and talin correlates frequently with that of the adhesion molecules A-CAM (or N-cadherin), L-CAM, and of integrin receptors. Thus, vinculin and talin, in association with the membrane components of adherens junctions, may actively participate both in the control of cellular interactions during early embryonic development and in cell differentiation.

Maintenance of compaction and adherent-type junctions in mouse morula-stage embryos

Timed morulae of different stages of development were exposed to cytochalasin B causing depolymerisation of microfilaments and to ECCD-1 antibodies interacting with Ca2(+)-dependent adhesion molecules or cultured in the absence of calcium. All three treatments decompacted mid-morula-stage embryos within one hour. Late morulae were resistant to ECCD-1 antibody treatment and relatively resistant to calcium-free cultivation, but not to cytochalasin B treatment. Scanning electron microscopy revealed that the decompacting treatments not only loosened the interblastomere contacts but also resulted in rearrangement of the cell surface microvilli. Transmission electron microscopy showed that normal, untreated embryos had specialized membrane junctions in the most apical regions of the interblastomere contacts. Immunoelectron microscopy revealed that these apical junction areas contained vinculin, a protein typical of adherent junctions. Upon decompaction the apical junctions disappeared completely. When transferred back to the normal medium, the embryos rapidly started to recompact. Simultaneously the apical junctions and cell surface microvilli reassumed the organization characteristic of the morula stage. Late morulae that were resistant to treatment had normal apical junctional areas. During subcultivation in the normal medium, the treated morulae developed into morphologically normal blastocysts. These data indicate that adherent-type junctions and cell surface microvilli participate in the initiation and maintenance of compaction of morula-stage embryos.

Talin and vinculin in the oocytes, eggs, and early embryos of Xenopus laevis: a developmentally regulated change in distribution

We have investigated the expression and distribution of talin and vinculin in the oocytes, eggs, and embryos of Xenopus laevis. Antibodies to the previously characterized avian proteins stain several different Xenopus cell types identically by immunofluorescence: adhesion plaques of cultured kidney (A6) cells, the cell peripheries of oviduct cells, and the postsynaptic neuromuscular junctions of tadpole tail muscle fibers. These antibodies also identify cognate proteins of the appropriate sizes on immunoblots of A6 cell and oviduct lysates. Using these antibodies on ovarian tissue, we find talin to be highly localized at the cortices of oocytes and vinculin to be in the oocyte cytoplasm and absent from the oocyte cortex. In the cells of the ovarian layers that surround the oocytes, talin and vinculin can be detected as soluble and cytoskeletal components. Vinculin is first detectable as a cytoskeletal component in eggs, appearing some time during or between oocyte maturation and oviposition. During early embryo development, talin and vinculin are colocalized in the cortex of cleavage furrows and blastomeres. Thus, Xenopus oocytes and eggs display different distributions of talin and vinculin. The change from unlinked localization to colocalization appears to be developmentally regulated, occurring during the transition from oocyte to egg.

Effects of cryoprotectants on actin filaments during the cryopreservation of one-cell rabbit embryos

A dynamic equilibrium between globular and filamentous actin plays a crucial role in cell structure and motility. Many factors such as pH, ionic strength, temperature, and divalent cations, are known to influence this equilibrium. Some organic solvents, such as those used for the cryopreservation of cells, may also alter the dynamic equilibrium of this system. Fluorescence staining with NBD-phallacidin permits polymerized actin to be visualized in embryos and provides evidence that propanediol depolymerizes actin, whereas dimethyl sulfoxide does not. This depolymerizing effect is reversible after propanediol removal. Biochemical techniques were used to study the influence of these solvents on rabbit skeletal actin. Results obtained by sedimentation, fluorescence, DNase inhibition, electron microscopy, and viscometry analysis demonstrate that propanediol has a dual effect on actin polymers in vitro: it decreases the proportion of filamentous actin and the remaining filaments appear shorter and aggregate to form bundles. In contrast, dimethyl sulfoxide does not alter dramatically the actin polymer integrity. Propanediol is shown to exert a good cryoprotective action on rabbit embryos, while dimethyl sulfoxide does not. We suggest that the depolymerization of actin filaments by propanediol prior to cooling may facilitate the cryopreservation of one-cell rabbit embryos.

Early development in the mouse: would it be affected by microgravity?

Gravity has been identified as a morphogenetic signal in Amphibian and bird embryonic development so it is plausible that it might be such in mammals as well. Since early mammalian development shows some apparently significant differences to these other groups, a brief summary of mouse embryogenesis will be given identifying events in which polarity is an important feature and consequently, in which gravity may be a causative factor. These include compaction and polarization during cleavage, establishment of the radial axis, the embryonic-abembryonic axis, the dorso-ventral axis, and the anterior-posterior axis, implantation, and the later rotation of the embryo. The experimental data on these morphogenetic steps will be discussed and an assessment of the possible involvement of gravity will be made.

Mammalian blastocyst: transport functions in a developing epithelium

This article reviews the current state of knowledge concerning morphological and physiological mechanisms important to growth and differentiation of the mammalian blastocyst between compaction and implantation. Morphological processes occur in conjunction with major changes in transport systems that control the movement of substances into and out of the embryo. Compaction is a morphological development that is associated with the formation of an outer squamous epithelium, the trophectoderm, which regulates the composition of the medium bathing the presumptive embryo (the inner cell mass). Implantation involves the interaction of two epithelia, the adhesion between the trophectoderm and the maternal endometrium. Before adhesion, the blastocyst lies free in the uterine fluid and exchanges occur between this fluid and the embryo. Apposition of these epithelia is brought about in part by expansion of the blastocyst and removal of the uterine fluid. Blastocyst physiology is an inherently important field because vectorial transport system development and the genes that regulate it can be studied.

Cytoskeleton in preimplantation mouse development

This paper reviews the constituents of the cytoskeleton in the cells of the preimplantation mouse embryo and how they change as the development proceeds. The cytoskeleton can be divided into two distinct groups, that in the cytosplasm and that associated with the membrane. The first and better-known group contains microfilaments, microtubules and intermediate filaments, the second such components of the cell and nuclear membrane as spectrin-like protein and nuclear lamin. The filamentous components of the cytoplasmic cytoskeleton adhere to the nuclear and cell membrane at attachment points where specific proteins such as vinculin may mediate the interaction. Each cell of the early embryo has all of these components, but their morphological organization and molecular constitution alter as the embryo develops. These modifications are especially pronounced when the cleavage-stage embryo compacts and when the blastocysts forms and differentiates. These events represent the most critical stages of morphogenesis and cytodifferentiation in the preimplantation embryo. The cytoskeleton may thus have an important role in the control of the early mammalian development.