Cell polarity and development of the first epithelium

Characterisation of the Hippo signalling pathway during bovine preimplantation embryo development

Blastocyst formation is an important milestone during preimplantation embryo development. During murine preimplantation embryogenesis, the Hippo signalling pathway is known to play a significant role in lineage segregation and henceforth the formation of blastocysts. However, the role of this cell signalling pathway during bovine embryogenesis remains unknown. Thus, the aim of the present study was to characterise the Hippo signalling pathway during bovine preimplantation embryo development. mRNA transcripts of Hippo signalling pathway constituents (i.e. crumbs cell polarity complex component 3 (CRB3), mammalian sterile 20-like 1 (MST1), mammalian sterile 20-like 2 (MST2), Yes associated protein 1 (YAP1), transcriptional coactivator with PDZ-binding motif (TAZ)) were observed during all stages of bovine preimplantation embryo development. To evaluate the localisation of Hippo pathway components, bovine embryos at timed stages of development were stained using specific antibodies and observed under a laser confocal microscope. Although MST1/2 proteins were in the cytoplasm during various stages of bovine embryonic development, TAZ and phosphorylated (p-) YAP were detected in the nucleus during the blastocyst stages. Localisation of TAZ and p-YAP proteins was distinct in the bovine compared with mouse model, suggesting that the Hippo signalling pathway is regulated differently in early bovine embryos.

The E-cadherin/AmotL2 complex organizes actin filaments required for epithelial hexagonal packing and blastocyst hatching

Epithelial cells connect via cell-cell junctions to form sheets of cells with separate cellular compartments. These cellular connections are essential for the generation of cellular forms and shapes consistent with organ function. Tissue modulation is dependent on the fine-tuning of mechanical forces that are transmitted in part through the actin connection to E-cadherin as well as other components in the adherens junctions. In this report we show that p100 amotL2 forms a complex with E-cadherin that associates with radial actin filaments connecting cells over multiple layers. Genetic inactivation or depletion of amotL2 in epithelial cells in vitro or zebrafish and mouse in vivo, resulted in the loss of contractile actin filaments and perturbed epithelial packing geometry. We further showed that AMOTL2 mRNA and protein was expressed in the trophectoderm of human and mouse blastocysts. Genetic inactivation of amotL2 did not affect cellular differentiation but blocked hatching of the blastocysts from the zona pellucida. These results were mimicked by treatment with the myosin II inhibitor blebbistatin. We propose that the tension generated by the E-cadherin/AmotL2/actin filaments plays a crucial role in developmental processes such as epithelial geometrical packing as well as generation of forces required for blastocyst hatching.

Changing expression of chloride channels during preimplantation mouse development

Plasma membrane chloride channels (ClCs) play important roles in a broad range of cellular processes including cell volume regulation, proliferation, and transepithelial transport, all of which are critical during preimplantation embryonic development. In this study, the molecular and functional expression of voltage-gated ClCs was analyzed throughout preimplantation development of the mouse conceptus. mRNA transcripts for allClcngenes were detected. OnlyClcn1mRNA showed differential expression in the blastocyst, being detected in the trophectoderm but not in the inner cell mass. CLCN3 protein was detected at low levels in the cytoplasm and plasma membrane in 4-cell embryos and was localized to the apical plasma membrane of the trophoblasts in the blastocyst. Whole-cell patch-clamp recordings demonstrated the presence of a DIDS-sensitive, outwardly rectifying Cl−current throughout development, with this conductance being large at the 1-cell, morula and blastocyst stages. A second DIDS-insensitive Cl−current, which was inactivated by membrane depolarization, was present in cells differentiating into the trophoblast lineage and during blastocyst expansion. Inhibition of the DIDS-sensitive current and the DIDS-insensitive current, with 9-AC, prevented blastocyst expansion.

Ouabain stimulates a Na+/K+-ATPase-mediated SFK-activated signalling pathway that regulates tight junction function in the mouse blastocyst

The Na(+)/K(+)-ATPase plays a pivotal role during preimplantation development; it establishes a trans-epithelial ionic gradient that facilitates the formation of the fluid-filled blastocyst cavity, crucial for implantation and successful pregnancy. The Na(+)/K(+)-ATPase is also implicated in regulating tight junctions and cardiotonic steroid (CTS)-induced signal transduction via SRC. We investigated the expression of SRC family kinase (SFK) members, Src and Yes, during preimplantation development and determined whether SFK activity is required for blastocyst formation. Embryos were collected following super-ovulation of CD1 or MF1 female mice. RT-PCR was used to detect SFK mRNAs encoding Src and Yes throughout preimplantation development. SRC and YES protein were localized throughout preimplantation development. Treatment of mouse morulae with the SFK inhibitors PP2 and SU6656 for 18 hours resulted in a reversible blockade of progression to the blastocyst stage. Blastocysts treated with 10(-3) M ouabain for 2 or 10 minutes and immediately immunostained for phosphorylation at SRC tyr418 displayed reduced phosphorylation while in contrast blastocysts treated with 10(-4) M displayed increased tyr418 fluorescence. SFK inhibition increased and SFK activation reduced trophectoderm tight junction permeability in blastocysts. The results demonstrate that SFKs are expressed during preimplantation development and that SFK activity is required for blastocyst formation and is an important mediator of trophectoderm tight junction permeability.

Preimplantation embryo programming: transcription, epigenetics, and culture environment

Preimplantation development directs the formation of an implantation- or attachment-competent embryo so that metabolic interactions with the uterus can occur, pregnancy can be initiated, and fetal development can be sustained. The preimplantation embryo exhibits a form of autonomous development fueled by products provided by the oocyte and also from activation of the embryo's genome. Despite this autonomy, the preimplantation embryo is highly influenced by factors in the external environment and in extreme situations, such as those presented by embryo culture or nuclear transfer, the ability of the embryo to adapt to the changing environmental conditions or chromatin to become reprogrammed can exceed its own adaptive capacity, resulting in aberrant embryonic development. Nuclear transfer or embryo culture-induced influences not only affect implantation and establishment of pregnancy but also can extend to fetal and postnatal development and affect susceptibility to disease in later life. It is therefore critical to define the basic program controlling preimplantation development, and also to utilize nuclear transfer and embryo culture models so that we may design healthier environments for preimplantation embryos to thrive in and also minimize the potential for negative consequences during pregnancy and post-gestational life. In addition, it is necessary to couple gene expression analysis with the investigation of gene function so that effects on gene expression can be fully understood. The purpose of this short review is to highlight our knowledge of the mechanisms controlling preimplantation development and report how those mechanisms may be influenced by nuclear transfer and embryo culture.

Early embryonic lethality of mice lacking ZO-2, but Not ZO-3, reveals critical and nonredundant roles for individual zonula occludens proteins in mammalian development

ZO-1, ZO-2, and ZO-3 are closely related scaffolding proteins that link tight junction (TJ) transmembrane proteins such as claudins, junctional adhesion molecules, and occludin to the actin cytoskeleton. Even though the zonula occludens (ZO) proteins are among the first TJ proteins to have been identified and have undergone extensive biochemical analysis, little is known about the physiological roles of individual ZO proteins in different tissues or during vertebrate development. Here, we show that ZO-3 knockout mice lack an obvious phenotype. In contrast, embryos deficient for ZO-2 die shortly after implantation due to an arrest in early gastrulation. ZO-2(-)(/)(-) embryos show decreased proliferation at embryonic day 6.5 (E6.5) and increased apoptosis at E7.5 compared to wild-type embryos. The asymmetric distribution of prominin and E-cadherin to the apical and lateral plasma membrane domains, respectively, is maintained in cells of ZO-2(-)(/)(-) embryos. However, the architecture of the apical junctional complex is altered, and paracellular permeability of a low-molecular-weight tracer is increased in ZO-2(-/-) embryos. Leaky TJs and, given the association of ZO-2 with connexins and several transcription factors, effects on gap junctions and gene expression, respectively, are likely causes for embryonic lethality. Thus, ZO-2 is required for mouse embryonic development, but ZO-3 is dispensable. This is to our knowledge the first report showing that an individual ZO protein plays a nonredundant and critical role in mammalian development.

Na+/K+-ATPase regulates tight junction formation and function during mouse preimplantation development

Research applied to the early embryo is required to effectively treat human infertility and to understand the primary mechanisms controlling development to the blastocyst stage. The present study investigated whether the Na(+)/K(+)-ATPase regulates tight junction formation and function during blastocyst formation. To investigate this hypothesis, three experimental series were conducted. The first experiments defined the optimal dose and treatment time intervals for ouabain (a potent and specific inhibitor of the Na(+)/K(+)-ATPase) treatment. The results demonstrated that mouse embryos maintained a normal development to the blastocyst stage following a 6-h ouabain treatment. The second experiments investigated the effects of ouabain treatment on the distribution of ZO-1 and occludin (tight junction associated proteins). Ouabain treatment (up to 6 h) or culture in K(+)-free medium (up to 6 h) resulted in the appearance of a discontinuous ZO-1 protein distribution and a loss of occludin immunofluorescence. The third set of experiments examined the influence of ouabain treatment on tight junction function. Ouabain treatment or culture in K(+)-free medium affected tight junction permeability as indicated by an increase in the proportion of treated embryos accumulating both 4 kDa and 40 kDa fluorescein isothiocyanate (FITC)-dextran into their blastocyst cavities. The results indicate that the Na(+)/K(+)-ATPase is a potent regulator of tight junction formation and function during mouse preimplantation development.

Roles of Na,K-ATPase in Early Development and Trophectoderm Differentiation

Before implantation into the uterine wall, the mammalian embryo undergoes a period of cell division, cell shape change, and cell differentiation leading to the formation of an outer epithelium, the trophectoderm. The trophectoderm is the part of the embryo that initiates uterine contact and, after transformation to become the trophoblast, uterine invasion. Similar to the kidney nephron, the trophectoderm is a transporting epithelium with distinct apical and basolateral membrane domains; its function is to facilitate transepithelial Na+ and fluid transport for blastocoel formation. That transport is driven by Na,K-adenosine triphosphatase (ATPase) localized in basolateral membranes of the trophectoderm. Preimplantation embryos express multiple alpha and beta subunit isoforms of Na,K-ATPase, potentially constituting multiple isozymes, but the basolaterally located alpha1beta1 isozyme appears to function uniquely to drive fluid transport. Embryos unable to express alpha1 subunits because of targeted deletion of the gene are able to form a blastocoel, but they fail to maintain their integrity and expire during the peri-implantation period. Preimplantation embryos also express the gamma subunit, a modulator of Na,K-ATPase activity, but targeted deletion of that gene did not reveal an essential developmental role. The preimplantation embryo offers a unique model for understanding the roles of Na,K-ATPase subunit isoforms in epithelial development and transepithelial transport.

Deletion of the Na/K-ATPase alpha1-subunit gene (Atp1a1) does not prevent cavitation of the preimplantation mouse embryo

Increases in Na/K-ATPase activity occur concurrently with the onset of cavitation and are associated with increases in Na(+)-pump subunit mRNA and protein expression. We have hypothesized that the alpha1-isozyme of the Na/K-ATPase is required to mediate blastocyst formation. We have tested this hypothesis by characterizing preimplantation development in mice with a targeted disruption of the Na/K-ATPase alpha1-subunit (Atp1a1) using embryos acquired from matings between Atp1a1 heterozygous mice. Mouse embryos homozygous for a null mutation in the Na/K-ATPase alpha1-subunit gene are able to undergo compaction and cavitation. These findings demonstrate that trophectoderm transport mechanisms are maintained in the absence of the predominant isozyme of the Na(+)-pump that has previously been localized to the basolateral membranes of mammalian trophectoderm cells. The presence of multiple isoforms of Na/K-ATPase alpha- and beta-subunits at the time of cavitation suggests that there may be a degree of genetic redundancy amongst isoforms of the catalytic alpha-subunit that allows blastocyst formation to progress in the absence of the alpha1-subunit.

Trophectoderm development and function: the roles of Na+/K(+)-ATPase subunit isoforms

Preimplantation development is a period of cell division, cell shape change, and cell differentiation leading to the formation of an epithelium, the trophectoderm. The trophectoderm is the part of the conceptus that initiates uterine contact and, after transformation to become the trophoblast, uterine invasion. Thus, trophectoderm development during preimplantation stages is a necessary antecedent to the events of implantation. The preimplantation trophectoderm is a transporting epithelium with distinct apical and basolateral membrane domains that facilitate transepithelial Na+ and fluid transport for blastocoel formation. That transport is driven by Na+/K(+)-ATPase localized in basolateral membranes of the trophectoderm. Preimplantation embryos express multiple alpha and beta subunit isoforms of Na+/K(+)-ATPase, potentially constituting multiple isozymes, but the basolaterally located alpha1beta1, isozyme uniquely functions to drive fluid transport. They also express the gamma subunit, which is a modulator of Na+/K(+)-ATPase activity. In the mouse, two splice variants of the gamma subunit, gammaa and gammab, are expressed in the trophectoderm. Antisense knockdown of gamma subunit accumulation caused a delay of cavitation, implying an important role in trophectoderm function. The preimplantation trophectoderm offers a unique model for understanding the roles of Na+/K(+)-ATPase subunit isoforms in transepithelial transport.

Amino acids and preimplantation development of the mouse in protein-free potassium simplex optimized medium

Development of outbred CF1 mouse zygotes in vitro was studied in a chemically defined, protein-free medium both with and without amino acids. The addition of amino acids to protein-free potassium simplex optimized medium (KSOM) had little effect on the proportion of embryos that developed at least to the zona-enclosed blastocyst stage. In contrast, amino acids stimulated very significantly, in a dilution-dependent way, the proportion of blastocysts that at least partially or completely hatched. Amino acids also stimulated cell proliferation in both the trophectoderm and inner cell mass (ICM) cells, at rates that favored proliferation of cells in the ICM; had no effect on the incidence of cell death (oncosis or apoptosis); and improved development of the basement membranes, which form on the blastocoelic surface of the trophectoderm and between the primitive endoderm and the primitive ectoderm. Thus, KSOM, supplemented with amino acids but containing no protein supplements, supports development of a newly fertilized ovum to the late blastocyst stage, in which its normal, three-dimensional structure is preserved and in which the ICM has been partitioned into the primitive ectoderm and primitive endoderm.

Differential involvement of Na(+),K(+)-ATPase isozymes in preimplantation development of the mouse

Na(+),K(+)-ATPase plays an essential role in mammalian blastocoel formation (cavitation) by driving trans-epithelial sodium transport. Previously, the alpha1 and beta1 subunit isoforms of this enzyme were identified in preimplantation mouse embryos and were assumed to be responsible for this function. Here we show that mRNAs encoding an additional alpha subunit isoform (alpha3) and the remaining two beta subunit isoforms are also present in preimplantation embryos. Whereas alpha3 mRNA accumulates between the four-cell and the blastocyst stages and thus results from embryonic transcription, the same could not be demonstrated for beta2 and beta3 mRNAs. Immunoblot analyses confirmed that these subunits are present in cavitating embryos. Using confocal immunofluorescence microscopy we found that alpha1 and beta1 subunits are concentrated in the basolateral membranes of the trophectoderm while being equally distributed in plasma membranes of the inner cell mass. In contrast, alpha3, beta2, and beta3 subunits were not detected in plasma membranes. Our current assessment, therefore, is that as many as six isozymes of Na(+),K(+)-ATPase could be involved in preimplantation development although it is primarily the alpha1beta1 isozyme that is responsible for blastocoel formation. Our findings imply that the regulation of sodium transport within the preimplantation mouse embryo is more complex than had been appreciated.

Apoptosis in rodent peri-implantation embryos: differential susceptibility of inner cell mass and trophectoderm cell lineages--a review

Inner cell mass (ICM) and trophectoderm cell lineages diverge early in cleavage in response to a complex combination of cellular and molecular determinative events. The resulting differences in metabolic requirements, cell positioning and micro-environments are considered as some of the major causes underlying the differential sensitivity of ICM and trophectoderm cell lines to embryotoxic agents. In most instances, ICM cells appear less resistant to disruption than trophectoderm cells, and past observations suggest that over-stimulation of apoptosis is probably one of the mechanisms leading to selective ICM depletion at the blastocyst stage. Disproportionate deficiency in this lineage below a certain threshold level may then prevent the ICM core from providing sufficient prefetal stem cells during gastrulation and from sending regulatory signals to the trophectoderm, leading to compromised post-implantation development. The aim of this review article is to discuss the above observations and to show the value of the impact of hyperglycaemia on blastocyst metabolism and development as an exciting model for further studies.

Dynamic rearrangement of the spectrin membrane skeleton during the generation of epithelial polarity in Drosophila

The origin of epithelial cell polarity during development is a fundamental problem in cell biology. Central to this process is the establishment of asymmetric membrane domains that will ultimately form the apical and basolateral surfaces. The spectrin-based membrane skeleton has long been thought to participate in the generation of this asymmetry. Drosophila melanogaster contains two known (beta)-spectrin isoforms: a conventional (beta)-spectrin chain, and the novel isoform (beta)(Heavy)-spectrin. These two proteins are restricted to the basolateral and apical membrane domains, respectively. To assay for the emergence of membrane asymmetry, we have characterized the distribution of these two (beta)-spectrins during the formation of the primary epithelium in the fly embryo. Our results show that the syncytial embryo contains a maternally established apical membrane skeleton containing (beta)(Heavy)-spectrin into which the basolateral (beta)-spectrin membrane skeleton is added. We have called this process basolateral interpolation. Although basolateral membrane skeleton addition begins during cellularization, it does not become fully established until the formation of a mature zonula adherens at mid to late gastrulation. The behavior of (beta)-spectrin is consistent with a primary role in establishing and/or maintaining the basolateral domain while the behavior of (beta)(Heavy)-spectrin suggests that its primary role is associated with a specialized DE-cadherin complex associated with the furrow canals and with the maturation of the zonula adherens. Thus, the apical spectrin membrane skeleton appears to play a distinct rather than analogous role to the basolateral spectrin membrane skeleton, during the emergence of cell polarity. We find that there are several parallels between our observations and previous studies on the establishment of primary epithelial polarity in vertebrates, suggesting that basolateral interpolation of the membrane skeleton may be a common mechanism in many organisms.

Gene expression regulating blastocyst formation

Development of embryos to the blastocyst stage is a critical event in the early lives of all eutherian mammalian species. Blastocyst formation is essential for implantation and is the principal morphological determinant of embryo quality prior to embryo transfer. The physiological events and roles of specific gene families that regulate blastocyst formation are subjects of intense research Recent findings have demonstrated that bovine embryos express multiple members of the Na/K-ATPase ion transporter gene family. Two members of this family have been co-localized to bovine trophectoderm, but each becomes largely confined to opposing cell membrane margins. Bovine blastocysts display a greater sensitivity to ouabain (potent inhibitor of the Na/K-ATPase) than murine blastocysts, and enzyme activity (ouabain sensitive 86Rb+ uptake) undergoes a 9-fold increase from the bovine morula to the blastocyst stage. Disruption of Na/K-ATPase gene expression by antisense oligodeoxynucleotide inhibition abolishes blastocyst formation. These results have implicated the Na/K-ATPase as a key regulator of bovine blastocyst formation and have provided insights necessary for the production of healthy bovine embryos by the application of in vitro maturation, in vitro fertilization and in vitro culture methods.

Increase of intracellular Ca2+ and relocation of E-cadherin during experimental decompaction of mouse embryos

To determine the role of intracellular Ca2+ in compaction, the first morphogenetic event in embryogenesis, we analyzed preimplantation mouse embryos under several decompacting conditions, including depletion of extracellular Ca2+, blocking of Ca2+ channels, and inhibition of microfilaments, calmodulin, and intracellular Ca2+ release. Those treatments induced decompaction of mouse morulae and simultaneously induced changes in cytosolic free Ca2+ concentration and deregionalization of E-cadherin and fodrin. When morulae were allowed to recompact, the location of both proteins recovered. In contrast, actin did not change its cortical location with compaction nor with decompaction-recompaction. Calmodulin localized in areas opposite to cell-cell contacts in eight-cell stage embryos before and after compaction. Inhibition of calmodulin with trifluoperazine induced its delocalization while morulae decompacted. A nonspecific rise of intracellular free Ca2+ provoked by ionomycin did not affect the compacted shape. Moreover, the same decompacting treatments when applied to uncompacted embryos did not produce any change in intracellular Ca2+. Our results demonstrate that in preimplantation mouse embryos experimentally induced stage-specific changes of cell shape are accompanied by changes of intracellular free Ca2+ and redistribution of the cytoskeleton-related proteins E-cadherin, fodrin, and calmodulin. We conclude that intracellular Ca2+ specifically is involved in compaction and probably regulates the function and localization of cytoskeleton elements.

Contributions of Na+/H+ exchanger isoforms to preimplantation development of the mouse

Previous work provided evidence of Na+/H+ exchanger activity in the apical domain of mouse trophectodermal plasma membranes that provides a route for entry of extracellular Na+ (Manejwala et al., 1989). This activity was hypothesized to contribute to the trans-trophectodermal Na+ flux that is required for blastocoel expansion. In the present work, we have used reverse transcriptase-polymerase chain reaction (RT-PCR) and immunocytochemistry to identify members of the Na+/H+ exchanger (NHE) family that are likely to participate in this process. When cDNA preparations from ovulated oocytes and several stages of preimplantation development were tested with PCR primers specific for the NHE-1, -2, -3, and -4 isoforms of the exchanger, only amplicons representing the NHE-1 and NHE-3 isoforms were detected. The identity of these amplicons was confirmed by direct sequencing. NHE-1 mRNA is present in oocytes and in all preimplantation stages, increasing threefold on a per embryo basis between the 4-cell and blastocyst stages. NHE-3 mRNA, on the other hand, was only detected in oocytes. Immunocytochemical analysis of blastocysts revealed that NHE-1 is localized in the basolateral domain of the trophectoderm, whereas NHE-3 is localized in the apical domain, a situation like that in epithelia of adult organs. We conclude that NHE-3, an oogenetic product that persists into the blastocyst stage, is the Na+/H+ exchanger isoform most likely to be involved in blastocoel expansion.

Na/K-ATPase-mediated 86Rb+ uptake and asymmetrical trophectoderm localization of alpha1 and alpha3 Na/K-ATPase isoforms during bovine preattachment development

This study evaluated Na/K-ATPase alpha 1- and alpha 3-subunit isoform polypeptide expression and localization during bovine preattachment development. Na/K-ATPase cation transport activity from the one-cell to blastocyst stage was also determined by measuring ouabain-sensitive 86Rb+ uptake. Both alpha1- and alpha 3-subunit polypeptides were detected by immunofluorescence to encircle the entire cell margins of each blastomere of inseminated zygotes, cleavage stage embryos, and morulae. Immunofluorescent localization of alpha1-subunit polypeptide in bovine blastocysts revealed an alpha1 immunofluorescence signal confined to the basolateral membrane margins of the trophectoderm and encircling the cell periphery of each inner cell mass (ICM) cell. In contrast, alpha 3-subunit polypeptide immunofluorescence was localized primarily to the apical cell surfaces of the trophectoderm with a reduced signal present in basolateral trophectoderm regions. There was no apparent alpha 3-subunit signal in the ICM. Analysis of 86Rb+ transport in vitro demonstrated ouabain-sensitive activity throughout development from the one-cell to the six- to eight-cell stage of bovine development. 86Rb+ uptake by morulae (day 6 postinsemination) did not vary significantly from uptake detected in cleavage stage embryos; however, a significant increase was measured at the blastocyst stage (P < 0.05). Treatment of embryos with cytochalasin D (5 micrograms/ml) did not influence 86Rb+ uptake in cleavage stage embryos. Cytochalasin D treatment however was associated with a significant rise in ion transport in morulae and blastocysts (13.49 and 61.57 fmol/embryo/min, respectively) compared to untreated controls (2.65 and 22.83 fmol/embryo/min, respectively). Our results, for the first time, demonstrate that multiple Na/K-ATPase alpha-subunit isoforms are distributed throughout the first week of mammalian development and raise the possibility that multiple isozymes of the Na/K-ATPase contribute to blastocyst formation.

Embryonic expression of the putative gamma subunit of the sodium pump is required for acquisition of fluid transport capacity during mouse blastocyst development

The sodium/potassium pump, Na+,K+-ATPase, is generally understood to function as a heterodimer of two subunits, a catalytic alpha subunit and a noncatalytic, glycosylated beta subunit. Recently, a putative third subunit, the gamma subunit, was cloned. This small protein (6.5 kD) coimmunoprecipitates with the alpha and beta subunits and is closely associated with the ouabain binding site on the holoenzyme, but its function is unknown. We have investigated the expression of the gamma subunit in preimplantation mouse development, where Na+, K+-ATPase plays a critical role as the driving force for blastocoel formation (cavitation). Using reverse transcriptase-polymerase chain reaction, we demonstrated that the gamma subunit mRNA accumulates continuously from the eight-cell stage onward and that it cosediments with polyribosomes from its time of first appearance. Confocal immunofluorescence microscopy revealed that the gamma subunit itself accumulates and is localized at the blastomere surfaces up to the blastocyst stage. In contrast with the alpha and beta subunits, the gamma subunit is not concentrated in the basolateral surface of the polarized trophectoderm layer, but is strongly expressed at the apical surface as well. When embryos were treated with antisense oligodeoxynucleotide complementary to the gamma subunit mRNA, ouabain-sensitive K+ transport (as indicated by 86Rb+ uptake) was reduced and cavitation delayed. However, Na+, K+-ATPase enzymatic activity was unaffected as determined by a direct phosphorylation assay ("back door" phosphorylation) applied to plasma membrane preparations. These results indicate that the gamma subunit, although not an integral component of Na+,K+-ATPase, is an important determinant of active cation transport and that, as such, its embryonic expression is essential for blastocoel formation in the mouse.

Dome formation induced by v-H-ras oncogene in a human choriocarcinoma cell line

To investigate the role of ras genes in trophoblastic cell lineage, we transfected the viral H- or K-ras oncogene into a human choriocarcinoma cell line, CCI, and analysed the biological properties of CCI cells expressing an activated ras oncogene. All v-H-ras-expressing clones distinctively formed the hemispherical domes, which represents an in vitro morphological expression of vectorial transport function and are characteristic of the polarized epithelial cells, but none of v-K-ras-expressing clones and control clones did. Microscopic observation demonstrated that those domes were cavities filled with fluid which accumulated between the cell layer and the surface of culture dish. Scanning electron microscopy revealed that the domes were aggregates of round cells with long numerous microvilli and were morphologically similar to a blastocyst. Furthermore, Na(+)-K(+)-ATPase activity, which is associated with the vectorial fluid transport in transporting epithelial cells, was significantly higher in the v-H-ras-expressing clones than that in the v-K-ras-expressing clones and the parental cells. Those domes flattened within 24 h after treatment with a specific inhibitor of Na(+)-K(+)-ATPase, ouabain, and the number of domes decreased in dose-dependent manner, indicating that Na(+)-K(+)-ATPase activity was required for maintainance of domes. These results suggest that up-regulated activity of H-ras but not of K-ras facilitates the vectorial fluid transport through a chorionic cell layer and leads to the dome formation. The function of II-ras in trophoblasts, may therefore, be essential for embryogenesis, especially for supplying the nutrients.

Multiple members of the connexin gene family participate in preimplantation development of the mouse

The connexin gene family, of which there are at least 12 members in rodents, encodes the protein subunits intercellular membrane channels (gap junction channels). Because of the diverse structural and biophysical properties exhibited by the different connexins, it has been proposed that each may play a unique role in development or homeostasis. We have begun to test this hypothesis in the preimplantation mouse embryo in which de novo gap junction assembly is a developmentally regulated event. As a first step, we have used reverse transcription-polymerase chain reaction (RT-PCR) to determine the connexin mRNA phenotype of mouse blastocysts, and have identified transcripts of connexins 30.3, 31, 31.1, 40, 43, and 45. Quantitative measurements indicated that all six of these connexin genes are transcribed after fertilization. They can be divided into two groups with respect to the timing of mRNA accumulation: Cx31, Cx43, and Cx45 mRNAs accumulate continuously from the two- or four-cell stage, whereas Cx30.3, Cx31.1, and Cx40 mRNAs accumulate beginning in the eight-cell stage. All six mRNAs were found to co-sediment with polyribosomes from their time of first appearance, indicating that all six are translated. The expression of Cx31.1 and Cx40 was examined by confocal immunofluorescence microscopy; whereas both could be detected in compacting embryos, only Cx31.1 could be seen in punctate membrane foci indicative of gap junctions. Taken together with other results (published or submitted), our findings indicate that at least four connexins (Cx31, 31.1, 43 and 45) contribute to gap junctions in preimplantation development. The expression of multiple connexin genes during this early period of embryogenesis (when there are only two distinct cell types) raises questions about the functional significance of connexin diversity in this context.

Isolation and characterization of a feeder-dependent, porcine trophectoderm cell line obtained from a 9-day blastocyst

We have established in culture a feeder-dependent cell line, termed TE1, from a 9 day, pre-implantation, porcine embryo. TE1 cells were observed by light and electron microscopy, and characterized by immunocytochemistry: the morphology, cytology and ultrastructure of this cell line are described. The cells display epithelial characteristics, as revealed using immunofluorescence microscopy with antibody against cytokeratins of simple epithelia, but not with antibody against vimentin. The cells demonstrate many morphological and cytochemical features in common with trophectoderm of the intact porcine blastocyst. For example, TE1 cells are polarized and possess tight junctions at their borders, similar to those found in trophectoderm of the pre-implantation embryo. Moreover, TE1 cells label positively for the porcine trophectoderm-specific monoclonal antibody, SN1/38. Thus, by several important criteria TE1 is deduced to be a porcine trophectoderm cell line.

Epidermal growth factor receptor function in early mammalian development

We review here the data indicating a role for epidermal growth factor receptor (EGF receptor) signalling in early mouse development. Embryonic development of the metazoan embryo generally begins with the formation of a cystic structure and epithelial layers that subsequently form anlagen of the definitive body parts and organs. For the mammalian embryo, this cystic structure is a blastocyst whose wall consists of trophectoderm, the first epithelium to develop during mammalian embryogenesis. The onset of expression and function of EGF receptors is coincident with the onset of trophectoderm development. Modulating EGF receptor expression and function modulates trophectoderm differentiation, leading to the hypothesis that functional EGF receptors participate in the induction of trophectoderm development and perhaps of other embryonic epithelial derivatives such as nervous tissues.

Expression of crumbs confers apical character on plasma membrane domains of ectodermal epithelia of Drosophila

The crumbs protein of Drosophila is an integral membrane protein, with 30 EGF-like and 4 laminin A G domain-like repeats in its extracellular segment, which is expressed on the apical plasma membrane of all ectodermally derived epithelia. Here, we present evidence to show that the insertion of crumbs into the plasma membrane is necessary and sufficient to confer apical character on a membrane domain. Overexpression of crumbs results in an enormous expansion of the apical plasma membrane and the concomitant reduction of the basolateral domain. This is followed by the redistribution of beta Heavy-spectrin, a component of the membrane cytoskeleton, and by the ectopic deposition of cuticle and other apical components into these areas. Strikingly, overexpression of the membrane-bound cytoplasmic portion of crumbs alone is sufficient to produce this dominant phenotype. Our results suggest that crumbs plays a key role in specifying the apical plasma membrane domain of ectodermal epithelial cells of Drosophila.

Beta 1- and beta 3-class integrins mediate fibronectin binding activity at the surface of developing mouse peri-implantation blastocysts. Regulation by ligand-induced mobilization of stored receptor

Implanting mouse blastocysts adhere through their abembryonic surfaces to the endometrial extracellular matrix. Because blastocysts cultured on fibronectin in vitro dissociate to form trophoblast outgrowths, it is unclear whether this adhesion is initially mediated by fibronectin receptors on the apical or basolateral surface of the trophectoderm. Intact blastocysts were examined in a ligand binding assay utilizing the fibronectin cell binding domain attached to fluorescent microspheres. Fibronectin binding activity on the apical surface of the trophectoderm was confined to the abembryonic pole of the blastocyst, where trophoblast differentiation initiates, and was regulated temporally in accordance with blastocyst outgrowth. Soluble fibronectin (IC50 = 0.2 microM) or Gly-Arg-Gly-Asp-Ser-Pro, but not laminin, competitively inhibited fibronectin binding activity. Addition of antibodies against the alpha v, alpha 5, beta 1, or beta 3 integrin subunits also inhibited binding activity. Blastocysts cultured in the absence of an adhesive substratum exhibited fibronectin binding activity only after exposure to immobilized or soluble ligand. Potentiation of binding activity by ligand was unaffected by cycloheximide but was sensitive to brefeldin A inhibition of protein trafficking. These findings suggest that the interaction of fibronectin with the trophectoderm induces a translocation event that up-regulates fibronectin binding beta 1- and beta 3-class integrins on the apical surface.

Factors controlling growth, motility, and morphogenesis of normal and malignant epithelial cells

Factors that control epithelial growth, motility, and morphogenesis play important roles in malignancy and in normal development. Here we discuss the molecular nature and the function of two types of molecules that control the development and maintenance of epithelia: Components that regulate epithelial cell adhesion; and soluble factors and their receptors that regulate growth, motility, differentiation, and morphogenesis. In development, the establishment of epithelial cell characteristics and organization is crucially dependent on cell adhesion and the formation of functional adherens junctions. The integrity of adherens junctions is frequently disturbed late in tumor progression, and the resulting loss of epithelial characteristics correlates with the metastatic potential of carcinoma cells. Various soluble factors that induce epithelial growth, motility, or differentiation in cell culture, function via tyrosine kinase receptors. We concentrate here on receptors that are expressed exclusively or predominantly on epithelia, and on ligands that are derived from the mesenchyme. In development, these receptors and their ligands function in mesenchymal-epithelial interactions, which are known to govern growth, morphogenesis, and differentiation of epithelia. During tumor development, mutations or overexpression of the receptors are frequently observed; these alterations contribute to the development and progression of carcinomas.

Polyadenylation of Na(+)-K(+)-ATPase beta 1-subunit during early development of Xenopus laevis

In fully grown Xenopus oocytes, the synthesis of beta-subunits is limiting for the formation of functional Na(+)-K(+)-adenosinetriphosphatase alpha/beta-complexes (Geering, K. FEBS Lett. 285: 189-193, 1991). In the present study, we show that during oocyte growth (from stage I to stage VI) alpha 1-, but not beta 1- or beta 3-isoform, mRNAs accumulate. In addition, beta-mRNAs are apparently sequestered in an untranslated pool in fully grown oocytes (stage VI). From fertilization to morulation, the total pools of alpha 1-, beta 1-, or beta 3-mRNAs vary little. Whereas polyadenylated [poly(A)+] alpha 1- and beta 3-isoform mRNAs did not change significantly, poly(A)+ beta 1-mRNA abundance increased three- to fourfold at morulation, accompanied by a parallel increase in beta 1-protein synthesis. After midblastula transition (i.e., at early gastrula) and during neurulation, poly(A)+ alpha 1- and beta 3-mRNAs accumulated rapidly, whereas poly(A)+ beta 1-mRNA accumulation was delayed by approximately 2 h, beginning only at early neurula. Our results indicate that 1) the abundance of poly(A)+ beta 1-mRNA is rate limiting during embryonic development for the assembly of alpha 1/beta 1-heterodimers, shown to be involved in the vectorial transport of sodium in kidney cells, and 2) the polyadenylation of beta 1-mRNA is a rate-limiting factor during morulation for the synthesis and assembly of new sodium pumps at the time of blastocoel fluid formation. The 3'-untranslated region of beta 1-mRNA (but not of alpha 1-mRNA) expresses cytoplasmic polyadenylation elements (CPEs) with the consensus sequence AXX-AUUUU(A/U)(A/U)(A/U). A role of CPE in the differential polyadenylation of alpha 1- and beta 1-mRNA is proposed.

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.

CRUMBS is involved in the control of apical protein targeting during Drosophila epithelial development

The gene crumbs (crb) of Drosophila encodes a transmembrane protein with 30 EGF-like and four laminin A G-domain-like repeats in its extracellular domain. Loss-of-function mutations lead to severe disorganization and degeneration of ectodermally derived embryonic epithelia. In embryos homozygous for crb8F105, an amorphic allele, the CRUMBS protein is diffusely distributed in the cytoplasm instead of being apically localized as in wild-type; this mislocation occurs before any morphologically detectable cellular phenotype becomes manifest, suggesting that apical targeting of proteins is affected in crb mutant embryos. This has been confirmed by using an antibody directed against YELLOW, another apically expressed protein. A single base exchange in crb8F105 leads to the introduction of a premature stop codon, thus eliminating the C-terminal part of the cytoplasmic domain. A possible role for crb in controlling apical-basal polarity is discussed.

Disease processes in epithelia: the role of the actin cytoskeleton and altered surface membrane polarity

The establishment and maintenance of cell polarity is essential for normal epithelial function. Disruption of the underlying processes, either as a primary inborn defect or as a secondary result of other pathologic processes, can lead to loss of epithelial polarity and further cellular and organ-level dysfunction. Continued elucidation of the processes involved may prove fruitful both in the understanding of basic cell biology and in the understanding and treatment of a variety of disease states.

Protein secretion by the mouse blastocyst: stimulatory effect on secretion into the blastocoel by transforming growth factor-alpha

We have previously demonstrated that newly synthesized proteins are secreted into the mouse blastocoel [Dardik and Schultz (1991): Biol Reprod 45:328-333]. In the present study we examine the effect of transforming growth factor-alpha (TGF-alpha) on these proteins. We observe that TGF-alpha stimulates secretion of these newly synthesized proteins into the blastocoel and apical medium, which faces the zona pellucida, by about 65%. Although one-dimensional gel electrophoretic analysis does not reveal any marked differences in the patterns of newly synthesized proteins secreted into the blastocoel in response to TGF-alpha, zymography reveals a marked stimulation in the secretion of several gelatinases into the blastocoel and apical medium. These results suggest additional functions for TGF-alpha in mouse preimplantation development.

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.

Modulation of mouse preimplantation development by epidermal growth factor receptor antibodies, antisense RNA, and deoxyoligonucleotides

Two-cell mouse preimplantation embryos were cultured for 48 h in four different reagents to modulate epidermal growth factor (EGF) receptor function. These were rabbit polyclonal and mouse monoclonal antibodies to EGF receptor, EGF receptor antisense RNA, and EGF receptor antisense deoxyoligonucleotides. Embryos were scored for two endpoints: onset of cavitation as a measure of trophectoderm differentiation and mean embryo cell number as a measure of cell proliferation. The consistent observations were that cavitation was significantly accelerated by antibodies and delayed by antisense RNA and antisense deoxyoligonucleotides. None of these reagents exerted a significant effect on mean embryo cell number, with one exception, the polyclonal antibody. Our interpretation of these observations is that the antibody binding facilitated cavitation by mimicking natural ligand-receptor binding and inducing the signal transduction cascade that is typical for the EGF receptor. In the case of antisense RNA or deoxyoligonucleotide, we propose that they delayed onset of cavitation by interfering with EGF receptor production. We hypothesize that during this period of development, EGF receptor is concerned predominantly with the regulation of differentiation more than with cell proliferation.

Colocalization of transforming growth factor-alpha and a functional epidermal growth factor receptor (EGFR) to the inner cell mass and preferential localization of the EGFR on the basolateral surface of the trophectoderm in the mouse blastocyst

Results of previous studies suggested that responses of mouse blastocysts to TGF-alpha/EGF treatment are mediated by EGF receptors (EGFR) located on the apical surface of the trophectoderm (TE). We report here results of experiments using gold-labeled EGF that confirm the presence of these apically located EGFRs. In addition, immunoelectron microscopy (IEM) studies using anti-EGFR antibodies indicate that the receptor is preferentially distributed on the basolateral surface of the TE. Furthermore, the receptor is also present on the inner cell mass (ICM) and is likely to be functional, since treatment of isolated ICMs with TGF-alpha affects [35S]methionine uptake and incorporation into acid-insoluble material. IEM was also used to demonstrate that EGF, which is not synthesized by the mouse preimplantation embryo, is present in both the oviduct and the uterus. Maternally derived EGF is present in both ICM and TE cells in freshly isolated blastocysts, but is present in greatly reduced amounts following overnight culture of blastocysts in vitro. Last, IEM was also used to demonstrate that TGF-alpha is preferentially localized to the ICM and polar TE. The co-localization of TGF-alpha and functional EGFRs to the ICM and polar TE suggests potential autocrine, juxtacrine, and paracrine roles for TGF-alpha in blastocyst development.

The cell biology of blastocyst development

Preimplantation development encompasses the "free"-living period of mammalian embryogenesis, which culminates in the formation of a fluid-filled structure, the blastocyst. Cavitation (blastocyst formation) is accompanied by the expression of a novel set of gene products that contribute directly to the attainment of cell polarity with the trophectoderm, which is both the first epithelium of development and the outer cell layer encircling the inner cell mass of the blastocyst. Several of these gene products have been identified and include the tight junction (ZO-1), Na/K-ATPase (alpha and beta subunits), uvomorulin, gap junction (connexin43), and growth factors such as transforming growth factor-alpha (TGF-alpha) and epidermal growth factor (EGF). This review will examine the role(s) of each of these gene products during the onset and progression of blastocyst formation. The trophectodermal tight junctional permeability seal regulates the leakage of blastocoel fluid and also assists in the maintenance of a polarized Na/K-ATPase distribution to the basolateral plasma membrane domain of the mural trophectoderm. The polarized distribution of the Na/K-ATPase plays an integral role in the establishment of a trans-trophectoderm Na+ gradient, which drives the osmotic accumulation of water across the epithelium into the nascent blastocoelic cavity. The cell adhesion provided by uvomorulin is necessary for the establishment of the tight junctional seal, as well as the maintenance of the polarized Na/K-ATPase distribution. Growth factors such as TGF-alpha and EGF stimulate an increase in the rate of blastocoel expansion, which could, in part, be mediated by secondary messengers that result in an increase in Na/K-ATPase activity. Insight into the mechanism of cavitation has, therefore, directly linked blastocyst formation to trophectoderm cell differentiation, which arises through fundamental cell biological processes that are directly involved in the attainment of epithelial cell polarity.

The establishment of polarized membrane traffic in Xenopus laevis embryos

Delineation of apical and basolateral membrane domains is a critical step in the epithelialization of the outer layer of cells in the embryo. We have examined the initiation of polarized membrane traffic in Xenopus and show that membrane traffic is not polarized in oocytes but polarized membrane domains appear at first cleavage. The following proteins encoded by injected RNA transcripts were used as markers to monitor membrane traffic: (a) VSV G, a transmembrane glycoprotein preferentially inserted into the basolateral surface of polarized epithelial cells; (b) GThy-1, a fusion protein of VSV G and Thy-1 that is localized to the apical domains of polarized epithelial cells; and (c) prolactin, a peptide hormone that is not polarly secreted. In immature oocytes, there is no polarity in the expression of VSV G or GThy-1, as shown by the constitutive expression of both proteins at the surface in the animal and vegetal hemispheres. At meiotic maturation, membrane traffic to the surface is blocked; the plasma membrane no longer accepts the vesicles synthesized by the oocyte (Leaf, D. L., S. J. Roberts, J. C. Gerhart, and H.-P. Moore. 1990. Dev. Biol. 141:1-12). When RNA transcripts are injected after fertilization, VSV G is expressed only in the internal cleavage membranes (basolateral orientation) and is excluded from the outer surface (apical orientation, original oocyte membrane). In contrast, GThy-1 and prolactin, when expressed in embryos, are inserted or released at both the outer membrane derived from the oocyte and the inner cleavage membranes. Furthermore, not all of the cleavage membrane comes from an embryonic pool of vesicles--some of the cleavage membrane comes from vesicles synthesized during oogenesis. Using prolactin as a marker, we found that a subset of vesicles synthesized during oogenesis was only released after fertilization. However, while embryonic prolactin was secreted from both apical and basolateral surfaces, the secretion of oogenic prolactin was polarized. Oogenic prolactin was secreted only into the blastocoel (from the cleavage membrane), none could be detected in the external medium (from the original oocyte membrane). These results provide the first direct evidence that the oocyte synthesizes a cache of vesicles for specific recruitment to the embryonic cleavage membranes which are polarized beginning with the first cleavage division.

Temporal changes in the expression and distribution of adhesion molecules during liver development and regeneration

We have compared by immunocytochemistry and immunoblotting the expression and distribution of adhesion molecules participating in cell-matrix and cell-cell interactions during embryonic development and regeneration of rat liver. Fibronectin and the fibronectin receptor, integrin alpha 5 beta 1, were distributed pericellularly and expressed at a steady level during development from the 16th day of gestation and in neonate and adult liver. AGp110, a nonintegrin fibronectin receptor was first detected on the 17th day of gestation in a similar, nonpolarized distribution on parenchymal cell surfaces. At that stage of development haemopoiesis is at a peak in rat liver and fibronectin and receptors alpha 5 beta 1 and AGp110 were prominent on the surface of blood cell precursors. During the last 2 d of gestation (20th and 21st day) hepatocytes assembled around lumina. AGp110 was initially depolarized on the surface of these acinar cells but then confined to the lumen and to newly-formed bile canaliculi. At birth, a marked increase occurred in the canalicular expression of AGp110 and in the branching of the canalicular network. Simultaneously, there was enhanced expression of ZO-1, a protein component of tight junctions. On the second day postpartum, presence of AGp110 and of protein constituents of desmosomes and intermediate junctions, DGI and E-cadherin, respectively, was notably enhanced in cellular fractions insoluble in nonionic detergents, presumably signifying linkage of AGp110 with the cytoskeleton and assembly of desmosomal and intermediate junctions. During liver regeneration after partial hepatectomy, AGp110 remained confined to apical surfaces, indicating a preservation of basic polarity in parenchymal cells. A decrease in the extent and continuity of the canalicular network occurred in proliferating parenchyma, starting 24 h after resection in areas close to the terminal afferent blood supply of portal veins and spreading to the rest of the liver within the next 24 h. Distinct acinar structures, similar to the ones in prenatal liver, appeared at 72 h after hepatectomy. Restoration of the normal branching of the biliary tree commenced at 72 h. At 7 d postoperatively acinar formation declined and one-cell-thick hepatic plates, as in normal liver, were observed.

Effects of human sera and human serum albumin on mouse embryo culture

Human proteins normally used to supplement human in vitro fertilization-embryo transfer (IVF-ET) culture media were tested for their effects on mouse embryo development from the zygote stage. These proteins included follicular and luteal-phase maternal sera, fetal cord sera, and both human and bovine serum albumin. Our results revealed that both maternal and fetal cord sera did not permit mouse blastocyst formation. Furthermore, predialysis of the human maternal sera and removal of IgG by protein A column chromatography did not improve their support of mouse embryonic development to the blastocyst stage. Similar detrimental effects were observed with maternal sera from term-pregnant IVF-ET patients. Interestingly, these serum samples had supported the in vitro growth of the human zygotes which resulted in these patients' pregnancies. Only some batches of human serum albumin supported mouse blastocyst formation, whereas all sources of bovine serum albumin were effective in this regard. These results raise the question of the suitability of the mouse embryo culture system as a quality control for the testing of protein supplements for human IVF-ET.

The genetic program for preimplantation development

This review summarizes information on accumulation profiles of individual gene transcripts in preimplantation development. Most of the information is from the mouse, but some data from other species are reviewed as well. The principal finding is that the transcription of most genes is not temporally linked with any of the three morphogenetic transitions (compaction, cavitation, and blastocoel expansion) that characterize this period. Most genes that are expressed during preimplantation development of the mouse are already being transcribed in the 4-cell stage, and some clearly begin as early as the 2-cell stage. Once activated, a gene continues to be transcribed at least into the blastocyst stage, resulting in continuous mRNA accumulation. Thus the pattern of gene transcription established at the time of genomic activation in the 2-cell stage is perpetuated into the blastocyst, with a few additions along the way. This information is interpreted in light of previous findings concerning the sensitivity of morphogenetic transitions to inhibition of gene expression. The lack of a clear relationship between the timing of expression of most genes and the schedule of morphogenesis leads one to conclude that temporal regulation is imposed downstream of transcription and translation. This conclusion is substantiated by a consideration of factors controlling the events of compaction.

Modulation of Na,K-ATPase expression during early development of Xenopus laevis

In amphibian and mammalian systems, regulation of Na+ transport via the Na,K-ATPase plays an important role in distinct developmental processes such as blastocoele formation and neurulation. In this study, we have followed the Na,K-ATPase activity, the biosynthesis, and the cellular accumulation of catalytic alpha-subunits after fertilization of Xenopus laevis eggs up to neurula formation. Our data show that Na,K-ATPase activity increases significantly between stages 4 and 6 and again between stages 13 and 24. The four-fold rise in Na,K-ATPase activity during blastocoele formation is not mediated by an increased cellular pool of alpha-subunits. On the other hand, a five-fold increase of the biosynthesis rate around midblastula precedes a progressive accumulation up to neurula stage mainly of alpha 1-subunits and to a lesser extent of a second alpha-immunoreactive species. In contrast, newly synthesized glycoproteinic beta 1-subunits of Na,K-ATPase cannot be detected up to late neurula. These data indicate that (1) upregulation of Na,K-ATPase activity during blastocoele and neurula formation are mediated by different regulation mechanisms and (2) alpha- and possibly beta-isoforms are expressed in a developmentally regulated fashion during early Xenopus development.

Antibodies to a renal Na+/glucose cotransport system localize to the apical plasma membrane domain of polar mouse embryo blastomeres

Mouse preimplantation embryos were examined for the cell surface expression of epitopes that cross-react with antibodies to a 75-kDa subunit of a purified porcine renal brush border Na+/glucose cotransport system. A Na+ cotransport system is hypothesized to reside in the apical plasma membrane domain of mouse polar blastomeres and to be associated with the induction of their apical-basal polarity. Western blot analysis showed that unfertilized oocytes as well as preimplantation embryos contain a cross-reacting antigen with an apparent molecular weight of about 75,000. Embryos and their isolated blastomeres were double-labeled and assayed by indirect immunofluorescence (IIF) for the expression of epitopes (visualized by labeling with rabbit antiserum or mouse monoclonal IgG to cotransporter followed by the appropriate rhodamine-conjugated second antibodies) and for the development of cell surface polarity (visualized by the apical restriction of fluoresceinated succinylated concanavalin A binding; FS Con A). IIF did not detect these epitopes until after the second cleavage when 4-cell embryos expressed low-to-moderate levels. Although epitopes were expressed on all surfaces of 4-cell blastomeres, some blastomeres expressed more epitopes on their apical surfaces than on their basolateral ones. All precompaction 8-cell embryos expressed epitopes, with expression being greater apically on some blastomeres. The level of expression appeared to reach a maximum on morulae and to decline on cavitating embryos. Assays performed on isolated blastomeres from postcompaction embryos showed that by the 16-cell stage epitope expression appeared to become restricted to FS Con A-labeled apical plasma membrane domains and was no longer evident on basolateral domains. This apparent apical restriction of epitope expression was confirmed by electron microscopic examination of immunogold-labeled isolated polar 16-cell blastomeres. These results demonstrate that preimplantation mouse embryos contain an antigen(s) that is immunologically and structurally similar to a 75-kDa renal Na+/glucose cotransporter. The onset of cell surface expression of this antigen precedes development of the stable polar phenotype.

Differentiation of an epithelium: factors affecting the polarized distribution of Na+,K(+)-ATPase in mouse trophectoderm

Na+,K(+)-ATPase is a marker of the basolateral plasma membrane domain of polarized epithelial cells, including the mural trophectoderm of the mammalian blastocyst (Watson and Kidder (1988). Dev. Biol. 126, 80-90). We have used this marker to explore the factors governing the establishment and maintenance of apical/basolateral polarity during differentiation of trophectoderm. A polyclonal antiserum (anti-GP80) against human cell-CAM 120/80, a homolog of the mouse cell-cell adhesion protein, uvomorulin, was used to prevent cell flattening (compaction) and formation of the epithelial junctional complex. The majority of treated embryos failed to develop a blastocoel; instead their blastomeres developed fluid-filled cavities that expanded while untreated control embryos were cavitating. Immunocytochemistry revealed that the catalytic subunit of Na+,K(+)-ATPase was contained within the membranes lining these cavities, as well as within numerous punctate foci in the cytoplasm. The down-regulation of expression of the enzyme that normally occurs in the ICM and polar trophectoderm did not take place, since the immunoreactivity remained equally strong in all blastomeres. The enzyme could not be detected in plasma membranes. We conclude that uvomorulin-mediated cell adhesion is involved in spatially restricting the expression of the catalytic subunit and is a prerequisite for the insertion of enzyme-laden vesicles into plasma membranes, but not for expression of the catalytic subunit gene. When fully developed blastocysts were treated with cytochalasins to disrupt the epithelial junctional complex, the catalytic subunit shifted from the basolateral to the apical plasma membrane. This finding suggests a primary role for the apical plasma membrane in the process of polarization, and implies that tight junctions are a manifestation of polarity that serve to maintain the separation between apical and basolateral markers.