Expression patterns of gap junctional proteins connexin 32 and 43 suggest new communication compartments in the gastrulating rabbit embryo

Pigmented macrophages and related aggregates in the spleen of european sea bass dosed with heavy metals: Ultrastructure and explorative morphometric analysis

The ultrastructure and morphometrics of pigmented macrophages (PMs) were assessed in the spleen of European sea bass experimentally dosed with Cd and Hg. PMs occurred either as solitary cells or as variably structured aggregations, defined as macrophage aggregates (MAs). Light microscopy revealed a high degree of morphological heterogeneity amongst MAs of all experimental groups. At the ultrastructural level, MAs showed a heterogeneous pigment content that was not influenced by the treatment. Cytoplasm rarefaction/vacuolation and euchromatic nuclei, were observed in PMs of dosed fish. Undosed and Cd-dosed samples differ significantly with regard to the following morphometric features: the Minor axis of the best fitting ellipse, Aspect Ratio, and Roundness. In Cd-dosed fish, MAs showed reduced size and complexity. Lacunarity showed significant differences between undosed and both Cd and Hg-dosed samples. These results suggest that heavy metals, and especially Cd, may influence the dynamics of PM aggregation/disaggregation. Variability in splenic MAs was observed both by light and electron microscopy. However, only the morphometric techniques adequately and objectively described the phenomenon, allowing a quantitative/statistical comparison of morphology among experimental groups. These morphometric analyses could be usefully applied in toxicological and ecotoxicological, as well as morpho-functional studies.

It's never too early to get it Right: A conserved role for the cytoskeleton in left-right asymmetry

For centuries, scientists and physicians have been captivated by the consistent left-right (LR) asymmetry of the heart, viscera, and brain. A recent study implicated tubulin proteins in establishing laterality in several experimental models, including asymmetric chemosensory receptor expression in C. elegans neurons, polarization of HL-60 human neutrophil-like cells in culture, and asymmetric organ placement in Xenopus. The same mutations that randomized asymmetry in these diverse systems also affect chirality in Arabidopsis, revealing a remarkable conservation of symmetry-breaking mechanisms among kingdoms. In Xenopus, tubulin mutants only affected LR patterning very early, suggesting that this axis is established shortly after fertilization. This addendum summarizes and extends the knowledge of the cytoskeleton's role in the patterning of the LR axis. Results from many species suggest a conserved role for the cytoskeleton as the initiator of asymmetry, and indicate that symmetry is first broken during early embryogenesis by an intracellular process.

Ultrastructural changes in goat interspecies and intraspecies reconstructed early embryos

The low efficiency of somatic cell nuclear transfer may be related to the ultrastructural deviations of reconstructed embryos. The present study investigated ultrastructural differences between in vivo-produced and cloned goat embryos, including intra- and interspecies embryos. Goat ear fibroblast cells were used as donors, while the enucleated bovine and goat oocytes matured in vitro as recipients. Goat–goat (GG), goat–cattle (GC) and goat in vivo-produced embryos at the 2-cell, 4-cell, 8-cell and 16-cell stages were compared using transmission electron microscopy. These results showed that the three types of embryos had a similar tendency for mitochondrial change. Nevertheless, changes in GG embryos were more similar to changes in in vivo-produced embryos than were GC embryos, which had more extreme mitochondrial deviation. The results indicate the effects of the cytoplast on mitochondria development. The zona pellucida (ZP) in all three types of embryos became thinner and ZP pores in both GC and GG embryos showed an increased rate of development, especially for GC embryos, while in vivo-produced embryos had smooth ZP. The Golgi apparatus (Gi) and rough endoplasmic reticulum (RER) of the two reconstructed embryos became apparent at the 8-cell stage, as was found for in vivo embryos. The results showed that the excretion of reconstructed embryos was activated on time. Lipid droplets (LD) of GC and GG embryos became bigger, and congregated. In in vivo-produced embryos LD changed little in volume and dispersed gradually from the 4-cell period. The nucleolus of GC and GG embryos changed from electron dense to a fibrillo-granular meshwork at the 16-cell stage, showing that nucleus function in the reconstructed embryos was activated. The broken nuclear envelope and multiple nucleoli in one blastomere illuminated that the nucleus function of reconstructed embryos was partly changed. In addition, at a later stage in GC embryos the nuclear envelope displayed infoldings and the chromatin was concentrated, implying that the blastomeres had an obvious trend towards apoptosis. The gap junctions of the three types of embryos changed differently and GG and GC embryos had bigger perivitelline and intercellular spaces than did in vivo-produced embryos. These results are indicative of normal intercellular communication at an early stage, but this became weaker in later stages in reconstructed embryos. In conclusion, inter- and intraspecies reconstructed embryos have a similar pattern of developmental change to that of in vivo-produced embryos for ZP, rough ER, Gi and nucleolus, but differ for mitochondria, LD, vesicles, nucleus and gap junction development. In particular, the interspecies cloned embryos showed more severe destruction. These ultrastructural deviations might contribute to the compromised developmental potential of reconstructed embryos.

Gap junctional communication in morphogenesis

Gap junctions permit the direct passage of small molecules from the cytosol of one cell to that of its neighbor, and thus form a system of cell-cell communication that exists alongside familiar secretion/receptor signaling. Because of the rich potential for regulation of junctional conductance, and directional and molecular gating (specificity), gap junctional communication (GJC) plays a crucial role in many aspects of normal tissue physiology. However, the most exciting role for GJC is in the regulation of information flow that takes place during embryonic development, regeneration, and tumor progression. The molecular mechanisms by which GJC establishes local and long-range instructive morphogenetic cues are just beginning to be understood. This review summarizes the current knowledge of the involvement of GJC in the patterning of both vertebrate and invertebrate systems and discusses in detail several morphogenetic systems in which the properties of this signaling have been molecularly characterized. One model consistent with existing data in the fields of vertebrate left-right patterning and anterior-posterior polarity in flatworm regeneration postulates electrophoretically guided movement of small molecule morphogens through long-range GJC paths. The discovery of mechanisms controlling embryonic and regenerative GJC-mediated signaling, and identification of the downstream targets of GJC-permeable molecules, represent exciting next areas of research in this fascinating field.

Novel gene expression patterns along the proximo-distal axis of the mouse embryo before gastrulation

BACKGROUND

To date, the earliest stage at which the orientation of the anterior-posterior axis in the mouse embryo is distinguishable by asymmetric gene expression is shortly after E5.5. At E5.5, prospective anterior markers are expressed at the distal tip of the embryo, whereas prospective posterior markers are expressed more proximally, close to the boundary with the extraembryonic region.

RESULTS

To contribute to elucidating the mechanisms underlying the events involved in early patterning of the mouse embryo, we have carried out a microarray screen to identify novel genes that are differentially expressed between the distal and proximal parts of the E5.5 embryo. Secondary screening of resulting candidates by in situ hybridisation at E5.5 and E6.5 revealed novel expression patterns for known and previously uncharacterised genes, including Peg10, Ctsz1, Cubilin, Jarid1b, Ndrg1, Sfmbt2, Gjb5, Talia and Plet1. The previously undescribed gene Talia and recently identified Plet1 are expressed specifically in the distal-most part of the extraembryonic ectoderm, adjacent to the epiblast, and are therefore potential candidates for regulating early patterning events. Talia and the previously described gene XE7 define a gene family highly conserved among metazoans and with a predicted protein structure suggestive of a post-transcriptional regulative function, whilst Plet1 appears to be mammal-specific and of unknown function.

CONCLUSION

Our approach has allowed us to compare expression between dissected parts of the egg cylinder and has identified multiple genes with novel expression patterns at this developmental stage. These genes are potential candidates for regulating tissue interactions following implantation.

Is the early left-right axis like a plant, a kidney, or a neuron? The integration of physiological signals in embryonic asymmetry

Embryonic morphogenesis occurs along three orthogonal axes. While the patterning of the anterior-posterior and dorsal-ventral axes has been increasingly well-characterized, the left-right (LR) axis has only relatively recently begun to be understood at the molecular level. The mechanisms that ensure invariant LR asymmetry of the heart, viscera, and brain involve fundamental aspects of cell biology, biophysics, and evolutionary biology, and are important not only for basic science but also for the biomedicine of a wide range of birth defects and human genetic syndromes. The LR axis links biomolecular chirality to embryonic development and ultimately to behavior and cognition, revealing feedback loops and conserved functional modules occurring as widely as plants and mammals. This review focuses on the unique and fascinating physiological aspects of LR patterning in a number of vertebrate and invertebrate species, discusses several profound mechanistic analogies between biological regulation in diverse systems (specifically proposing a nonciliary parallel between kidney cells and the LR axis based on subcellular regulation of ion transporter targeting), highlights the possible importance of early, highly-conserved intracellular events that are magnified to embryo-wide scales, and lays out the most important open questions about the function, evolutionary origin, and conservation of mechanisms underlying embryonic asymmetry.

Epithelial-mesenchymal transition in colonies of rhesus monkey embryonic stem cells: a model for processes involved in gastrulation

Rhesus monkey embryonic stem (rhES) cells were grown on mouse embryonic fibroblast (MEF) feeder layers for up to 10 days to form multilayered colonies. Within this period, stem cell colonies differentiated transiently into complex structures with a disc-like morphology. These complex colonies were characterized by morphology, immunohistochemistry, and marker mRNA expression to identify processes of epithelialization as well as epithelial-mesenchymal transition (EMT) and pattern formation. Typically, differentiated colonies were comprised of an upper and a lower ES cell layer, the former growing on top of the layer of MEF cells whereas the lower ES cell layer spread out underneath the MEF cells. Interestingly, in the central part of the colonies, a roundish pit developed. Here the feeder layer disappeared, and upper layer cells seemed to ingress and migrate through the pit downward to form the lower layer while undergoing a transition from the epithelial to the mesenchymal phenotype, which was indicated by the loss of the marker proteins E-cadherin and ZO-1 in the lower layer. In support of this, we found a concomitant 10-fold upregulation of the gene Snail2, which is a key regulator of the EMT process. Conversion of epiblast to mesoderm was also indicated by the regulated expression of the mesoderm marker Brachyury. An EMT is a characteristic process of vertebrate gastrulation. Thus, these rhES cell colonies may be an interesting model for studies on some basic processes involved in early primate embryogenesis and may open new ways to study the regulation of EMT in vitro.

Polarity in the rabbit embryo

The main aim of the gastrulation process is commonly regarded to be the generation of the definitive germ layers known as mesoderm, endoderm and ectoderm. Here we discuss how the topography of gene expression, cellular migration and proliferative activity in the preliminary germ layers (hypoblast and epiblast) of the rabbit embryo reveal the sequence of events that establishes the three major body axes. We present a testable model in which a combination of cellular movement in the hypoblast with a morphogen gradient created by the (extraembryonic) trophoblast creates morphological polarity in the embryo and, hence, the co-ordinates for germ layer formation.

The embryonic origins of left-right asymmetry

The bilaterally symmetric body plan of vertebrates features several consistent asymmetries in the placement, structure, and function of organs such as the heart, intestine, and brain. Deviations from the normal pattern result in situs inversus, isomerisms, or heterotaxia (independent randomization), which have significant clinical implications. The invariance of the left-right (LR) asymmetry of normal morphology, neuronal function, and phenotype of several syndromes raises fascinating and fundamental questions in cell, developmental, evolutionary, and neurobiology. While a pathway of asymmetrically expressed signaling factors has been well-characterized in several model systems, very early steps in the establishment of LR asymmetry remain poorly understood. In particular, the origin of consistently oriented asymmetry is unknown. Recently, a candidate for the origins of asymmetry has been suggested: bulk transport of extracellular morphogens by rotating primary cilia during gastrulation. This model is appealing because it 'bootstraps' morphological asymmetry of the embryo from the intrinsic structural (molecular) chirality of motile cilia. However, conceptual and practical problems remain with this hypothesis. Indeed, the genetic data are also consistent with a different mechanism: cytoplasmic transport roles of motor proteins. This review outlines the progress and remaining questions in the field of left-right asymmetry, and focuses on an alternative model for 'Step 1' of asymmetry. More specifically, based on wide-ranging data on ion fluxes and motor protein function in several species, it is suggested that laterality is driven by pH/voltage gradients across the midline, which are established by chiral movement of motor proteins with respect to the cytoskeleton.

Motor protein control of ion flux is an early step in embryonic left-right asymmetry

The invariant left-right asymmetry of animal body plans raises fascinating questions in cell, developmental, evolutionary, and neuro-biology. While intermediate mechanisms (e.g., asymmetric gene expression) have been well-characterized, very early steps remain elusive. Recent studies suggested a candidate for the origins of asymmetry: rotary movement of extracellular morphogens by cilia during gastrulation. This model is intellectually satisfying, because it bootstraps asymmetry from the intrinsic biochemical chirality of cilia. However, conceptual and practical problems remain with this hypothesis, and the genetic data is consistent with a different mechanism. Based on wide-ranging data on ion fluxes and motor protein action in a number of species, a model is proposed whereby laterality is generated much earlier, by asymmetric transport of ions, which results in pH/voltage gradients across the midline. These asymmetries are in turn generated by a new candidate for "step 1": asymmetric localization of electrogenic proteins by cytoplasmic motors.

Cadmium-induced alterations of connexin expression in the promotion stage of in vitro two-stage transformation

During the multistage carcinogenesis, functions of several key genes involved in the cell cycle control and cell-cell communication can be damaged. Gap junction intercellular communication (GJIC) is known to transfer small, water-soluble molecules through intercellular channels composed of proteins called connexins (Cxs). Therefore, aberrant expression of Cx may be one of the critical factors for the clonal expansion of initiated cells during the two-stage transformation. We already improved the classical in vitro two-stage transformation method using N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) as an initiator and cadmium as a promoter on Balb/3T3 A31 cells, and reconfirmed the promotional effect of cadmium with this method (Fang, M.Z., Cho, M.H., Lee, H.W., 2001. Improvement of in vitro two-stage transformation assay and detection of the promotional effect of cadmium, Toxicol. In Vitro (in press). In this study, precise roles of Cd on Cx expression in normal Balb/3T3 A31 and during the promotion stage of the in vitro two-stage transformation were elucidated. For this purpose, the Cx43, Cx32 and Cx26 protein levels, Cx43 and Cx26 mRNA levels and the cellular distribution location of Cx43 protein were determined. Normal Balb/3T3 cells expressed Cx43 and Cx32, but not Cx26. After a short-term treatment of cadmium on normal cells, phosphorylation of Cx43 protein increased and Cx32 protein level decreased. However, during the promotion stage of the in vitro two-stage transformation, transformed cells treated with cadmium for long periods expressed Cx43 and Cx32 highly, similar to the level of normal Balb/3T3 cells, compared to the nontransformed cells. Moreover, Cx43 of the transformed cells was distributed mostly in the perinuclear region rather than the intercellular membrane. These data suggest that cadmium may inhibit the GJIC by increasing the phosphorylation of Cx43 and decreasing the expression of Cx32 in the normal Balb/3T3 A31 cells. Our results also suggest that these changes are not associated with the cell transformation; transformed cells may reexpress Cx43 and Cx32 similar to the normal cells, though Cx43 protein is distributed aberrantly during the transformation process. Further studies are needed to clarify the relationship between transformation and posttranslational modification of the Cx proteins.