Cell polarity in sea urchin embryos: reorientation of cells occurs quickly in aggregates

Simulations of sea urchin early development delineate the role of oriented cell division in the morula-to-blastula transition

The sea urchin morula to blastula transition has long been thought to require oriented cell divisions and blastomere adherence to the enveloping hyaline layer. In a computer simulation model, cell divisions constrained by a surface plane division rule are adequate to effect morphological transition. The hyaline membrane acts as an enhancer but is not essential. The model is consistent with the orientation of micromere divisions and the open blastulae of direct developing species. The surface plane division rule precedes overt epithelization of surface cells and acts to organize the developing epithelium. It is a universal feature of early metazoan development and simulations of non-echinoid cleavage patterns support its role throughout Metazoa. The surface plane division rule requires only local cues and cells need not reference global positional information or embryonic axes.

Cell polarity emerges at first cleavage in sea urchin embryos

In protostomes, cell polarity is present after fertilization whereas most deuterostome embryos show minimal polarity during the early cleavages. We now show establishment of cell polarity as early as the first cleavage division in sea urchin embryos. We find, using the apical markers G(M1), integrins, and the aPKC-PAR6 complex, that cells are polarized upon insertion of distinct basolateral membrane at the first division. This early apical-basolateral polarity, similar to that found in much larger cleaving amphibian zygotes, reflects precocious functional epithelial cell polarity. Isolated cleavage blastomeres exhibit polarized actin-dependent fluid phase endocytosis only on the G(M1), integrin, microvillus-containing apical surface. A role for a functional PAR complex in cleavage plane determination was shown with experiments interfering with aPKC activity, which results in several spindle defects and compromised blastula development. These studies suggest that cell and embryonic polarity is established at the first cleavage, mediated in part by the Par complex of proteins, and is achieved by directed insertion of basolateral membrane in the cleavage furrow.

Focal adhesion kinase (FAK) expression and phosphorylation in sea urchin embryos

We have cloned three cDNA isoforms of focal adhesion kinase (FAK) from the sea urchin, Lytechinus variegatus. The sea urchin FAK is more closely related to FAK from other deuterostomes than from invertebrate protostomes or to cell adhesion kinase beta (CAKbeta/Pyk2/FAK2). FAK is expressed in all cells of sea urchin embryos by the 120-cell stage and strongly in blastulae. Phospho-FAK concentrates on basal surfaces of epithelial cells in early blastulae and occurs in syncytial cables of primary mesenchyme cells (PMC). Inhibition of FAK by constructs of FAK-related non-kinase delays blastocoel expansion and early PMC ingression. These results suggest that FAK has roles in cell adhesion and in the shape and integrity of the epithelial cells in sea urchin embryos.

Molecular networks controlling epithelial cell polarity in development

During embryonic development, polarized epithelial cells are either formed during cleavage or formed from mesenchymal cells. Because the formation of epithelia during embryogenesis has to occur with high fidelity to ensure proper development, embryos allow a functional approach to study epithelial cell polarization in vivo. In particular, genetic model organisms have greatly advanced our understanding of the generation and maintenance of epithelial cell polarity. Many novel and important polarity genes have been identified and characterized in invertebrate systems, like Drosophila melanogaster and Caenorhabditis elegans. With the rapid identification of mammalian homologues of these invertebrate polarity genes, it has become clear that many important protein domains, single proteins and even entire protein complexes are evolutionarily conserved. It is to be expected that the field of epithelial cell polarity is just experiencing the 'top of the iceberg' of a large protein network that is fundamental for the specific adhesive, cell signalling and transport functions of epithelial cells.

The biology of cortical granules

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

Early morphogenesis of the Caenorhabditis elegans pharynx

We investigated the cellular behaviors that accompany the early stages of pharyngeal morphogenesis in Caenorhabditis elegans. The embryonic pharynx develops from a ball of cells into a linear tube connected anteriorly to the buccal cavity and posteriorly to the midgut. By using GFP reporters localized to discrete subcellular regions, we show that pharyngeal morphogenesis can be divided into three stages: (1) lengthening of the nascent pharyngeal lumen by reorientation of apicobasal polarity of anterior pharyngeal cells ("Reorientation"), (2) formation of an epithelium by the buccal cavity cells, which mechanically couples the buccal cavity to the pharynx and anterior epidermis ("Epithelialization"), and (3) a concomitant movement of the pharynx anteriorly and the epidermis of the mouth posteriorly to bring the pharynx, buccal cavity, and mouth into close apposition ("Contraction"). Several models can account for these cellular behaviors, and we distinguish between them by physically or genetically ablating cells within the digestive tract. These studies provide the first description of how the pharynx primordium develops into an epithelial tube, and reveal that pharyngeal morphogenesis resembles aspects of mammalian kidney tubulogenesis.

Change in the adhesive properties of blastomeres during early cleavage stages in sea urchin embryo

Blastomeres of sea urchin embryo change their shape from spherical to columnar during the early cleavage stage. It is suspected that this cell shape change might be caused by the increase in the adhesiveness between blastomeres. By cell electrophoresis, it was found that the amount of negative cell surface charges decreased during the early cleavage stages, especially from the 32-cell stage. It was also found that blastomeres formed lobopodium-like protrusions if the embryos were dissociated in the presence of Ca2+. Interestingly, a decrease in negative cell surface charges and pseudopodia formation first occurred in the descendants of micromeres and then in mesomeres, and last in macromeres. By examining the morphology of cell aggregates derived from the isolated blastomeres of the 8-cell stage embryo, it was found that blastomeres derived from the animal hemisphere (mesomere lineage) increased their adhesiveness one cell cycle earlier than those of the vegetal hemisphere (macromere lineage). The timing of the initiation of close cell contact in the descendants of micro-, meso- and macromeres was estimated to be 16-, 32- and 60-cell stage, respectively. Conversely, the nucleus-to-cell-volume ratios, which are calculated from the diameters of the nucleus and cell, were about 0.1 when blastomeres became adhesive, irrespective of the lineage.

Apical, lateral, and basal polarization cues contribute to the development of the follicular epithelium during Drosophila oogenesis

Analysis of the mechanisms that control epithelial polarization has revealed that cues for polarization are mediated by transmembrane proteins that operate at the apical, lateral, or basal surface of epithelial cells. Whereas for any given epithelial cell type only one or two polarization systems have been identified to date, we report here that the follicular epithelium in Drosophila ovaries uses three different polarization mechanisms, each operating at one of the three main epithelial surface domains. The follicular epithelium arises through a mesenchymal-epithelial transition. Contact with the basement membrane provides an initial polarization cue that leads to the formation of a basal membrane domain. Moreover, we use mosaic analysis to show that Crumbs (Crb) is required for the formation and maintenance of the follicular epithelium. Crb localizes to the apical membrane of follicle cells that is in contact with germline cells. Contact to the germline is required for the accumulation of Crb in follicle cells. Discs Lost (Dlt), a cytoplasmic PDZ domain protein that was shown to interact with the cytoplasmic tail of Crb, overlaps precisely in its distribution with Crb, as shown by immunoelectron microscopy. Crb localization depends on Dlt, whereas Dlt uses Crb-dependent and -independent mechanisms for apical targeting. Finally, we show that the cadherin-catenin complex is not required for the formation of the follicular epithelium, but only for its maintenance. Loss of cadherin-based adherens junctions caused by armadillo (beta-catenin) mutations results in a disruption of the lateral spectrin and actin cytoskeleton. Also Crb and the apical spectrin cytoskeleton are lost from armadillo mutant follicle cells. Together with previous data showing that Crb is required for the formation of a zonula adherens, these findings indicate a mutual dependency of apical and lateral polarization mechanisms.

Characterization of sea urchin unconventional myosins and analysis of their patterns of expression during early embryogenesis

Early sea urchin development requires a dynamic reorganization of both the actin cytoskeleton and cytoskeletal interactions with cellular membranes. These events may involve the activities of multiple members of the superfamily of myosin motor proteins. Using RT-PCR with degenerate myosin primers, we identified 11 myosin mRNAs expressed in unfertilized eggs and coelomocytes of the sea urchin Strongylocentrotus purpuratus. Seven of these sea urchin myosins belonged to myosin classes Igamma, II, V, VI, VII, IX, and amoeboid-type I, and the remaining four may be from novel classes. Sea urchin myosins-V, -VI, -VII, and amoeboid-type-I were either completely or partially cloned and their molecular structures characterized. Sea urchin myosins-V, -VI, -VII, and amoeboid-type-I shared a high degree of sequence identity with their respective family members from vertebrates and they retained their class-specific structure and domain organization. Analysis of expression of myosin-V, -VI, -VII, and amoeboid-type-I mRNAs during development revealed that each myosin mRNA displayed a distinct temporal pattern of expression, suggesting that myosins might be involved in specific events of early embryogenesis. Interestingly, the onset of gastrulation appeared to be a pivotal point in modulation of myosin mRNA expression. The presence of multiple myosin mRNAs in eggs and embryos provides insight into the potential involvement of multiple specific motor proteins in the actin-dependent events of embryo development.

Developmental characterization of the gene for laminin alpha-chain in sea urchin embryos

We describe the isolation and characterization of a cDNA clone encoding a region of the carboxy terminal globular domain (G domain) of the alpha-1 chain of laminin from the sea urchin, Strongylocentrotus purpuratus. Sequence analysis indicates that the 1.3 kb cDNA (spLAM-alpha) encodes the complete G2 and G3 subdomains of sea urchin a-laminin. The 11 kb spLAM-alpha mRNA is present in the egg and declines slightly in abundance during development to the pluteus larva. The spLAM-alpha gene is also expressed in a variety of adult tissues. Whole mount in situ hybridization of gastrula stage embryos indicates that ectodermal and endodermal epithelia and mesenchyme cells contain the spLAM-alpha mRNA. Immunoprecipitation experiments using an antibody made to a recombinant fusion protein indicates spLAM-alpha protein is synthesized continuously from fertilization as a 420 kDa protein which accumulates from low levels in the egg to elevated levels in the pluteus larva. Light and electron microscopy identify spLAM-alpha as a component of the basal lamina. Blastocoelic microinjection of an antibody to recombinant spLAM-alpha perturbs gastrulation and skeleton formation by primary mesenchyme cells suggesting an important role for laminin in endodermal and mesodermal morphogenesis.

Characterization of the role of cadherin in regulating cell adhesion during sea urchin development

During development, the modulation of cadherin adhesive function is proposed to control various morphogenetic events including epithelial-mesenchymal conversions and tubulogenesis, although the mechanisms responsible for regulating cadherin activity during these events remain unclear. In order to gain insights into the regulation of cadherin function during morphogenesis, we utilized the sea urchin embryo as a model system to study the regulation of cadherin localization during epithelial-mesenchymal conversion and convergent-extension movements. Polyclonal antibodies raised against the cytoplasmic domain of a cloned sea urchin cadherin recognize three major polypeptides of M(r) 320, 140, and 125 kDa and specifically stain adherens junctions, and to a lesser extent, lateral membrane domains in all epithelial tissues of the embryo. Analysis of embryos during gastrulation demonstrates that changes in cadherin localization are observed in cells undergoing an epithelial-mesenchymal conversion. Ingression of primary mesenchyme cells is accompanied by the rapid loss of junctional cadherin staining and the coincident accumulation of cadherin in intracellular organelles. These data are consistent with the idea that the deadhesion of mesenchymal cells from neighboring epithelial cells involves the regulated endocytosis of cell surface cadherin molecules. Conversely, neither cadherin abundance nor localization is altered in cells of the gut which undergo convergent-extension movements during the formation of the archenteron. This observation indicates that these movements do not require the loss of junctional cadherin molecules. Instead, the necessary balance between adhesion and motility may be achieved by regulating the expression of different subtypes of cadherin molecules or modifying interactions between cadherins and catenins, proteins that bind the cytoplasmic domain of cadherin and are necessary for cadherin adhesive function. To address cadherin function at the molecular level, we used a partial cDNA representing the conserved cytoplasmic domain to identify a novel cadherin molecule in the sea urchin Lytechinus variegatus. The deduced amino acid sequence of LvG-cadherin (for Goliath-cadherin) predicts that it is a transmembrane protein with an apparent relative molecular mass of 303 kDa. The cytoplasmic domain shows significant sequence identity to that of vertebrate classic cadherins. However, the extracellular domain is distinguished from its vertebrate counterparts by both an increased number of cadherin-specific repeats and the presence of four EGF-like repeats proximal to the transmembrane domain. Taken together, these data are consistent with the hypothesis that the sea urchin possesses several cadherins, including a novel member of the cadherin family, and that the dynamic regulation of cadherin localization plays a role in epithelial to mesenchymal conversions during gastrulation.

Regulated exocytosis and sequential construction of the extracellular matrix surrounding the sea urchin zygote

After fertilization most eggs become surrounded by a complex extracellular matrix. This study examines those matrix assembly processes that are triggered by fertilization of the sea urchin egg. The study uses antibodies that identify five different storage compartments in the egg. These compartments release their protein contents in a highly regulated fashion to assemble and modify the extraembryonic layers. The exocytosis sequence begins with a fertilization wave that progresses from the site of sperm entry and elevates the fertilization envelope above a water-filled perivitelline space. The immediate surface of the zygote then becomes covered by a newly secreted hyaline layer. Prior to fertilization some of the antigens are localized to cortical granules. Others are found in "basal laminar vesicles" that are released in a wave beginning at about 30 sec, or roughly at the same time as cortical granule exocytosis. The remaining antigens are exocytosed with a rather precise timing, but with a delay of several to tens of minutes relative to the first wave of exocytosis. "Apical vesicles," so named because antigens from this class are preferentially exocytosed toward the apical cell surface of polarized cells, include antigens that are exocytosed beginning at about 5 min postfertilization. The fourth compartment, named "echinonectin vesicles" release echinonectin, a protein that is deposited to the inner side of the hyaline layer. Surface staining of echinonectin is first detected about 10-15 min following sperm contact. Finally, maternal cadherin, which is stored in yet a fifth distinct compartment, is not detected on the surface until at least 30 min following fertilization. The data are also consistent with the notion that the tightly regulated timing of exocytosis contributes to the ordered assembly of the hyaline layer and elevation of the fertilization envelope. Finally, two of the vesicle classes continue to exocytose after the cells become polarized. In polarized cells apical and basal laminar antigens are trafficked toward opposite sides of the same cell after passing through the same trans-Golgi network-like compartment.

Characterization of moesin in the sea urchin Lytechinus variegatus: redistribution to the plasma membrane following fertilization is inhibited by cytochalasin B

We have investigated the distribution and function of an ezrin-radixin-moesin-like (ERM) molecule in the sea urchin. A sea urchin homologue of moesin was cloned that shares 75% amino acid similarity in the conserved N-terminal region to other moesin molecules. A 6.3 kb message is transcribed late in embryogenesis and is present in adult tissues. Polyclonal antibodies were generated to proteins expressed by a bacterial expression vector, and affinity purified. These antibodies recognize a single 75 kDa protein that is present throughout development in approximately equal abundance, and specifically they immuno-precipitate a single protein. We show by immunolocalization that SUmoesin has two predominant patterns during development. First, SUmoesin is rapidly redistributed after fertilization from a location throughout the egg cytoplasm to a location in the egg cortex. Later in embryogenesis, SUmoesin is localized to the apical ends of cells in the regions of cell-cell junctions. We show that SUmoesin is present in actin-rich regions of the embryo. Finally, we show that the location of SUmoesin requires an intact actin-based cytoskeleton. SUmoesin fails to localize to the plasma membrane after fertilization in the presence of cytochalasin B. Furthermore, SUmoesin loses its apical position in the region of cell-cell junctions in the presence of cytochalasin B in later stages of embryogenesis. This effect is reversible, and the microtubule inhibitor colchicine has no effect. These results show that SUmoesin becomes associated with apical plasma membrane structures early in development, and that SUmoesin is both coincident with actin and requires the assembly of actin filaments to maintain its localization.

Deployment of extracellular matrix proteins in sea urchin embryogenesis

The apical extracellular matrix of the sea urchin embryo, known as the hyaline layer (HL), is a multi-laminate organelle composed of at least 10 polypeptides. Although integrated into one ECM, HL proteins exhibit individual temporal and spatial dynamics throughout development. These molecules are stockpiled in the oocyte during vitellogenesis in at least four distinct vesicle populations. They are released onto the cell surface at fertilization in a specific order, and interact differentially with embryonic cells as development proceeds. Many experiments have suggested that the HL is vital for embryogenesis, but relatively little is known about the functions and interactions of its constituent molecules. The purpose of the present review has been to gather information on the basic characteristics of the known HL proteins together with data on their expression in the embryo, and where possible, their biological activities. Compiled, these observations may provide some insight into the workings of a uniquely embryonic organelle.

Centrifugal elutriation of large fragile cells: isolation of RNA from fixed embryonic blastomeres

In order to analyze the RNA populations present in different cells of very early embryos, we have developed a protocol to purify these large blastomeres using counterflow centrifugal elutriation (CCE). This procedure employs ethanol fixation to stabilize the cells against shear forces encountered during CCE. Using this method, we fractionated the three different blastomere types of the 16-cell sea urchin embryo, the micromeres, mesomeres, and macromeres, achieving 96, 94, and 96% mean purities, respectively. We show here that intact RNA is recovered with equal efficiency from each blastomere preparation. Using this method, we have identified several RNAs that are distributed non-uniformly among these cells.

Structural and functional polarity of starfish blastomeres

The cortex of the blastomeres of Asterina pectinifera are structurally polarized so that some kinds of granules in the cortex, which can be stained vitally with Nile blue (Nile blue-positive granules, NBGs), and microvilli were distributed mainly in the apical region. The blastomeres always faced the adjoining blastomeres and blastocoel with the NBG-free, smooth region during embryogenesis. To confirm whether such blastomeres are functionally polarized, we rotated one of the blastomeres in the 2-cell-stage embryo so that it faced the other with the NBG-containing region. As a result, all embryos developed into twin or partitioned blastulae. This shows that the blastomeres are functionally polarized and have to orient the basal cortex toward the inner side of the embryo in order to be integrated into a blastula together with the others. The cortical polarity was formed and maintained even in blastomeres of dissociated embryos. In such blastomeres the cleavage furrows were formed along the axis of polarity. When the blastomeres began to adhere closely to each other at the 256-cell stage, only the NBG-free (basal) region acquired adhesiveness. These facts make it possible to infer why the correct apicobasal orientation of blastomeres is necessary for embryonic integration, without considering intercellular communication during the cleavage stage.

Embryonic cellular organization: differential restriction of fates as revealed by cell aggregates and lineage markers

Cleavage-stage Lytechinus variegatus embryos were dissociated and the cells were aggregated in an experimental system designed to address questions of embryonic organizational capability. Using monoclonal antibodies against stage- and structure-specific antigens, it was determined that germ-layer specific molecules were expressed in the expected locations in aggregates and that the germ layers differentiated in the normal temporal sequence, but the dorsoventral axis of the embryo was not reestablished properly. In other experiments individual rhodamine-labeled blastomeres were incorporated into unlabeled aggregates. Micromeres localized and differentiated normally. Mesomeres, however, which in normal embryos form only ectoderm, were found to change their specified fate and participate in gut formation. The sequence of aggregate organization revealed other properties of the embryo. Ectodermal and endodermal epithelia formed via two temporally distinct epithelialization events. Ectoderm separated from a mass of interior cells at about 12 hrs, and endoderm compacted from the interior cells at about 20 hr. A lathrytic agent that prevents gut formation in normal embryos also prevented gut formation in aggregates; however, it did not affect formation of the ectoderm. Hence, formation of the triploblastic structure in aggregates appears to be dependent upon developmentally regulated, distinct cell adhesion events.