Reaggregation of dissociated cells of sea urchin embryos

Cellular pathways of calcium transport and concentration toward mineral formation in sea urchin larvae

Sea urchin larvae have an endoskeleton consisting of two calcitic spicules. The primary mesenchyme cells (PMCs) are the cells that are responsible for spicule formation. PMCs endocytose sea water from the larval internal body cavity into a network of vacuoles and vesicles, where calcium ions are concentrated until they precipitate in the form of amorphous calcium carbonate (ACC). The mineral is subsequently transferred to the syncytium, where the spicule forms. Using cryo-soft X-ray microscopy we imaged intracellular calcium-containing particles in the PMCs and acquired Ca-L2,3 X-ray absorption near-edge spectra of these Ca-rich particles. Using the prepeak/main peak (L2'/ L2) intensity ratio, which reflects the atomic order in the first Ca coordination shell, we determined the state of the calcium ions in each particle. The concentration of Ca in each of the particles was also determined by the integrated area in the main Ca absorption peak. We observed about 700 Ca-rich particles with order parameters, L2'/ L2, ranging from solution to hydrated and anhydrous ACC, and with concentrations ranging between 1 and 15 M. We conclude that in each cell the calcium ions exist in a continuum of states. This implies that most, but not all, water is expelled from the particles. This cellular process of calcium concentration may represent a widespread pathway in mineralizing organisms.

Self-organization and branching morphogenesis of primary salivary epithelial cells

Embryonic tissues may provide clues about mechanisms required for tissue reassembly and regeneration, but few studies have utilized primary embryonic tissue to study tissue assembly. To test the capacity of tissue fragments to regenerate, we cultured fragments of embryonic day 13 (E13) mouse submandibular gland (SMG) epithelium and found that fragments as small as a quarter-bud retain the ability to branch. Further, we found that completely dissociated SMG epithelial cells self-organize into structures that undergo significant branching. Investigation into the mechanisms involved in tissue self-assembly demonstrated that inhibition of beta(1) integrin prevents cell aggregation, while inhibition of E-cadherin hinders aggregate compaction. Immunostaining showed that the cellular architecture and expression patterns of E-cadherin, beta-catenin, and actin in the reassembled aggregates mirror those seen in intact glands. Adding SMG mesenchymal cells to the epithelial cell cultures facilitates branching and morphological differentiation. Quantitative real-time RT-PCR indicated that the aggregates express the differentiation markers aquaporin-5 (AQP5), prolactin-inducible protein (PIP), and SMG protein C (SMGC). Together, these data show that dissociated SMG epithelial cells self-organize and undergo branching morphogenesis to form tissues with structural features and differentiation markers characteristic of the intact gland. These findings provide insights into self-assembly and branching that will facilitate future regeneration strategies in the salivary gland and other organs.

Homeobox-containing gene transiently expressed in a spatially restricted pattern in the early sea urchin embryo

In the sea urchin embryo, the lineage founder cells whose polyclonal progenies will give rise to five different territories are segregated at the sixth division. To investigate the mechanisms by which the fates of embryonic cells are first established, we looked for temporal and spatial expression of homeobox genes in the very early cleavage embryos. We report evidence that PlHbox12, a paired homeobox-containing gene, is expressed in the embryo from the 4-cell stage. The abundance of the transcripts reaches its maximum when the embryo has been divided into the five polyclonal territories--namely at the 64-cell stage--and it abruptly declines at later stages of development. Blastomere dissociation experiments indicate that maximal expression of PlHbox12 is dependent on intercellular interactions, thus suggesting that signal transduction mechanisms are responsible for its transcriptional activation in the early cleavage embryo. Spatial expression of PlHbox12 was determined by whole-mount in situ hybridization. PlHbox12 transcripts in embryos at the fourth, fifth, and sixth divisions seem to be restricted to the conditionally specified ectodermal lineages. These results suggest a possible role of the PlHbox12 gene in the early events of cell specification of the presumptive ectodermal territories.

Characterization of bep1 and bep4 antigens involved in cell interactions during Paracentrotus lividus development

We have identified and partially characterised two antigens, extracted with 3% butanol, from Paracentrotus lividus embryos dissociated at the blastula stage, and encoded by the cDNA clones previously described as bep1 and bep4 (bep-butanol extracted proteins). The cDNA fragments containing the specific central portions of bep1 and bep4 were expressed as MS2 polymerase fusion proteins in Escherichia coli. These two fusion proteins, called 1C1 (bep1) and 4A1 (bep4), were injected subcutaneously into rabbits and the corresponding polyclonal antibodies generated. Western blot analysis of proteins, extracted with 3% butanol, from sea urchin embryos at the blastula stage (b.e.p.), established that both antibodies recognize two 33 KDa proteins. Reducing and non-reducing electrophoretic conditions show that both antibodies against bep1 and bep4 related proteins react also with a protein band of a molecular weight 66 KDa, indicating that these two antigens probably exist as dimers. Immunolocalization with anti 1C1 and 4A1 antibodies shows the presence of the related antigens also on the cell surface. Fab fragments of the polyclonal antibodies against 1C1 and 4A1 inhibited reaggregation of sea urchin embryonic cells, dissociated from blastula stage embryos. This prevention of reaggregation indicates that these proteins probably play a role in cell interaction during sea urchin embryonic development.

The contractility of elongated microvilli in early sea urchin embryos

Elongated microvilli attach the early sea urchin embryo to the fertilization envelope and support it in a concentric position within the perivitelline space. The contractility of the elongated microvilli was demonstrated in several ways. (1) During normal cleavage, these microvilli change their length to adapt to the change in shape and numbers of blastomeres. (2) When treated with calcium-free sea water, embryos become eccentrically located and the microvilli extend further than normal on one side; when returned to normal sea water, the embryos become centered again. (3) Several agents cause the fertilization envelope to become higher and thinner than normal and the elongated microvilli to extend correspondingly if treated within ten min after fertilization. In some cases, both elongated microvilli and fertilization envelope return to normal size when returned to normal sea water. (4) Fertilization in a papain solution causes the elongated microvilli and the fertilization envelope to contract to the surface of the embryo. (5) Refertilization after the papain-induced contraction can bring about the elongation of these microvilli and the elevation of the fertilization envelope a second time. It was also shown that elongated microvilli are extended immediately upon fertilization, at the same time as the short microvilli. The firm adherence of the tips of elongated microvilli to the fertilization envelope by means of extracellular matrix fibers is shown in a high voltage electron microscope stereoimage. This allows us to understand why it is that when the elongated microvilli extend or contract, the fertilization envelope also extends and contracts accordingly.

Analysis of the sequence and expression during sea urchin development of two members of a multigenic family, coding for butanol-extractable proteins

Two cDNA clones related to Paracentrotus lividus butanol-extracted proteins, presumably belonging to cell surface proteins, were isolated by a lambda gt11 expression library of ovary poly A+ RNA. These clones, called bep1 and bep4, of 1,110 and 1,071 bp, respectively, belong to a multigene family. By sequencing analysis, a special structural organization in the coding region is detected. A single copy region is inserted between two regions different from each other but similar in the two clones, which constitute two perfectly preserved domains in the genome and are not always present together in the various members of this gene family. The bep1 and bep4 clones derive from two single genes that are polymorphic in the sea urchin genome. Expression of these clones was studied by Northern blot analysis. Both bep1 and bep4 are transcribed during oogenesis into mRNAs of 1.4 kb, which are stored in eggs and utilized during early embryogenesis. None of these RNAs is, in fact, detectable after the gastrula stage. Moreover, the transcripts of three other members of the family are present in eggs and at the 32 cell stage, but they are also synthesized in the early developmental stages.

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.

Extracellular matrix of sea urchin and other marine invertebrate embryos

The extracellular matrix surrounding the sea urchin embryo (outer ECM) contains fibers and granules of various sizes which are organized in recognizable patterns as shown by ultrastructural studies, particularly stereoimaging techniques. The use of the ruthenium red method for retaining and staining the ECM, with modifications of the Luft (Anatomical Record 171:347-368, 1971) method for invertebrate embryos, allows for the clarification of certain structures, particularly fiber compaction in the interzonal region, and microvillus-associated bodies. The inner ECM in the sea urchin embryo includes the basal lamina and blastocoel matrix. Stereoimages show that the fibers which are loosely distributed in the blastocoel matrix become compacted around the periphery of the blastocoel to form the basal lamina. The ruthenium red method was also used on a number of marine invertebrate embryos and larvae, representing different phyla, to facilitate comparisons between their surface coats. The similarities observed in the specimens shown suggest that ECMs are widely found on marine invertebrate eggs, embryos, and larvae, and that they resemble vertebrate ECMs and may, therefore, have similar functions.

[3H]serotonin binding to blastula, gastrula, prism, and pluteus sea urchin embryo cells

1. The presence of serotonin binding sites in blastula, gastrula, prism, and pluteus embryos of the sea urchin, Arbacia punctulata, was investigated by the binding of radiolabelled serotonin to dissociated embryo cells. 2. [3H]serotonin binding sites were identified in prism, early pluteus, and advanced pluteus larvae, but not in blastula or gastrula embryos. 3. The ontogeny of [3H]serotonin binding activity closely parallels that of serotonin content as previously reported in Paracentrotus lividus embryos (Toneby, 1977a). 4. Results of this study support a regulatory role of serotonin in developmental processes in postgastrula sea urchin embryos.

The factors that promote the development of symmetry properties in aggregates from dissociated echinoid embryos

Embryos of Hemicentrotus pulcherrimus at the 16 cell, 400 cell or mesenchyme blastula stage of development were dissociated into single cells. The cells were reaggregated, and the development of individual aggregates was monitored. Only aggregates from 16 cell embryos developed into pluteus-like larvae with radial or bilateral symmetry. When embryos at these three developmental stages were incompletely dissociated so that there were mixtures of single cells and groups of undissociated cells, the percentage of aggregates from 16 cell embryos that developed in a pluteus-like manner was greater than in aggregates from completely dissociated 16 cell embryos. Also a small percentage of aggregates from 400 cell embryos now developed into pluteus-like larvae. In both of these experiments small aggregates tend to develop in a more normal manner than larger aggregates.In order to test the role of undissociated cells in promoting pluteus-like development in aggregates from incompletely dissociated blastula stage embryos, pieces of intact animal, lateral, or vegetal blastula wall were grafted to aggregates formed from completely dissociated embryos. While each kind of graft improved the ability of the aggregate to develop in a pluteus-like manner, grafts of vegetal blastula wall were most effective. In an aggregate, a graft differentiates according to its presumptive fate and influences the cells of the aggregate to differentiate in an appropriate manner. The ability of the graft to influence the development of the other cells in the aggregate depends on the developmental stage of the cells that make up the aggregate and the size of the aggregate.

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

Four apical components were used as markers for the apical end of the cell in studies centering on cell polarity in the early blastula stage of sea urchin embryos and in aggregates of cleavage stage cells. Cells were observed to maintain their polarity for several hours if dissociated and cultured in suspension. Orientation of cells in aggregates initially is random; however, within 3 hr the cells have reoriented so that their apical-basal axis corresponds to the correct inside-outside position in the aggregate. This reorientation occurs before formation of a basal lamina or a new hyalin layer in the aggregate, and appears to take place by a rotation or other movement of individual cells. The polarity within each cell is maintained during reorientation. An apical surface antigen is colocalized with concentrations of filamentous actin. Treatment of isolated cells with cytochalasin B causes the antigen to lose its apical position and eventually become distributed around the outside of the cell. Microtubules are visible radiating from two foci closely associated with the nucleus in untreated cells. Treatment of isolated cells with nocodazole leaves the apical cell surface marker and its associated actin undisturbed, but causes the nucleus to lose its apical position. Cytochalasin B and colchicine both prevent reorientation of cells in aggregates. Thus polarity appears to be a constant for the cells, and their reorientation in aggregates occurs prior to the polarized release of extraembryonic matrix and basal lamina.

Embryo dissociation, cell isolation, and cell reassociation

In resolving the role of cell recognition events in the process of morphogenesis it is necessary to focus on single events against a background of many complex interactions. This article presents a series of approaches that are designed to do just that. It should be noted that with simplification there is a danger of oversight. Nevertheless, in learning about mechanisms of cellular movement at gastrulation we have found that cell separation techniques, simplified adhesion assays, and predictable antibody activities are helpful for approaching the complex mechanisms of morphogenesis.

The coincident time-space patterns of septate junction development in normal and exogastrulated sea urchin embryos

Earlier studies have shown that two types of septate junction are formed during early sea urchin morphogenesis. One type is the straight, unbranched, double septum septate (SUDS) which is found in the ectodermal layer throughout early development. The second type is formed only in cells which invaginate to become endoderm and to form the digestive tract. This junction is characterized by pleated, anastomosing, single septum septates (PASS). In order to ascertain in which parts of the digestive tract these junctions are formed, we studied exogastrulae because the endoderm is everted and forms constricted areas of the gut which are easily recognizable. Our results show that, in control embryos, SUDS septates are found in the mouth, esophagus and coelom and that PASS septates are found in the stomach, intestine and anus. These junctional types are also found in the same areas in exogastrulae; SUDS septates are found in the stomadeum, esophagus and coelom, and PASS septates are found in the stomach and intestine. The transition from SUDS to PASS junctions takes place within the same time period in exogastrulae as in normal embryos, i.e., from the time of mid-gastrulation through the pluteus stage. These results indicate that septate junction formation in the sea urchin embryo digestive tract may be genetically programmed in terms of both time and spatial location. This program is not altered either by the major dislocation of cells from their normal position within the embryo or from normal contacts with neighboring cells.

Dissociation of cells from sea urchin embryos alters the synthesis of actins and other proteins

The effects of altered cellular microenvironments on patterns of protein synthesis at various periods during sea urchin development were quantitated by comparing the relative incorporation of [35S]methionine into selected polypeptides of intact embryos and cells dissociated from them. The effects of increasing times of reassociation were also determined. Quantitative, but not qualitative, differences in incorporation were noted. Actins, as well as heterogeneous acidic polypeptides with an Mr of about 80 kDa, showed increased incorporation in dissociated cells labeled at the time control embryos were recently hatched blastulae. Labeling of another acidic group of polypeptides with an Mr of about 100 kDa was decreased. Possible mechanisms regulating these shifts in incorporation were investigated by the use of inhibitors. The dissociation-triggered changes were insensitive to actinomycin D, cordycepin, dibutyryl cAMP, 3-isobutyl-1-methylxanthine, and trifluoperazine; however, the latter two stimulated incorporation into some polypeptides in intact blastulae. Age-dependent shifts in incorporation were also detected in both intact embryos and dissociated/reassociating cells.

Protein composition of the hyaline layer of sea urchin embryos and reaggregating cells

The protein composition of the sea urchin embryo hyaline layer has been studied by 125I surface labeling. The electrophoretic patterns of iodinated proteins indicate that the composition of the hyaline layer is species and stage-specific. Dissociation of iodinated embryos removes some labeled proteins of apparent molecular weights of 290,000, 175,000, 145,000, 110,000, 70,000, and 50,000. The electrophoretic pattern of labeled proteins found in the intact embryo is reestablished on the surface of aggregates following reaggregation. The possible function of these proteins with respect to cell adhesion, cell-sorting behavior and morphogenesis is discussed.

Cell sociology: a way of reconsidering the current concepts of morphogenesis

Research in the field of planarian regeneration on the one hand, and a general survey of embryology on the other, throw doubt upon the reality of supra-cellular controls, which are still at the basis of all modern concepts of morphogenesis. The necessity of referring to such controls, which have never been convincingly demonstrated, is probably due to the fact that two aspects of cell behaviour have been underestimated: 1) the capacity of cells to change their individualities for a time independently of other cells; 2) the social behaviour of cells, which is the consequence of the reciprocal exchange of information. Pattern formation and pattern remodeling in normal development results from readjustments of cell populations to local or global changes. The common specific syntheses and cell migration. In the young embryo these may promptly restore the unity of the injured primordium, leading to so-called restitution; this is based on a normal sequence of further readjustments in the primordium. In older organisms the same responses give rise to cell interactions which may be the starting point for further sequential readjustments (regeneration)--in some instances these are comparable to those that originally organized the primordium in question during development. The desirability of giving up the notion of morphogenetic field is discussed.

Isolation of the plasma membrane from sea urchin embryos

A method for isolation of sea urchin embryos plasma membranes is described. Purification of the obtained fraction was assayed by several enzymatic markers and electron microscopy. The isolated plasma membranes appear to be pure from contamination of other cell membranes (endoplasmic reticulum and mitochondria), and they can therefore be used for analytical studies on the composition and structure of plasma membrane.