Serine 785 phosphorylation of the beta1 cytoplasmic domain modulates beta1A-integrin-dependent functions.

The integrin beta1 cytoplasmic domain plays a key role in a variety of integrin-mediated events including adhesion, migration and signaling. A number of studies suggest that phosphorylation may modify the functional state of the cytoplasmic domain, but these studies frequently only examine the effect of substituting amino acid mimics that cannot be phosphorylated. We now demonstrate, using site directed mutagenesis, that substituting either an unphosphorylated (S to M) or a phosphorylated (S to D) mimic in place of serine can modify integrin function. Specifically, we show that expressing a residue that mimics a dephosphorylated form of the protein promotes cell spreading and directed cell migration, whereas a residue mimicking a phosphorylated form of the protein promotes attachment but inhibits cell spreading or migration. The significance of these observations is strengthened by the fact that the beta1 mutations display the same properties in both a fibroblast cell line (GD25) and a teratocarcinoma cell line (F9). The results indicate that changes in the phosphorylation state of S785 modulates beta1 integrin function.

The involvement of tissue-type plasminogen activator in parietal endoderm outgrowth.

When F9 stem cells are treated in suspension with retinoic acid, they differentiate into embryoid bodies (EBs) consisting of an inner core of undifferentiated stem cells surrounded by an outer layer of visceral endoderm (VE). When these EBs are plated onto a fibronectin (FN)-coated substrate, VE-derived parietal endoderm (PE) cells migrate onto the substrate. It has been suggested that increased levels of tPA associated with the emerging PE cells may help mediate PE outgrowth. We now show that goat anti-human tPA, an anticatalytic antibody that crossreacts with mouse tPA, and a panel of serine protease inhibitors partially inhibit PE outgrowth. Extracellular matrix (ECM) degradation analysis demonstrates that PE cell-mediated degradation of [3H]proline-labeled ECM is time- and cell concentration-dependent. A serine protease inhibitor reduced the extent of degradation, suggesting that tPA might play a role in PE outgrowth by cleaving the ECM. In support of this contention, we demonstrate that incubation of purified FN with conditioned medium plus plasminogen results in FN proteolysis. The degradation of FN is blocked by either serine protease inhibitors or goat anti-human tPA. Our data suggest that enhanced production of tPA during PE outgrowth may facilitate the migratory behavior of PE cells by mediating the degradation of ECM components such as FN.

The role of phosphorylation in modulating beta 1 integrin localization.

The beta 1-integrin subunit localizes to focal contacts during F9 teratocarcinoma stem cell differentiation to parietal endoderm. Concomitantly, this integrin subunit becomes dephosphorylated at serine 790, the only serine in the well-conserved cytoplasmic domain of beta 1. We hypothesized that this dephosphorylation is required for this subunit to move to a focal contact. To test this, we transfected F9 stem cells with distinct cDNAs encoding three forms of chicken beta 1: the wild-type, and two site-specific mutants possessing amino acid substitutions at residue 790 which included methionine and aspartate. The negatively charged aspartate was selected to mimic the phosphorylated serine found at position 790 in the wildtype protein but in a form that cannot be dephosphorylated upon differentiation. The chicken beta 1 subunits heterodimerized with endogenous mouse alpha subunits and, using a chicken beta 1-specific monoclonal antibody, we examined movements of these chimeric integrins to focal contacts using immunofluorescence microscopy. The chimeric integrins possessing either the wildtype or methionine mutant beta 1 subunits localized to focal contacts upon F9 differentiation. In contrast, the aspartate chimeric integrins failed to localize. These data suggest that the dephosphorylation of serine 790 is required for the beta 1 subunit to localize to focal contacts during F9 differentiation.

Expression of syndecan-1 changes during the differentiation of visceral and parietal endoderm from murine F9 teratocarcinoma cells.

F9 teratocarcinoma stem cells treated with retinoic acid differentiate in suspension into embryoid bodies with an outer layer of visceral endoderm surrounding a core of largely undifferentiated cells. The visceral endoderm-containing embryoid bodies, when plated onto an extracellular matrix coating, give rise to parietal endoderm outgrowth. These in vitro cell cultures mimic both geometrically and biochemically the differentiation of visceral and parietal endoderm in the early mouse embryo and, thus, were used as a model system for the study of molecular and cellular mechanisms underlying the differentiation of the extraembryonic endoderm lineages. We have investigated the expression of syndecan-1, an integral membrane proteoglycan that binds to multiple components of the extracellular matrix and basic FGF, during visceral endoderm differentiation and parietal endoderm outgrowth. Syndecan-1 immunostaining is detected on all cell surfaces in the undifferentiated embryoid bodies and in the differentiating embryoid bodies prior to the formation of the visceral endoderm. Following the differentiation of visceral endoderm, syndecan-1 localizes predominantly to the basal surface of this epithelial layer, while syndecan-1 staining in the core of differentiated embryoid bodies is faint. Quantitation of cell associated syndecan-1 indicates that syndecan-1 is down-regulated during embryoid body differentiation. However, northern analysis shows that the amounts of steady-state syndecan-1 mRNA are the same in undifferentiated versus differentiated embryoid bodies, suggesting post-transcriptional regulation of syndecan-1 expression in the differentiating embryoid body. Analysis of syndecan-1 distribution in the outgrowth culture by immunofluorescence demonstrates that syndecan-1 is absent from the cell surface of parietal endoderm. However, a substantial amount of syndecan-1 is detected inside parietal endoderm cells. While all three cell types release syndecan-1 ectodomain into the culture medium, the parietal endoderm outgrowth releases more syndecan-1 ectodomain than the differentiated embryoid body. These data suggest that the post-transcriptional control and post-translational shedding of syndecan-1 from the cell surface are developmentally regulated during the differentiation of visceral to parietal endoderm and the migration of parietal endoderm.

Two CDC25 homologues are differentially expressed during mouse development.

The cdc25 gene product is a tyrosine phosphatase that acts as an initiator of M-phase in eukaryotic cell cycles by activating p34cdc2. Here we describe the cloning and characterization of the developmental expression pattern of two mouse cdc25 homologs. Sequence comparison of the mouse genes with human CDC25 genes reveal that they are most likely the mouse homologs of human CDC25A and CDC25B respectively. Mouse cdc25a, which has not been described previously, shares 84% sequence identity with human CDC25A and has a highly conserved phosphatase domain characteristic of all cdc25 genes. A glutathione-S-transferase-cdc25a fusion protein can hydrolyze para-nitro-phenylphosphate confirming that cdc25a is a phosphatase. In adult mice, cdc25a transcripts are expressed at high levels in the testis and at lower levels in the ovary, particularly in germ cells; a pattern similar to that of twn, a Drosophila homolog of cdc25. Lower levels of transcript are also observed in kidney, liver, heart and muscle, a transcription pattern that partially overlaps, but is distinct from that of cdc25b. Similarly, in the postimplantation embryo cdc25a transcripts are expressed in a pattern that differs from that of cdc25b. cdc25a expression is observed in most developing embryonic organs while cdc25b expression is more restricted. An extended analysis of cdc25a and cdc25b expression in preimplantation embryos has also been carried out. These studies reveal that cdc25b transcripts are expressed in the one-cell embryo, decline at the two-cell stage and are re-expressed at the four-cell stage, following the switch from maternal to zygotic transcription which mirrors the expression of string, another Drosophila homolog of cdc25. In comparison, cdc25a is not expressed in the preimplantation embryo until the late blastocyst stage of development, correlating with the establishment of a more typical G1 phase in the embryonic cell cycles. Both cdc25a and cdc25b transcripts are expressed at high levels in the inner cell mass and the trophectoderm, which proliferate rapidly prior to implantation. These data suggest the cdc25 genes may have distinct roles in regulating the pattern of cell division during mouse embryogensis and gametogenesis.

Function and differential regulation of the alpha 6 integrin isoforms during parietal endoderm differentiation.

F9 embryonal carcinoma cells treated with retinoic acid differentiate in monolayer into parietal endoderm (PE) or in suspension into embryoid bodies with an outer layer of visceral endoderm (VE) surrounding a core of largely undifferentiated cells. Previous reports have shown that cell-extracellular matrix interactions mediated by the beta 1 integrins play a critical role in the differentiation and migration of PE. In the present study we investigated the pattern of expression and function of the integrin alpha 6 beta 1 during the differentiation of F9 cells into VE and PE. F9 cells express integrin subunits alpha 3, alpha 5, alpha 6, and beta 1. Cell adhesion and migration assays demonstrate that alpha 6 beta 1 is the major laminin receptor in undifferentiated F9 cells as well as F9-derived PE cells. However, the amount of alpha 6 protein decreases significantly upon F9 cell differentiation into either VE or PE, as revealed by immunofluorescent staining and immunoprecipitation analysis. In contrast, the amount of steady-state alpha 6 message stays constant before and after F9 cell differentiation, suggesting that the down-regulation of alpha 6 beta 1 occurs post-transcriptionally. In view of previous reports of two alpha 6 isoforms generated by alternative RNA processing, we carried out reverse transcription-PCR analysis and show that, while alpha 6B is the major mRNA isoform before and after F9 cell differentiation, alpha 6A mRNA is weakly expressed in undifferentiated F9 cells and is substantially increased following F9 differentiation into PE. Immunoprecipitations using the isoform-specific antibodies show an increase in alpha 6A and a dramatic decrease in alpha 6B protein following PE differentiation. Pulse-chase experiments indicate that, whereas the stability of alpha 6B protein is unaltered, synthesis of alpha 6B protein is decreased at least threefold following PE differentiation. Further experiments demonstrate that alpha 6A localizes to focal contacts in PE cells. The switch from alpha 6B to alpha 6A and the localization of alpha 6A at focal contacts correlate with the acquisition of PE cell motility, which suggests distinct functions for the two alpha 6 isoforms.

The Pem homeobox gene is X-linked and exclusively expressed in extraembryonic tissues during early murine development.

We previously reported the isolation of a cDNA clone for a homeobox-containing gene designated Pem, shown by Northern analysis of Day 7 through Day 16 mouse embryos to be expressed in extraembryonic tissues. In this study, Pem gene expression was further examined using in situ hybridization and immunocytochemistry to determine the spatial distribution of Pem transcripts and protein in peri-implantation embryos and in embryoid bodies (EBs). Low amounts of Pem mRNA were detected in undifferentiated EBs. When EBs were induced to differentiate, the outer cell layer of visceral or parietal endoderm expressed both Pem mRNA and protein. In developing embryos, no Pem protein was detectable in the uncompacted morula; 12% of the nuclei in compacted morulae were Pem positive, while 25% of the blastocyst trophectoderm and 15% of inner cell mass cells expressed Pem protein. Shortly after implantation, in 5.5 and 6.5 d.p.c. embryos, Pem expression was limited to extraembryonic tissues and was present in distal and proximal visceral endoderm, parietal endoderm, and ectoplacental cone. By 7.5-8.5 d.p.c. neither Pem RNA nor protein was found in the distal squamous visceral endoderm, which surrounds the embryonic region of the egg cylinder, nor in the parietal endoderm. Expression was retained in the proximal columnar epithelium of the visceral endoderm. Prominent Pem expression was observed in the chorion, in trophoblast-derived cells of the ectoplacental cone, and in secondary giant cells, localized in the nuclear compartment. Pem was localized to the X chromosome and found to be expressed in cell lineages where only the maternal X chromosome is active. The data indicate a possible role for Pem in regulating genes involved in the differentiation of extraembryonic tissues.

Homeobox-containing genes in teratocarcinoma embryoid bodies: a possible role for Hox-D12 (Hox-4.7) in establishing the extraembryonic endoderm lineage in the mouse.

We wish to identify genes involved in mediating early lineage decisions in the mouse embryo. F9 teratocarcinoma cells treated with retinoic acid (RA) in suspension culture develop into embryoid bodies (EBs) with an outer layer of visceral endoderm. In order to identify genes that are involved in establishing this extraembryonic endoderm lineage we have employed a PCR-based approach using cDNAs from early EBs as templates. PCR reactions were performed with degenerate oligonucleotide primers coding for the highly conserved regions of the homeodomains of the Drosophila Antennapedia, bicoid, and zerknüllt proteins. Among the PCR products were representatives of previously identified mouse genes, including Hox-A5 (1.3), A1 (1.6), A9 (1.7), B8 (2.4), B2 (2.8), C8 (3.1), and D12 (4.7). Whole mount in situ hybridization analysis, performed to examine the temporal and spatial distribution of transcripts, suggests a possible role for the Hox-D12 gene during endoderm differentiation in F9 EBs. Whereas the expression patterns of several other homeobox genes are essentially uniform throughout the aggregates, Hox-D12 expression is restricted to the outer surface of early EBs at a time when lineage decisions may be occurring. In order to establish the relationship between the Hox-D12 expression pattern and the role of RA in inducing F9 EB differentiation, we examined PSA-1 EBs that differentiate in the absence of added RA. PSA-1 EBs show similar temporal and spatial localization of Hox-D12 when compared to F9 EBs. These data suggest that the pattern of Hox-D12 expression correlates with endoderm differentiation and not with RA treatment and point to a possible role for homeobox-containing genes during the early stages of mouse embryogenesis.

Disruption of the cytoskeleton-extracellular matrix linkage promotes the accumulation of plasminogen activators in F9 derived parietal endoderm.

When F9 teratocarcinoma cells are treated with retinoic acid plus cyclic AMP (RACF9) they differentiate into parietal endoderm. Differentiation is accompanied by the acquisition of substrate adhesion sites and a change in the pattern of gene expression, including the synthesis of tissue-type plasminogen activator (tPA). We demonstrate here that dihydrocytochalasin B (DHCB) treatment of differentiating F9 cells prevents the assembly of a structured actin cytoskeleton and generates a more rounded and stellate cell morphology. This morphological change is accompanied by the accumulation of the usually visceral endoderm-specific marker urokinase-type plasminogen activator (uPA) and an increase in tPA levels in comparison to untreated RACF9 controls. The increase in tPA accumulation is preceded by an increase in tPA mRNA levels. These effects are reversible, with a lag, when DHCB is removed, and PA accumulation can be stimulated within 24 h when differentiated cells are exposed to DHCB. Exposure to the microtubule disrupting agent colchicine has no effect on uPA or tPA accumulation. In addition, antibody directed against the beta 1 integrin subunit can also specifically elicit increased PA production. Thus disturbing the cytoskeleton and cytoskeleton associated substrate adhesions promotes PA accumulation.

Plasminogen activator expression in F9 teratocarcinoma embryoid bodies and their endoderm derivatives.

Plasminogen activators are believed to play an important role in tissue remodeling and cell migration. During mouse embryogenesis, visceral endoderm secretes urokinase-type plasminogen activator (uPA) whereas parietal endoderm secretes tissue-type plasminogen activator (tPA). Visceral endoderm from F9 embryoid bodies can transdifferentiate into parietal endoderm under the appropriate culture conditions. We have examined at the protein and mRNA levels the type of plasminogen activator expressed in whole embryoid bodies, visceral endoderm and its parietal endoderm derivatives. Our experiments show that the visceral endoderm on F9 embryoid bodies synthesizes and secretes substantial amounts of both tPA and uPA. In contrast, the parietal endoderm derived directly from the visceral endoderm secretes dramatically increased levels of tPA and decreases production of uPA to low or below detectable levels. These data support the finding that visceral endoderm can transdifferentiate to parietal endoderm. In addition, this transition provides an excellent model for studying the molecular basis of the coincident down- and upregulation of the two plasminogen activators as well as their potential function during embryogenesis.

Integrin phosphorylation is modulated during the differentiation of F-9 teratocarcinoma stem cells.

The retinoic acid-induced differentiation of F-9 teratocarcinoma cells in monolayer culture is accompanied by the accumulation of fibrillar fibronectin deposits, the appearance of a highly structured actin cytoskeleton, and the redistribution of integrin to apparent sites of substrate contact. We have studied the 140-kD fibronectin receptor during this process and report that although the integrin molecule is present in equivalent amounts before and after differentiation, the level of integrin phosphorylation decreases dramatically as the cells differentiate. This loss of phosphorylation coincides temporally with the observed changes in actin, fibronectin, and integrin organization. The phosphorylation state of integrin thus may mediate developmentally regulated cell-matrix interactions.

The role of cell interactions in the differentiation of teratocarcinoma-derived parietal and visceral endoderm.

Cell interactions have been implicated in the differentiation of visceral and parietal endoderm in the developing mouse embryo. Embryoid bodies formed from F9 embryonal carcinoma cells have been useful in characterizing the events which lead to endoderm formation. As part of our effort to specify the interactions which may be involved in this process we have isolated visceral endoderm-like cells (VE) from F9 embryoid bodies and cultured them under various conditions. Using a combination of immunoprecipitation and enzyme-linked immunosorbent assay, we demonstrate that monolayer culture of these cells on a number of different substrates leads to a dramatic decrease in the level of alphafetoprotein (AFP), a VE-specific marker. Northern blot analysis of AFP mRNA indicates very low levels of this message are present after 48 hr in monolayer culture. Coincident with the drop in AFP levels is an increase in the levels of the cytokeratin Endo C and tissue plasminogen activator, both markers for parietal endoderm (PE). Morphological evidence at the ultrastructural level supports a transition from VE to PE. In contrast, the VE phenotype can be maintained in vitro by interaction with aggregates, but not monolayers, of stem cells. In addition, culturing the cells on the curved surface of gelatin-coated dextran beads, but not on a flat gelatin surface facilitates AFP expression and the cells are morphologically intermediate between VE and PE cells. The potential role of junctional complexes and cell shape are discussed.

Altered accumulations of fibronectin are not dependent on fibronectin modifications during the differentiation of F-9 teratocarcinoma stem cells.

F-9 teratocarcinoma stem cells differentiate into parietal endoderm when monolayer cultures are treated with retinoic acid. This change in phenotype is accompanied by increased accumulation and altered organization of fibronectin deposits. Although both stem cells and treated cells synthesize and accumulate fibronectin, only the treated cells deposit a fibrillar array of the protein. We have monitored the accumulation of fibronectin in nontreated and treated F-9 cells with indirect immunofluorescence and have biochemically analyzed the fibronectin synthesized by each cell type with one- and two-dimensional acrylamide gels and peptide maps. Our data suggest that no differences exist between these fibronectins to account for the observed changes in accumulation. Thus, another mechanism may regulate the organization of matrix deposition.

The role of extracellular matrix in the migration and differentiation of parietal endoderm from teratocarcinoma embryoid bodies.

Embryoid bodies formed from teratocarcinoma stem cells differentiate an outer layer consisting of parietal and visceral endoderm or of visceral endoderm exclusively. We have previously shown that when these embryoid bodies are plated on collagen-coated substrates a parietal endoderm-like cell migrates onto the substrate, whereas all of the visceral endoderm remains associated with the stem cell mass, suggesting a role for substrate contact in parietal endoderm differentiation. We now identify fibronectin as the migration-promoting component in these cultures, and note that laminin and collagen type IV are 10-fold less effective at promoting both attachment and endoderm outgrowth. The RGDS tetrapeptide (arg-gly-asp-ser) from the cell attachment domain of fibronectin can specifically block attachment and outgrowth on both fibronectin- and laminin-coated substrates. In addition, the involvement of the 140-kD fibronectin receptor is demonstrated using an antibody directed against this molecule.

The outgrowth of parietal endoderm from mouse teratocarcinoma stem-cell embryoid bodies.

Teratocarcinoma stem cells can be used to study certain events occurring during early mouse embryogenesis. We report that the outgrowth of parietal endoderm from teratocarcinoma stem-cell embryoid bodies in vitro is analogous to the same process in vivo in terms of the spatial distribution of endoderm types: only parietal endoderm migrates away from the aggregate, whereas visceral endoderm remains associated with the embryoid body. The outgrowths generated on a substrate of type-I collagen from PSA-1 and retinoic-acid-treated F 9 embryoid bodies were found to be comparable, even though these aggregates express different endoderm types. We demonstrated that retinoic-acid-treated F 9 embryoid bodies that contain essentially only visceral endoderm in suspension culture can nonetheless generate parietal-endoderm outgrowth when plated on type-I collagen, suggesting that substrate interaction plays an important role in inducing parietal-endoderm differentiation. These data indicate the usefulness and relevance of studying endoderm differentiation and outgrowth in vitro employing the teratocarcinoma model system.

Isolation of a teratocarcinoma stem cell lectin implicated in intercellular adhesion.

We have previously identified a cell surface teratocarcinoma stem cell lectin with a fucan/mannan specificity. We now report the purification of the hemagglutinin (lectin) from stem cell conditioned medium by exclusion on a Sepharose 2B column, followed by elution with 0.5M NaCl from DEAE-cellulose, providing an overall purification of about 90-fold. When this material was analyzed, by SDS-polyacrylamide gel electrophoresis, a major band of Mr 56000 was consistently observed. Hemagglutination activity was renatured from the gels and localized exclusively to a region of the gel that, as detected by fluorography, contains only the 56-kDa component. This suggested that this polypeptide comprises the lectin.

Isolation of a putative cell adhesion mediating lectin from teratocarcinoma stem cells and its possible role in differentiation.

We have identified a cell surface teratocarcinoma stem cell lectin with a fucan/mannan specificity by the use of an erythrocyte rosetting assay as well as hemagglutination assay. We have also described experiments that suggest that the lectin is involved in mediating divalent cation-independent adhesion of the stem cells. This molecule has been purified from stem cell conditioned medium and identified as a polypeptide of 56 000 apparent molecular weight. An antibody has been raised to this 56K polypeptide (using material eluted from an SDS-polyacrylamide gel as the immunogen) and its specificity determined by protein blot analysis. In addition, we have recently observed that only carbohydrates recognized by the lectin interfere with in vitro embryoid body formation by the stem cells, suggesting that the lectin may be involved in differentiation.

Teratocarcinoma stem cell adhesion: the role of divalent cations and a cell surface lectin.

We describe two additive systems of intercellular adhesion in teratocarcinoma stem cells (Nulli cell line). One component is divalent cation-dependent (Ca++ or Mg++) and the other involves a cell surface fucan/mannan-specific lectin, previously identified on stem cells by an erythrocyte rosetting assay. The existence of these two systems is inferred from the observation that reaggregation of stem cells was partially inhibited by the removal of divalent cations or by the presence of lectin inhibitors such as fucoidan, but reaggregation was completely blocked when the two conditions were combined. Our results are related to recent work describing a calcium-dependent system of intercellular adhesion in teratocarcinoma stem cells.

Tunicamycin reversibly inhibits the terminal differentiation of teratocarcinoma stem cells to endoderm.

The differentiation of aggregates of certain teratocarcinoma stem cell lines begins with the formation of an outer layer of primary endoderm cells characterized by the production of plasminogen activator and the absence of histochemically detectable alkaline phosphatase activity. After several days of culture these outer cells develop into a mixture of two types of terminally differentiated endoderm: parietal endoderm which produces a thick layer of underlying basement membrane and visceral endoderm which produces alpha-fetoprotein (AFP). We report here that in the presence of tunicamycin, a drug that inhibits glycosylation of N-asparagine linked glycoproteins, a primary endoderm-like cell is formed which is alkaline phosphatase negative and plasminogen activator positive. However, terminal differentiation of these cells is inhibited as manifested by the lack of accumulation of a thick basement membrane and the absence of immunologically detected AFP. Such inhibition is reversible following removal of the tunicamycin. Terminal differentiation of endoderm depends, therefore, upon N-asparagine linked glycoproteins.

Carbohydrate specificity of sea urchin sperm bindin: a cell surface lectin mediating sperm-egg adhesion.

We have examined the carbohydrate specificity of bindin, a sperm protein responsible for the adhesion of sea urchin sperm to eggs, by investigating the interaction of a number of polysaccharides and glycoconjugates with isolated bindin. Several of these polysaccharides inhibit the agglutination of eggs by bindin particles. An egg surface polysaccharide was found to be the most potent inhibitor of bindin-mediated egg agglutination. Fucoidin, a sulfated fucose heteropolysaccharide, was the next most potent inhibitor, followed by the egg jelly fucan, a sulfated fucose homopolysaccharide, and xylan, a beta(1 leads to 4) linked xylose polysaccharide. A wide variety of other polysaccharides and glycoconjugates were found to have no effect on egg agglutination. We also report that isolated bindin has a soluble lectinlike activity which is assayed by agglutination of erythrocytes. The bindin lectin activity is inhibited by the same polysaccharides that inhibit egg agglutination by particulate bindin. This suggests that the egg adhesion activity of bindin is directly related to its lectin activity. We have established that fucoidin binds specifically to bindin particles with a high apparent affinity (Kd = 5.5 X 10(-8) M). The other polysaccharides that inhibit egg agglutination also inhibit the binding of 125I-fucoidin to bindin particles, suggesting that they compete for the same site on bindin. The observation that polysaccharides of different composition and linkage type interact with bindin suggests that the critical structural features required for binding may reside at a higher level of organization. Together, these findings strengthen the hypothesis that sperm-egg adhesion in sea urchins is mediated by a lectin-polysaccharide type of interaction.