Interaction of PC-3 cells with fibronectin adsorbed on sulfonated polystyrene surfaces

The ability of cancer cells to invade neighboring tissues is crucial for cell dissemination and tumor metastasis. It is generally assumed that cell adhesion to extracellular matrix proteins is an important stage of cancer progression. Hence, adhesion of cancer cells under in vitro conditions to proteins adsorbed on a substratum surface has been studied to provide a better understanding of cell-protein interaction mechanisms. A protein, adsorbed in an appropriate conformation on a substratum surface, creates a biologically active layer that regulates such cell functions as adhesion, spreading, proliferation and migration. In our study, we examined the interaction of PC-3 cells under in vitro conditions with fibronectin adsorbed on sulfonated polystyrene surfaces of a defined chemical composition and topography. We investigated cell adhesion to fibronectin and cell spreading. Using automatic, sequential microscopic image registration, we are the first to present observations of the dynamics of PC-3 cell spreading and the cell shape during this process. Our results show that cell adhesion and the shape of spreading cells strongly depend on the time interaction with fibronectin. The analysis of images of cytoskeletal protein distribution in the cell region near the cell-substratum interface revealed that induction of a signal cascade took place, which led to the reorganization of the cytoskeletal proteins and the activation of focal adhesion kinase (FAK).     

Influence of substratum hydration and adsorbed macromolecules on bacterial attachment to surfaces

The attachment of Pseudomonas fluorescens and an Acinetobacter sp. to hydrogel and polystyrene surfaces was investigated to evaluate the influence of adsorbed water and macromolecules on adhesion. With both organisms, there was a decrease in attachment numbers with increasing water content of the hydrogels. There was also a decrease in attachment with a decrease in water contact angle on untreated, tissue culture and sulfonated polystyrene surfaces; however, the attachment numbers were higher than would be expected on the basis of the hydrogel data. With P. fluorescens, attachment to untreated and tissue culture polystyrene was inhibited by bovine serum albumin, Escherichia coli lipopolysaccharide, and the supernatant from spent medium, both when the conditioning substances were added to the suspension of attaching cells and when they were preadsorbed onto the surfaces. Dextran inhibited attachment only when added to the bacterial suspension. Supernatants from centrifuged natural freshwater samples had no effect. Thus, hydration of a surface and the adsorption of macromolecules can reduce bacterial attachment; however, additional factors relating to the chemical composition of the substratum and polymeric stabilization of suspended cells can affect the adhesion interaction and resultant numbers of attached cells.     

Influence of substratum hydration and adsorbed macromolecules on bacterial attachment to surfaces.

The attachment of Pseudomonas fluorescens and an Acinetobacter sp. to hydrogel and polystyrene surfaces was investigated to evaluate the influence of adsorbed water and macromolecules on adhesion. With both organisms, there was a decrease in attachment numbers with increasing water content of the hydrogels. There was also a decrease in attachment with a decrease in water contact angle on untreated, tissue culture and sulfonated polystyrene surfaces; however, the attachment numbers were higher than would be expected on the basis of the hydrogel data. With P. fluorescens, attachment to untreated and tissue culture polystyrene was inhibited by bovine serum albumin, Escherichia coli lipopolysaccharide, and the supernatant from spent medium, both when the conditioning substances were added to the suspension of attaching cells and when they were preadsorbed onto the surfaces. Dextran inhibited attachment only when added to the bacterial suspension. Supernatants from centrifuged natural freshwater samples had no effect. Thus, hydration of a surface and the adsorption of macromolecules can reduce bacterial attachment; however, additional factors relating to the chemical composition of the substratum and polymeric stabilization of suspended cells can affect the adhesion interaction and resultant numbers of attached cells.     

The relative importance of topography and RGD ligand density for endothelial cell adhesion

The morphology and function of endothelial cells depends on the physical and chemical characteristics of the extracellular environment. Here, we designed silicon surfaces on which topographical features and surface densities of the integrin binding peptide arginine-glycine-aspartic acid (RGD) could be independently controlled. We used these surfaces to investigate the relative importance of the surface chemistry of ligand presentation versus surface topography in endothelial cell adhesion. We compared cell adhesion, spreading and migration on surfaces with nano- to micro-scaled pyramids and average densities of 6×10(2)-6×10(11) RGD/mm(2). We found that fewer cells adhered onto rough than flat surfaces and that the optimal average RGD density for cell adhesion was 6×10(5) RGD/mm(2) on flat surfaces and substrata with nano-scaled roughness. Only on surfaces with micro-scaled pyramids did the topography hinder cell migration and a lower average RGD density was optimal for adhesion. In contrast, cell spreading was greatest on surfaces with 6×10(8) RGD/mm(2) irrespectively of presence of feature and their size. In summary, our data suggest that the size of pyramids predominately control the number of endothelial cells that adhere to the substratum but the average RGD density governs the degree of cell spreading and length of focal adhesion within adherent cells. The data points towards a two-step model of cell adhesion: the initial contact of cells with a substratum may be guided by the topography while the engagement of cell surface receptors is predominately controlled by the surface chemistry.     

Adhesion sites of neural tumor cells : Morphogenesis of substratum-attached material

An analysis by scanning electron microscopy (SEM) has been performed of the attachment, neurite outgrowth, EGTA-mediated detachment, and morphological characteristics of substratum-attached material (SAM) for non-neurite- or neurite-containing rat neuroblastoma cells growing on serum-coated plastic coverslips. Attachment is initiated by filopodial contact with the substratum and with subsequent broad spreading of the surface membrane; footpad-type adhesion sites commonly observed in fibroblasts are not apparent at the periphery of these neuronal cells. During serum starvation, neurite extension occurs by elongation into bipolar cells, membrane ruffling and filopodial extension at these polar ends, and growth cone extension over the substratum. With time, some growth cones terminate membrane ruffling and spread extensively into a footpad-like morphology. EGTA-mediated detachment occurs by cell body rounding and pulling away from small focal areas of contact between the surface membrane and the substratum. After complete detachment, two morphologically different classes of SAM are identified. Non-neurite-containing neuroblastoma cells leave large membranous pools of SAM which are rigid and raised off the substratum, revealing small focal contact areas. A second morphological class of SAM is identified in neurite-containing cultures as small pools of membranous material tightly bound to the substratum and reminiscent of the footpad SAM deposited by fibroblasts. Along with the biochemical differences noted previously for the SAMs from non-neurite- or neurite-containing cultures, these studies indicate that the adhesion between the growth cone of neurites and the serum-coated substratum is significantly different from the adhesion processes occurring between the cell body and the substratum.     

The effect of a range of biological polymers and synthetic surfactants on the adhesion of a marine Pseudomonas sp. strain NCMB 2021 to hydrophilic and hydrophobic surfaces

Abstract The effect of a range of biological polymers and synthetic surfactants on the adhesion of a marine Pseudomonas sp. strain NCMB2021 to hydrophilic glass and hydrophobic polystyrene has been investigated. Brij 56 (polyethylene oxide (10) cetyl ether) was the only compound that had a significant effect, almost totally inhibiting the adhesion of Pseudomonas sp. NCMB2021 to hydrophobic polystyrene, but having little or no effect on hydrophilic glass. The surfactant was demonstrated to be effective both when present in the bacterial suspension at low concentrations (approx. 5 ppm), and when pre-adsorbed onto the substratum. Brij 56 was shown to prevent the adhesion of a range of marine and fresh-water bacteria to polystyrene.
It is proposed that on a hydrophobic substratum Brij 56 is adsorbed via its hydrophobe in such a way that the polyethylene glycol chains are pointing outwards into the aqueous phase giving a surface with a high density of uncharged, highly hydrated hydrophilic chains, forming a steric barrier which inhibits the adhesion of bacteria.     

Intramembrane events during the nerve impulse

In current literature the cell adhesion to solid surfaces has been treated in the context of basic physicochemical forces. However, in all these reports the concept of solid surface force has not been properly analyzed. The surface forces acting across an interface formed when two phases meet has been shown to consist of dispersion (attraction) forces and polar forces (arising from different interactions). Current theories have repeatedly neglected the role of polar forces in the cell adhesion. In order to clarify this concept, the particular case, i.e. adhesion of cells on polystyrene surfaces with varying degree of polar groups is described. In this case, the adhesion of cells was reported to increase with polarity of polystyrene, and this agrees with the present study that the solid polar force component increased in the same manner.     

Contact interactions of spreading thrombocytes

Contact interactions of rabbit platelets was studied in the course of their spreading over the siliconized glass surface. When the density of cells adhered to the substratum surface is high enough, the platelets are spreading in such a way that their edges are in a close proximity, while the overlapping of cells is very rare and scarcely pronounced. A morphometric analysis of the probability of such an organization of platelets at their random and independent disposition on the substratum proves the existence of a contact inhibition of platelets spreading on the substratum. Thus, a phenomenon of the contact inhibition seems to be a common feature not only of large nucleated cells but also of small anuclear platelets.     

Production of factors required for cell attachment and spreading is a constitutive property in mouse A9 cells.

It is demonstrated here that cells in a suspension culture of an established mammalian cell line release non-dialyzable factors into their growth medium. These factors are capable of promoting the adhesion and spreading of these cells on a generally non-attachable substratum and also promote spreading on an adhering substrate. Evidence is presented which demonstrates that the spreading promotion activity of the condition medium is dependent on the cell density of the culture from which it was derived. Dilution of the conditioned medium results in a proportionate dilution of the spreading promotion activity. The results clearly demonstrate that the production of this spreading promotion factor is continued even in the absence of cell to substrate attachment.     

Adhesion of cells to polystyrene surfaces

The surface treatment of polystyrene, which is required to make polystyrene suitable for cell adhesion and spreading, was investigated. Examination of surfaces treated with sulfuric acid or various oxidizing agents using (a) x-ray photoelectron and attenuated total reflection spectroscopy and (b) measurement of surface carboxyl-, hydroxyl-, and sulfur-containing groups by various radiochemical methods showed that sulfuric acid produces an insignificant number of sulfonic acid groups on polystyrene. This technique together with various oxidation techniques that render surfaces suitable for cell culture generated high surface densities of hydroxyl groups. The importance of surface hydroxyl groups for the adhesion of baby hamster kidney cells or leukocytes was demonstrated by the inhibition of adhesion when these groups were blocked: blocking of carboxyl groups did not inhibit adhesion and may raise the adhesion of a surface. These results applied to cell adhesion in the presence and absence of serum. The relative unimportance of fibronectin for the adhesion and spreading of baby hamster kidney cells to hydroxyl-rich surfaces was concluded when cells spread on such surfaces after protein synthesis was inhibited with cycloheximide, fibronectin was removed by trypsinization, and trypsin activity was stopped with leupeptin.     

Models of normal and transformed cell adhesion,and capping and locomotion in vitro

It is proposed that patching, capping and endocytosis, and cell locomotion are manifestations of a single process whereby the cell discards foreign materials. Capping results from the binding to the cell surface of particulate (or molecular) objects which cannot function as immovable substratum. This might be described as unsuccessful or abortive cell adhesion in that the particles adhere to the cell rather than the cell adhering to the substratum. Lateral particle movements on the cell surface membrane are effected by the submembranous microfilament-microtubule system, resulting in capping without displacement of the cell. Successful adhesion of the cell to a substratum renders capping and endocytosis impossible and the cell attempts to discard the substratum by mechanisms analogous to capping. The cell achieves this by lateral movement and detachment of the trailing edge.The concept of abortive adhesion leading to capping has been amplified by the development of molecular models of normal and neoplastic cell adhesion in vitro in the presence and absence of serum. In these models, the normal cells have molecule A (adhesion sites) on their surface; they can spread on the substratum in the absence of serum. In the presence of serum, the A molecules on the normal cell surface bind with B molecules in serum, which may be substratum-bound or free in suspension. Binding of free B molecules with cell surface A molecules results in blockage of adhesion sites; these are cleared via capping. New adhesion sites (A molecules) are produced at the active edges of the cell. Binding of cell surface A molecules with the substratum bound B molecules results in cell adhesion. Transformed cells do not have A molecules on their surface; they cannot spread in the absence of serum. The transformed cells may recruit A molecules from the serum to attain deformability and spreading.These models also relate to capping of gold or resin particles, cell locomotion and regulation of cell division, and lectin-induced agglutination of transformed cells.     

Initial adhesion of human fibroblasts in serum-free medium: possible role of secreted fibronectin.

Experiments were carried out to test the hypothesis that the initial attachment and spreading of human fibroblasts in serum-free medium occurs to cell fibronectin which has been secretd spread on tissue culture substrata in serum-free medium in 60 min. When potential protein adsorption sites on the substratum were covered with bovine serum albumin before initial human fibroblasts attachment, their subsequent attachment to the substratum was prevented. When substratum adsorption sites were covered immediately after initial attachment, subsequent cell spreading was prevented. The distribution of fibronectin on human fibroblast surfaces during initial attachment and spreading was studied by indirect immunofluorescence analysis using a monospecific anti-cold-insoluble globulin antiserum. The initial appearance (10 min) of fibronectin was in spots over the entire cell surface. Concomitant with human fibroblast spreading, the random distribution of sites disappeared, and most fibronectin was subsequently observed in spots at the cell substratum interface (60 min). A fibrillar pattern of fibronectin appeared later (2-8 hr). The sites beneath the cells could be visualized as footprints on the substratum following treatment of the attached human fibroblasts with 0.1 M NaOH. A second fluorescence pattern of fibronectin secreted on the substratum was characterized by a diffuse halo around the cells and a very faint, diffuse staining elsewhere on the substratum. Another cell type (baby hamster kideny cells) was used to assay biologically for the presence or absence of the factor secreted by human fibroblasts on the substratum. Human fibroblasts were found to secrete an adhesion factor for baby hamster kidney cells into the substratum in a time- and temperature-dependent fashion, and immunological studies indicated that the factor secreted by human fibroblasts was cross-reactive with cold-in-soluble globulin, the plasma form of fibronectin. The conditioning factor secreted by the human fibroblasts was also found to be an attachment and spreading factor for human fibroblasts in experiments measuring human fibroblast adhesion to fibronectin footprints of human fibroblasts. Substratum-adsorbed cold-insoluble globulin was also found to be an attachment and spreading factor for human fibroblasts. Based upon the timing of appearance of conditioning factors on the substratum and the immunofluorescence patterns, it seems that the diffusely organized fibronectin on the substratum constitutes the sites to which cell attachment occurs. The bright spots of fibronectin that appear beneath the cells may represent fibronectin reorganization during cell spreading.     

Fibrinogen induces adhesion, spreading, and microfilament organization of human endothelial cells in vitro

Human umbilical vein endothelial cells (ECs) have been shown to attach to a substratum of fibrinogen (fg). Later, ECs undergo spreading, organization of thick microfilament bundles of the stress fiber type, and formation of focal contacts (adhesion plaques) that correspond to accumulation of vinculin at the cytoplasmic aspect of the ventral membrane. The rate of attachment to fg and the type of spreading is virtually identical to that obtained on substrata coated with fibronectin (FN). Antibodies to fg, but not to FN, prevent EC adhesion to fg; conversely, antibodies to FN, but not to fg, prevent adhesion of ECs to a FN-coated substratum. The removal of residual FN contamination from fg preparations by means of DEAE-cellulose chromatography does not result in any difference in EC adhesion on fg. Moreover, pretreatment of cells with inhibitors of synthesis and release of proteins does not impair their adhesion capacity on an fg-coated substratum. In contrast, human arterial smooth muscle cells do not adhere and spread on fg substrata but do so on FN. The synthetic peptides (Gly-Arg-Gly-Asp[GRGD] and Gly-Arg-Gly-Asp-Ser-Pro[GRGDSP]) containing the tripeptide Arg-Gly-Asp (RGD), originally found to be responsible for the cell binding activity of FN, have been found to inhibit EC spreading and the redistribution of their cytoskeleton, including the formation of stress fibers and the localization of vinculin either on fg or on FN. Conversely, the synthetic peptide Arg-Gly-Gly (RGG) was completely uneffective in inhibiting the adhesion and the sequence of events leading to spreading and cytoskeletal organization. These results indicate that ECs, but not smooth muscle cells, specifically adhere and spread on an fg substratum and this occurs by recognition mechanisms similar to those reported for FN.     

Variability of the Influence of Physicochemical Factors Affecting Bacterial Adhesion to Polystyrene Substrata

The role of electrostatic and hydrophobic interactions and solid and liquid surface tensions in the adhesion of four bacterial species (Pseudomonas fluorescens, Enterobacter cloacae, Chromobacterium sp., and Flexibacter sp.) to hydrophobic polystyrene petri dishes and to more hydrophilic polystyrene tissue culture dishes was investigated. The effect of electrostatic interactions was investigated by determining the effects of different electrolyte solutions on attachment to and of different electrolyte and pH solutions on detachment from the polystyrene substrate. The significance of solid and liquid surface tensions and hydrophobic interactions was investigated by measuring the effects of different surfactants (including a concentration series of dimethyl sulfoxide) on adhesion and detachment. Adhesion varied with bacterial species, substratum, and electrolyte type and concentration, with no apparent correlation between adhesion and electrolyte valence or concentration. The influence of different pH and detergent solutions on bacterial detachment also varied with species, substratum, pH, and detergent type; however, the greatest degree of detachment of all strains from the surfaces was produced by detergent treatment. The results suggest that adhesion cannot be attributed to any one type of adhesive interaction. There was some evidence for both electrostatic and hydrophobic interactions, but neither interaction could wholly account for the data.     

Association between fibronectin receptor and the substratum: Spare receptors for cell adhesion

We have investigated the association of the recently described 140-kDa cell membrane receptor for fibronectin with the cytoskeleton or with substratum-bound fibronectin. Using a monospecific polyclonal antibody to the 140-kDa receptor, we have demonstrated that most of the receptor molecules are soluble in nonionic detergent either in suspension culture CHO cells or in CHO cells attached to and spread on a fibronectin-coated substratum. This may suggest that putative linkages of the receptor either to fibronectin or to detergent-insoluble cytoskeletal components are labile to nonionic detergent and thus are rather weak. Alternatively, it may mean that only a small fraction of the cell's receptors are needed to mediate adhesion. In order to explore this latter concept, we have coated substrata with various concentrations of PB1, a monoclonal antibody with a high affinity for fibronectin receptor. We demonstrate that coating the substratum with increasing concentrations of PB1 results in increasing amounts of 140-kDa receptor becoming bound to the substratum in detergent-insoluble form. However, the amount of receptor bound does not necessarily correlate with the degree of cell adhesion and spreading. Thus, coating the substratum with 5 μg/ml of PB1 results in essentially complete attachment and spreading of CHO cells, but only a small fraction of the 140-kDa receptor becomes substratum bound. Coating with 50 μg/ml of PB1 produces no further increase in cell adhesion and spreading, but results in the detergent-stable association of a large fraction of the total receptor pool with the substratum. These observations suggest the possibility of there being “spare” receptors for cell adhesion processes.     

Surface movements during the spreading of blood platelets

When human blood platelets spread on a substratum they increase their surface area as much as 4-fold. We investigated the mechanism of spreading by light microscopy and by scanning and transmission electron microscopy. Contact of a platelet with a glass surface induces formation of thin extensions which spread out over the substratum. These extensions resemble the actin-containing microspikes and lammelipodia of tissue cells in culture and appear to be drawn from the peripheral cortical layer associated with the plasma membrane. If platelets are initially labeled on their external surface with cationic ferritin or lentil-conjugated gold particles and then allowed to spread, the labels are retained in the central region, or granulomere. Proteins released by the spreading platelet--fibronectin and fibrinogen--also remain in this central unspread region. Peripheral regions of spread platelet surface (hyalomere) were unlabeled following the above procedures but could be labeled with cationic ferritin or lentil-conjugated gold provided these were applied after spreading was completed. These markers are cleared with time from the periphery, moving centripetally to accumulate at the granulomere. We suggest, on the basis of these observations, that platelets spread onto a substratum by a closely similar mechanism to that used by cells such as fibroblasts. In both cases the spreading involves the peripheral actin cortex and is accompanied by a continual centripetal movement of surface components--a "membrane flow"--which continues even after spreading is completed.     

Adhesion of Azospirillum brasilense to polystyrene: Influence of ionic strength through protein adsorption

The influence of ionic strength on the adhesion of Azospirillum brasilense to polystyrene has been examined by comparing water and phosphate buffer saline (PBS) as suspending media. Polystyrene supports analysed by X‐ray photoelectron spectroscopy (XPS) after adhesion in PBS for 2 h or 24 h and detachment of adhering cells showed a higher protein surface concentration, reflected by the N/C atomic concentration ratio, compared to supports analysed after adhesion in water. It was shown that PBS both favours protein release by the cells into the solution and enhances the tendency of proteins to adsorb at the support surface.

After 2 h contact time, the increase in the concentration of adsorbed proteins in PBS was related to an increase in adhesion density. However, the observation that the adhesion density after 24 h was lower in PBS than in water indicated that the amount of proteins adsorbed at the support surface controls cell adhesion in a complex way. In PBS, a thick layer of proteinaceous material retaining the bacterial cells is formed; this leads to underestimation of the density of adhering cells as well as to a heterogeneous adhesion pattern and to a relatively low adhesion density due to detachment of pellicles upon rinsing.

The ionic strength thus influences bacterial adhesion in a more subtle way than simply through double layer interactions between the cells and the support.     

Shark cartilage extract interferes with cell adhesion and induces reorganization of focal adhesions in cultured endothelial cells

In this study, we examined the effects of shark cartilage extract on the attachment and spreading properties and the focal adhesion structure of cultured bovine pulmonary artery endothelial cells. Treatment with cartilage extract resulted in cell detachment from the substratum. Immunofluorescence staining of those treated cells that remained attached showed that, instead of being present in both central and peripheral focal adhesions as in control cells, both integrin alpha(v)beta(3) and vinculin were found only in peripheral focal adhesion and thinner actin filament bundles were seen. In addition to causing cell detachment, cartilage extract partially inhibited the initial adherence of the cells to the substratum in a dose-dependent manner. Integrin alpha(v)beta(3) and vinculin staining of these cells also showed a peripheral focal adhesion distribution pattern. Vitronectin induced cell spreading in the absence of serum, but was blocked by simultaneous incubation with cartilage extract, which was shown to inhibit both integrin alpha(v)beta(3) and vinculin recruitment to focal adhesion and the formation of stress fibers. Dot binding assays showed that these inhibitory effects on cell attachment and spreading were not due to direct binding of cartilage extract components to integrin alpha(v)beta(3) or vitronectin. Shark cartilage chondroitin sulfate had no inhibitory effect on either cell attachment or spreading of endothelial cells. These results show that the inhibitory effects of cartilage extract on cell attachment and spreading are mediated by modification of the organization of focal adhesion proteins.     

The organization of actin in spreading macrophages : The actin-cytoskeleton of peritoneal macrophages is linked to the substratum via transmembrane connections

The cytoskeleton of murine peritoneal macrophages has been examined by a combination of morphological techniques, including phase-contrast light microscopy, scanning electron microscopy (SEM), and several transmission electron microscopic (TEM) methods. The cytoskeleton of cells spreading on glass, Formvar-carbon, and polystyrene substrata was exposed by brief extraction with non-ionic detergent, and stabilized by exposure to heavy meromyosin, myosin subfragment-1 or tropomyosin. In the spreading lamellae and lamellipodia the cytoskeleton is principally composed of filamentous actin, which appears as dense foci, interconnected by radiating filaments and filament bundles. The actin of the foci, as well as individual actin filaments, are connected to the substratum by transmembrane linkages which appear as filaments that pass through the plane of the (extracted) plasma membrane. Thus, the results of this study indicate that the adhesion of macrophages to substrata for the purposes of spreading and motility may be a function of transmembrane elements which link actin to substrata. Further, the formation of actin foci may serve to stiffen and stabilize the cytoskeleton, conditioning it to function in cell adhesion, spreading and locomotion.     

Dorsoventral differences in cell-cell interactions modulate the motile behaviour of cells from the Xenopus gastrula.

When groups of cells from the inner marginal zone (mesendoderm) of the early Xenopus gastrula are placed on a fibronectin-coated substratum, the explants of the dorsal region spread into monolayers whereas those from the ventral region, though they adhere to the substratum, do not show this spreading reaction. This different behaviour is not reflected in the in vitro behaviour of the respective cells kept in isolation. No difference between dorsal and ventral cells was observed, when they were tested for lamellipodia-driven spreading, movement over the substratum or properties of integrin- and cadherin-mediated adhesion. However, cell contacts between individual dorsal cells are significantly less stable than those between ventral cells. The higher flexibility of the cell-cell contacts seems to determine the spreading behaviour of the dorsal explants, which includes lamellipodia-driven outward movement of the peripheral cells, rearrangements of the cells, building up a horizontal tension within the aggregate and intercalation of cells from above into the bottom layer. Ventral explants lack these properties. Staining for F-actin revealed a decisive difference of the supracellular organisation of the cytoskeleton that underlies the morphology of the different types of explants. Evidence for a higher flexibility of cell-cell contacts in the dorsal mesendoderm was also obtained in SEM studies on gastrulating embryos. Dorsal mesendodermal cells show stronger protrusive activity as compared to ventral mesendodermal cells. The meaning of these observations for the mechanisms of morphogenetic movements during gastrulation is central to the discussion.