Interaction of human skin fibroblasts with moderate wettable polyacrylonitrile--copolymer membranes

The development of a bioartificial skin is a step toward the treatment of patients with deep burns or nonhealing skin ulcers. One possible approach is based on growing dermal cells on membranes to obtain appropriate living cellular stroma (sheets) to cover the wound. New membrane-forming copolymers were synthesized, based on acrylonitrile (AN) copolymerization with hydrophilic N-vinylpyrrolidone (NVP) monomer, in different percentage ratios, such as 5, 20, and 30% w/w, and with two other relatively high polar comonomers--namely, sodium 2-methyl-2-propene-1-sulfonic acid (NaMAS) and aminoethylmethacrylate (AeMA). All these copolymers were characterized for their bulk composition and number average molecular weight, and used to prepare ultrafiltration membranes. Water contact angles and water uptake were estimated to characterize the wettability and scanning force microscopy to visualize the morphology of the resulting polymer surface. Cytotoxicity was estimated according to the international standard regulations, and the materials were found to be nontoxic. The interaction of the membranes with human skin fibroblasts was investigated considering that these cells are among the first to colonize membranes upon implantation or with prolonged external contact. The overall cell morphology, formation of focal adhesion contacts, and cell proliferation were estimated to characterize the cell material interactions. It was found that the pure polyacrylonitrile homopolymer (PAN) membrane provides excellent conditions for seeding with fibroblasts, comparable only to a copolymer containing AeMA. In contrast, the presence of NaMAS with acidic ionic groups decreased both the attachment and proliferation of fibroblasts. Low content of NVP in the copolymer, up to about 5%, still enabled good attachment and spreading of cells, as well as subsequent proliferation of fibroblasts, but higher ratios of 20 and 30% resulted in a significant decrease of these cellular activities.     

The role of surface wettability on hepatocyte adhesive interactions and function

In this paper the effect of surface wettability on hepatocyte morphology and function was studied, using clean and octadecylsylane (ODS)-coated glass as a model for hydrophilic and hydrophobic surfaces, respectively. C3A cells--a hepatoblastoma cell line, and freshly obtained porcine hepatocytes were cultured for a short-time period of up to 4 days on the above substrata. Hepatocyte adhesive interactions were characterized monitoring the initial cell attachment, the overall cell morphology, the formation of focal adhesions, and actin filaments. Since hepatocytes showed a clear tendency for homotypic adhesion on ODS, specific E-cadherin staining was used to visualize the intercellular contacts by immunofluorescence microscopy. Additionally, functional assays were carried out to monitor proliferation, metabolic activity, and albumin synthesis of C3A cells. It could be shown that both C3A cells and normal porcine hepatocytes spread better on hydrophilic glass; spreading being accompanied by the development of pronounced actin stress fibers and focal adhesion contacts. In contrast, on hydrophobic substrata predominant cell-cell interactions took place which led to intense E-cadherin staining in the intercellular contacts of porcine hepatocytes but not in C3A cells. On the other hand, metabolic activity and growth of C3A cells were reduced on hydrophobic ODS, but albumin synthesis was similar on both surfaces. It was concluded that the wettability of materials has a strong influence on the attachment and morphology of hepatocytes while the influence of surface properties on the functional activity of hepatocytes still remains to be elucidated.     

The role of surface wettability on hepatocyte adhesive interactions and function

In this paper the effect of surface wettability on hepatocyte morphology and function was studied, using clean and octadecylsylane (ODS)-coated glass as a model for hydrophilic and hydrophobic surfaces, respectively. C3A cells - a hepatoblastoma cell line, and freshly obtained porcine hepatocytes were cultured for a short-time period of up to 4 days on the above substrata. Hepatocyte adhesive interactions were characterized monitoring the initial cell attachment, the overall cell morphology, the formation of focal adhesions, and actin filaments. Since hepatocytes showed a clear tendency for homotypic adhesion on ODS, specific E-cadherin staining was used to visualize the intercellular contacts by immunofluorescence microscopy. Additionally, functional assays were carried out to monitor proliferation, metabolic activity, and albumin synthesis of C3A cells. It could be shown that both C3A cells and normal porcine hepatocytes spread better on hydrophilic glass; spreading being accompanied by the development of pronounced actin stress fibers and focal adhesion contacts. In contrast, on hydrophobic substrata predominant cell-cell interactions took place which led to intense E-cadherin staining in the intercellular contacts of porcine hepatocytes but not in C3A cells. On the other hand, metabolic activity and growth of C3A cells were reduced on hydrophobic ODS, but albumin synthesis was similar on both surfaces. It was concluded that the wettability of materials has a strong influence on the attachment and morphology of hepatocytes while the influence of surface properties on the functional activity of hepatocytes still remains to be elucidated.     

Membranes for biohybrid liver support systems--investigations on hepatocyte attachment,morphology and growth

The biological properties of four different membranes were studied regarding their possible application in biohybrid liver support systems. Two of them, one made of polyetherimide (PEI), and a second based on polyacrylonitrile-N-vinylpyrollidone co-polymer (P(AN-NVP)), were recently developed in our lab and studied for the first time. Together with pure polyacrylonitrile (PAN) membranes, the three preparations were characterised as ultra-filtration membranes. Their ability to support cell attachment, morphology, proliferation and function of human hepatoblastoma C3A cells was studied. The role of surface morphology for the interaction with hepatocytes was highlighted using a commercial, moderately wettable polyvinylidendifluoride (PVDF) membrane with micro-filtration properties. Comparative investigations showed strongest interaction of C3A cells with PAN membranes, as the focal adhesion contacts were more expressed and cell growth was also high. However, the functional activity in terms of albumin synthesis was reduced. Very similar results were obtained with the most hydrophobic PEI membrane. In contrast, the most hydrophilic membrane P(AN-NVP) was found to provoke stronger homotypic adhesion (E-cadherin expression) of C3A cells and less substratum attachment (focal adhesions), but enhanced albumin secretion. However, proliferation of C3A cells was lowered. Micro-porous PVDF membrane showed very good initial attachment, but the resulting cell material and cell-cell interaction were relatively poor developed. Among four membranes tested, PEI seems to be the most attractive membrane for biohybrid liver devices, as it provides good surface properties for hepatocytes interaction, but in addition it is highly thermostable, which would permit steam sterilisation. No simple relationship, however, between the wettability of the membranes and their ability to support hepatocyte adhesion and function was found in this study.     

Membranes from acrylonitrile-based polymers for selective cultivation of human keratinocytes

A cell carrier made from synthetic material supporting selective growth of keratinocytes is a promising approach to avoid the phenomenon of fibroblast overgrowth during in vitro culture of skin substitutes. Therefore, we investigated polymer membranes made of polyacrylonitrile and copolymers of acrylonitrile and N-vinylpyrrolidone (NVP) for their ability to support selectively the growth of keratinocytes. It was found that a copolymer with an NVP-content of 30% (NVP30) supports growth of human keratinocyte cell line (HaCaT) cells and inhibits fibroblast growth under serum-containing conditions. Cell proliferation of HaCaT cells was measured over 14 days. If both cell types were cultured under serum-free conditions for initial adhesion over 6 h on these NVP30 polymers, they adhered to the same extent. Long-term experiments over 7 days were performed as a coculture of both cell types showing that HaCaT cells had a growth advantage that seems to be related to the paracrine activity of contaminating fibroblasts. As a result, confluent layers of HaCaT cells were obtained with small numbers of remaining fibroblasts. The new poly [acrylonitrile-co(NVP) membranes seem to be a promising culture system for the production of epidermal transplants.     

Isolation and preliminary in vitro characterization of the porcine pulmonary intravascular macrophage

Porcine intravascular macrophages were isolated by perfusion of the pulmonary vasculature with 0.1% collagenase solution. The isolated cells formed intercellular adhesion plaques with endothelial cells when incubated with porcine pulmonary artery, aorta, and corneal cups. Intercellular adhesion plaques were focal junctionlike membrane specializations consisting of paired submembranous amorphous densities subjacent to 15-20 nm gaps between parallel apposing cell membranes. The intermembranous space was filled with moderately electron dense, finely granular material. Adhesion plaques formed in 4-8 hours and resembled the adhesion plaques formed between pulmonary intravascular macrophages and endothelium in vivo. Alveolar macrophages and peripheral blood monocytes did not form intercellular adhesion plaques with endothelial cells. Intravascular macrophages had histologic and ultrastructural features of macrophages, were alpha naphthyl butyrate esterase positive, adhered to plastic coverslips after 1 hour of incubation, and were smaller than alveolar macrophages and endothelial cells. The formation of intercellular adhesion plaques in vivo and in vitro by cells with morphologic and histochemical features of macrophages distinguishes intravascular macrophages from monocytes and alveolar macrophages.     

Influence of polymer membrane porosity on C3A hepatoblastoma cell adhesive interaction and function

The effect of the porosity of acrylonitrile-N-vinylpyrrolidone copolymer membranes on human C3A hepatoblastoma cell adhesive interaction and functioning is investigated on four membranes with an average pore size ranging between 6 and 12 nm. Adhesion of C3A cells was quantified and characterized by studying overall cell morphology and focal adhesion formation. Cell-cell interactions were characterized by E-cadherin expression and organization. Cell growth, fibronectin synthesis and cytochrome P450 activity were estimated as criteria of functional cell activity. The results suggest that membrane porosity influences the initial cell-surface interactions since an increasing pore size augmented cell adhesion and aggregate formation. Cell growth after 7 d was diminished on membranes with an average pore size of 12 nm. The activity of P450 measured by 7-ethoxycoumarin conversion at day 7 was influenced by membrane topography representing a clear optimum in the range of 7-10 nm pore size. These results indicate that membrane porosity is a determinant for the function of hepatocytes in extracorporal liver assist devices.     

Electron microscope observations on the formation of primitive villi in rat small intestine with special reference to intercellular junctions.

Formation and fusion of intraepithelial cavities have long been considered an essential process in the histogenesis of the intestinal mucosa. By electron microscope observation of thin sections and freeze-fracture replicas of the small intestine of rat fetuses, we first demonstrated the initial steps in the formation of intraepithelial cavities: A focal tight junction (macula occludens) was formed in the abluminal part of the epithelium, after which the membrane of an intracellular cavity was fused with that of the focal tight junction to form an intercellular (intraepithelial) cavity enclosed by a zonula occludens. The present study also revealed that gap junctions appeared and enlarged simultaneously with the formation of primitive villi and differentiation of absorptive cells. These gap junctions gradually came to be confined in the epithelium of intervillous regions where proliferation and differentiation of epithelial cells took place. Absorptive cells in villi rarely had gap junctions. These results suggest that tight and gap junctions play important roles in the histogenesis of the intestinal mucosa, and in the proliferation and differentiation of epithelial cells.     

A study of intercellular relationships between trabecular bone and marrow stromal cells in the murine femoral metaphysis

The cellular relationship between the substantia spongiosa of bone (cancellous or trabecular bone) and the haematopoietic bone marrow in the femoral metaphysis of C57BL/6NJCL mice was studied by transmission electron microscopy (TEM). Special attention was directed to intercellular junctions between osteocytes, osteoblasts, and bone marrow reticular cells. These were gap junctions and adhesive devices of simple architecture referred to as primitive junctions or zonula adherens-like junctions. Gap junctions were observed between osteocytes (within the trabeculae) and osteoblasts (at the trabecular surface) and between osteoblasts and marrow reticular cells. Gap junctions were also observed between the same cell type within each of these categories. These junctions involved the plasmalemmal membranes of adjacent cell bodies and of processes. Primitive cell junctions had a similar cellular distribution. Quantitative analysis of the cell types covering or positioned around the trabecular bones and of gap junctions between these and other cells was carried out by TEM. It was found that osteoblasts were the most numerous cell type, occupying 31% of the total of each cell type positively identified around the trabeculae (31%), while pre-osteoblasts, (flattened bone marrow reticular cells) took up 26%. These data emphasise the intimate relationship of the various mesenchymal cells based on processes and intercellular junctions, and point to an anatomical and probably functional integration of trabeculae and marrow. The functional significance and putative regulatory activity of this unit are discussed.     

Early stages in the reorganization of dissociated embryonic chick skeletal muscle cells

Summary Cell surface material and contact specializations were observed in embryonic chick skeletal muscle cells at early stages (prior to fusion) in monolayer culture. Ruthenium red-staining surface material was largely absent after prior treatment with trypsin. During reorganization into a tissue pattern dense staining amorphous material was seen at the cell surfaces and in the extracellular spaces of clustered cells; the free surface material was clumped, that between the cells more compact. This material appeared to be mucopolysaccharide and could be involved in adhesion. Numerous close junctions (intercellular space, 25–100 Å), as well as occasional focal tight junctions (no apparent intercellular space), were observed between apposed cells. These junctions semmed related to cellular adhesion and perhaps also to electrical coupling.This research was supported by grants from the Muscular Dystrophy Associations of America, Inc. and Japanese Ministry of Education. The author thanks Dr. D. A. Fischman for the opportunity to use an AEI electron microscope.     

Tight junction contact events and temporary gap junctions in the sciatic nerve fibres of the chicken during Wallerian degeneration and subsequent regeneration

Summary Tight and gap junctions are described on the basis of freeze-fractures in normal chicken sciatic nerves as well as during Wallerian degeneration and subsequent regeneration. 1. Small calibre nerve fibres display a fairly continuous tight junction contact zone in the membranes of the mesaxons, paranodal loops and Schmidt-Lanterman incisures. Large fibres with more than 40 lamellae have only focal tight junction contacts in the mesaxonal membranes. 2. With the onset of Wallerian degeneration (days 2–4 post-crush, distal stump) myelinic tight junctions become arranged as maculae composed of one circular or several polygonally oriented strands that are criss-crossed by other tight junctional strands. These maculae are subsequently found in the membranes of cytoplasmic vacuoles of the Schwann cells, indicating an endocytotic mode of uptake. Tight junctions are not found between the 5th and 6th day after crush. 3. During the proliferation phase of the Schwann cells and the arrangement of these cells into Büngner cell bands (2 to 8 days post-crush) gap junctions appear between the Schwann cells of the bands. These junctions then disappear with the onset of remyelination (8 days post-crush). 4. With the onset of remyelination (from the 8th day onwards) short focal tight junctions appear in the membranes of the outer mesaxons. Shortly thereafter, when the sheaths possess 4 to 8 lamellae, tight junctions also appear in the membranes of the inner mesaxons, the paranodal loops and the cytoplasmic inclusions. The characteristic differences of tight junction elaboration in small versus large nerve fibres are re-established after three months of regeneration.The elaborated tight junctions in small and early remyelinating fibres point to a specific function; in small fibres (versus large fibres) the tight junctions might effect a separation of the intramyelinic extracellular space as a single compartment. The tight junction contacts in early remyelinating fibres support the hypothesis that myelin growth occurs within the myelin spiral and not by a free rotation and elongation of the Schwann cell tongues. It is assumed that the gap junctions between the Schwann cells contribute to the co-ordination of the Schwann cell band formation, which is involved in the guidance of sprouting axons.     

Electrochemical modulation of epithelia formation using conducting polymers

Conducting polymers are soft, flexible materials, exhibiting material properties that can be reversibly changed by electrochemically altering the redox state. Surface chemistry is an important determinant for the molecular events of cell adhesion. Therefore, we analyzed whether the redox state of the conducting polymer PEDOT:Tosylate can be used to control epithelial cell adhesion and proliferation. A functionalized cell culture dish comprising two adjacent electrode surfaces was developed. Upon electronic addressing, reduced and oxidized surfaces are created within the same device. Simultaneous analysis of how a homogenous epithelial MDCK cell population responded to the electrodes revealed distinct surface-specific differences. Presentation of functional fibronectin on the reduced electrode promoted focal adhesion formation, involving α_vβ₃ integrin, cell proliferation, and ensuing formation of polarized monolayers. In contrast, the oxidized surface harbored only few cells with deranged morphology showing no indication of proliferation. This stems from the altered fibronectin conformation, induced by the different surface chemistry of the PEDOT:Tosylate electrode in the oxidized state. Our results demonstrate a novel use of PEDOT:Tosylate as a cell-hosting material in multiple-electrode systems, where cell adhesion and proliferation can be controlled by electrochemical modulation of surface properties.     

Analysis of the biological response of endothelial and fibroblast cells cultured on synthetic scaffolds with various hydrophilic/hydrophobic ratios: influence of fibronectin adsorption and conformation

In this study we developed polymer scaffolds intended as anchorage rings for cornea prostheses among other applications, and examined their cell compatibility. In particular, a series of interconnected porous polymer scaffolds with pore sizes from 80 to 110 microns were manufactured varying the ratio of hydrophobic to hydrophilic monomeric units along the polymer chains. Further, the effects of fibronectin precoating, a physiological adhesion molecule, were tested. The interactions between the normal human fibroblast cell line MRC-5 and primary human umbilical vein endothelial cells (HUVECs) with the scaffold surfaces were evaluated. Adhesion and growth of the cells was examined by confocal laser scanning microscopy. Whereas MRC-5 fibroblasts showed adhesion and spreading to the scaffolds without any precoating, HUVECs required a fibronectin precoating for adhesion and spreading. Although both cell types attached and spread on scaffold surfaces with a content of up to a 20% hydrophilic monomers, cell adhesion, spreading, and proliferation increased with increasing hydrophobicity of the substrate. This effect is likely due to better adsorption of serum proteins to hydrophobic substrates, which then facilitate cell adhesion. In fact, atomic force microscopy measurements of fibronectin on surfaces representative of our scaffolds revealed that the amount of fibronectin adsorption correlated directly with the hydrophobicity of the surface. Besides cell adhesion we also examined the inflammatory state of HUVECs in contact with the scaffolds. Typical patterns of platelet/endothelial cell adhesion molecule-1 expression were observed at intercellular boarders. HUVECs adhering on the scaffolds retained their proinflammatory response potential as shown by E-selectin mRNA expression after stimulation with lipopolyssacharide (LPS). The proinflammatory activation occurred in most of the cells, thus confirming the presence of a functionally intact endothelium. Little or no expression of the proinflammatory activation markers in the absence of LPS stimulation was observed for HUVECs growing on scaffolds with up to a 20% of hydrophilic component, whereas activation of these markers was observed after stimulation. In conclusion, scaffolds containing up to 20% hydrophilic monomers exhibited excellent cell compatibility toward human fibroblast cell line MRC-5 and human endothelial cells. Atomic force microscopy confirmed that adsorbed serum proteins such as fibronectin probably accounted for the positive correlation of HUVEC adhesion and surface hydrophobicity.     

The viability and function of primary rat hepatocytes cultured on polymeric membranes developed for hybrid artificial liver devices

Bioartificial liver devices require membranes to support the function and viability of hepatocytes because they are anchorage-dependent cells. This study investigated the ability of several polymeric membranes to support the functions of primary hepatocyte cultures. Tailor-made membranes were sought by synthesizing acrylonitrile copolymers with different comonomers resulting in ionic, hydrophilic, or reactive functional groups on the polymer surface. Hepatocyte morphology and viability were assessed by confocal microscopy, and function by the content and activities of cytochrome P450, and the expression of glutathione S-transferases. Hydrophilic membranes (polyacrylonitrile and acrylonitrile copolymerized with 2-acrylamino-2-methyl-propane sulfonic acid) were more biocompatible than hydrophobic membranes such as polysulfone. The chemistry of the hydrophilic group was important; amine groups had a deleterious effect on maintenance of the primary hepatocytes. The biocompatibility of hydrophobic membranes was improved by collagen coating. Improving the chemistry of membranes for artificial liver devices will enhance the phenotypic stability of the cells, enabling us to prolong treatment times for patients.     

Direct quantification of the modulation of interaction between cell- or surface-bound LFA-1 and ICAM-1

The functional activity of leukocyte integrins is highly regulated by several mechanisms related to intrinsic molecular properties and receptor interaction with the cell membrane. Here, we present a microkinetic study of the lymphocyte function-associated antigen-1-mediated interaction between flowing Jurkat cells and surface- or cell-bound intercellular adhesion molecule-1 (ICAM-1). We conclude that adhesion is initiated by the formation of a single bond with approximately 0.3 s(-1) dissociation rate, and attachment is subsequently strengthened by the formation of additional bonds during the next 10 s; exposing cells to Mg2+ or Mn2+ resulted in up to a 16-fold increase of the binding frequency, in line with reported measurements performed on isolated molecules with surface plasmon resonance methodology; cell-bound ICAM-1 molecules were more efficient in mediating adhesion than Fc-ICAM-1, properly oriented and bound by surface-adsorbed protein A; and quantitative analysis of binding frequency suggested that adhesion efficiency was ten- to 100-fold lower than the maximum value allowed by previously determined association rates of soluble molecules. It is concluded that the presented methodology provides a simple and unique way of dissecting the initial step of cell adhesion and discriminating between affinity and avidity modulation of adhesion receptors.     

E-cadherin in the mature and developing organ of Corti of the mouse

Summary The reticular lamina of the mammalian cochlea is formed by the tightly joined apical surfaces of hair cells and supporting cells. This lamina creates a barrier separating endolymph and perilymph, two fluids of different composition. The preservation of this fluid barrier is a requirement for cochlear function. This study aimed to determine whether the calcium-dependent, cell adhesion molecule, E-cadherin was appropriately placed both temporally and spatially to contribute to the formation and maintenance of the reticular lamina. Cochleas aged E15 to P31 were stained immunocytochemically for E-cadherin. In the P31 organ of Corti, E-cadherin is localized to the apical intercellular junctions of supporting cells, but is absent from supporting cell-hair cell borders. During development, E-cadherin is present only on the apices and lateral edges of those cells which will eventually lie adjacent to fluid spaces — pillar, outer hair and Deiters cells. The molecule disappears from the lateral cell membranes at about the time in development that fluid spaces form. These data suggest that E-cadherin plays a role in maintaining the reticular lamina by mediating inter-supporting cell adhesion and raise the possibility that fluid space opening in the organ of Corti is facilitated by the down-regulation or redistribution of E-cadherin.     

Surface modification of poly(<Emphasis Type="SmallCaps">l</Emphasis>-lactic acid) affects initial cell attachment,cell morphology,and cell growth

The object of this study was to develop a highly porous scaffold to be used in regeneration of blood vessels, nerves, and other hollow tissues with small openings. Using the phase-inversion method and a mixture of water and methanol as a coagulating agent, we prepared highly porous flat membranes from poly(l-lactic acid) (PLLA) with numerous pores both on the surface and in the interior of the membranes. Chinese hamster ovary (CHO) cells were cultured on the membranes to evaluate initial cell adhesion, cell proliferation, and cell morphology. Adhesion of CHO cells to PLLA was poor: the cells adhered at approximately half the rate observed with a tissue culture polystyrene dish (TCPS). In contrast, adhesion of cells to PLLA treated with a low-temperature oxygen plasma was good; the adhesion rate was the same as that on TCPS. The rate of cell proliferation on the treated membranes was no different from that on the nontreated membranes, but cell morphologies were quite different. The cells on the nontreated membranes were small and round and proliferated separately from one another. In contrast, the cells on the plasma-treated membranes proliferated in close contact with other cells, spreading out extensively in sheet-like formations. Since the plasma treatment not only accelerated cell adhesion but also enabled cells to proliferate in the form of sheets resembling biological tissue, we believe that oxygen-plasma treatment is extremely effective for modifying surfaces of materials used for tissue regeneration.     

Response of MG63 osteoblast-like cells onto polycarbonate membrane surfaces with different micropore sizes

Response of different types of cells on materials is important for the applications of tissue engineering and regenerative medicine. It is recognized that the behavior of the cell adhesion, proliferation, and differentiation on materials depends largely on surface characteristics such as wettability, chemistry, charge, rigidity, and roughness. In this study, we examined the behavior of MG63 osteoblast-like cells cultured on a polycarbonate (PC) membrane surfaces with different micropore sizes (0.2-8.0 microm in diameter). Cell adhesion and proliferation to the PC membrane surfaces were determined by cell counting and MTT assay. The effect of surface micropore on the MG63 cells was evaluated by cell morphology, protein content, and alkaline phosphatase (ALP) specific activity. It seems that the cell adhesion and proliferation were progressively inhibited as the PC membranes had micropores with increasing size, probably due to surface discontinuities produced by track-etched pores. Increasing micropore size of the PC membrane results in improved protein synthesis and ALP specific activity in isolated cells. There was a statistically significant difference (P<0.05) between different micropore sizes. The MG63 cells also maintained their phenotype under conditions that support a round cell shape. RT-PCR analysis further confirmed the osteogenic phenotype of the MG63 cells onto the PC membranes with different micropore sizes. In results, as micropore size is getting larger, cell number is reduced and cell differentiation and matrix production is increased. This study demonstrated that the surface topography plays an important role for phenotypic expression of the MG63 osteoblast-like cells.     

The distribution of intercellular junctions in the developing myotomes of the clawed toad.

The distribution of intercellular junctions between premyotomal and myotomal cells of Xenopus embryos has been described. Intermediate junctions, gap junctions, and small desmosomes are found between unsegmented mesoderm cells. In the region of myotome formation gap junctions appear to be absent, although close appositions of cell membranes are found. Both gap and intermediate junctions are numerous between cells of the segmented myotomes; they occur within the myotomes close to the myosepts and at interdigitations between the myotomes across the myosepts. Gap junctions between the myotomes are found in embryos that have hatched and can swim. The observed distribution of gap junctions is correlated with physiological data on electrical coupling. Electrical connections via gap junctions may produce contraction of the axial musculature remote from motor innervation.