The role of connexon aggregate fusion in gap junction growth

Gap junctions are thought to grow in size by the incorporation of single connexons and by the fusion of connexon aggregates but the relative importance of these two growth mechanisms, particularly in the formation of large gap junctions, remains unclear. We have used a quantitative approach to distinguish between these two growth mechanisms and present evidence here that gap junctions in Drosophila melanogaster wing discs have a periodic, (approximately equal to 0.025 micron 2), multimodal areal distribution which suggests that gap junctions grow by aggregate fusion. Previously published gap junction areal frequency distributions from another invertebrate, Manduca sexta, and from several vertebrate animals also show periodic (approximately equal to 0.030 micron 2), multimodal areal distributions. We suggest that the interpeak spacings on the frequency distributions represent average areas of 'unit aggregates' of connexons and that the fusion of such unit aggregates with one another and with integral multiples of unit aggregates may be a general mechanism for gap junction growth. Unit aggregates in mammals and insects are similar in surface area rather than in connexon number suggesting that a common molecular mechanism, perhaps operating at the level of the plasma membrane, may be involved in their formation and stabilization.

Gap junction distribution in the Drosophila wing disc mutants vg, l(2)gd, l(3)c43hs1, and l(2)gl4

The density of gap junctions in four Drosophila melanogaster mutants with abnormal wing disc development has been determined using quantitative electron microscopy and compared with the gap junction density in wild-type wing discs. No appreciable differences relative to wild-type controls were found in the cell death mutant vestigial or in the mildly hyperplastic mutant lethal giant disc which could not be accounted for in terms of altered lateral plasma membrane surface density or as an extension of the gap junction growth which normally occurs during the third larval stage of development in wild-type wing discs. However, both the severely hyperplastic mutant l(3)c43hs1 and the neoplastic mutant lethal giant larva have significant reductions in the gap junction surface density, the number of gap junctions, and the gap junction areal fraction of the lateral plasma membrane compared with wild-type controls. These differences cannot be attributed to altered lateral plasma membrane surface densities which are not significantly different from wild-type control wing discs. The reduced gap junction density in severely hyperplastic and neoplastic wing discs suggests that alterations in the number or distribution of gap junctions may be as disruptive to normal growth and development as their complete absence.

Ultrastructural study of the myocardial wall of the atrio-ventricular canal during the development of the embryonic chick heart

In the early stages of chick embryo development (days 3-5) the myocardium of the atrio-ventricular canal (AV) is continuous with the atrial and ventricular muscles; however, interruption of muscular continuity is observed at later stages (from day 6 to day 8). The most relevant event occurring at the AV canal region is the dissociation of the myocytes due to the loss of their cellular attachments, rather than an invasion of connective tissue cells (endocardial and sub-epicardial) located on both sides of the myocardium. In this study, particular attention was paid to the sequential changes that take place in the myocardium of this region, these being (1) a reduction in the number of desmosomes and intercalated discs with the subsequent appearance of large, inter-cellular spaces between myocytes; (2) migration of these cells through a complex extra-cellular matrix, to which it appears to be closely related, suggesting that the macromolecules of this matrix may be being synthesized by the myocytes, and may take part in the process of cardiac cell separation; (3) incorporation of the myocytes in the developing tricuspid valve, where they co-exist with fibroblasts. The results of the study correspond remarkably well to those previously carried out on the left AV canal myocardium, suggesting that the behaviour of the muscle is the same, at all points around the AV canal.

Pericyte endothelial gap junctions in human cerebral capillaries

Human cerebral tissue has been ultrastructurally studied and gap junctions have been visualized between endothelial cells and pericytes that permit ion exchange. We propose that the functional interrelationship between endothelium and pericytes may play a role in the alteration of capillary diameter for the control of local cerebral blood flow.

Degradation of the epidermal-dermal junction by proteolytic enzymes from human skin and human polymorphonuclear leukocytes

The degradation of normal human skin by the human polymorphonuclear leukocyte proteinases cathepsin G and elastase, and by a human skin chymotrypsin-like proteinase that appears to be a mast cell constituent, was examined. Enzymes were incubated with fresh, split-thickness skin for up to 8 h; the tissue was examined ultrastructurally and immunohistochemically using antibodies to known basement membrane constituents. In all cases, the primary damage observed was at the epidermal-dermal junction. Elastase degraded the lamina densa leaving scattered and disorganized anchoring fibrils, dermal microfibril bundles, and normal-appearing collagen fibers. Immunohistochemically, type IV collagen, laminin, KF1 antigen, and EBA antigen were absent. The bullous pemphigoid antigen was present and localized on the basal cells. Epidermal-dermal separation produced by the chymotrypsin-like proteinases, cathepsin G, and the human skin proteinase, was confined to the lamina lucida. The lamina densa and sub-lamina densa fibrillar network remained intact. The human skin chymotrypsin-like proteinase produced extensive epidermal-dermal separation, while cathepsin G, at comparable concentrations, produced only focal separations. Immunohistochemically, all antigens were present after incubation with enzyme. The bullous pemphigoid antigen, however, was found on the epidermal side of the split, while laminin was found on the dermal side. These results show that the epidermal-dermal junction is highly susceptible to neutral serine proteinases located in mast cells and polymorphonuclear leukocytes. Although all the proteinases produce epidermal-dermal separation, the patterns and extent of degradation are different. The distinctive patterns of degradation may provide a clue to the involvement of these proteinases in skin diseases.

Chick embryonic pigmented retina is one of the group of epithelioid tissues that lack cytokeratins and desmosomes and have intermediate filaments composed of vimentin

Using sodium dodecyl sulphate/polyacrylamide gels to analyse detergent-insoluble residues, and indirect immunofluorescence, we have found that the major protein of intermediate filaments in cultures and freshly explanted fragments of chick embryonic retinal pigment epithelium (RPE) is vimentin. Moreover, these cells also fail to stain with antibodies against cytokeratins and most components of true desmosomes (maculae adhaerentes). Staining with anti-vinculin antibody suggests that the principal intercellular junction is the zonula adherens. Thus although RPE is an epithelium according to all other criteria, it belongs to a group of tissues (including vascular endothelium, iris and lens-forming epithelium) that have intermediate filaments composed of vimentin and possess neither cytokeratins nor desmosomes. That a tissue can be fully epithelial by other criteria, whilst lacking these components, is in agreement with other work, which has shown a lack of effect of micro-injection of antibodies to cytokeratin, and of the suppression of desmosome formation, on epithelial organization in culture. Although our observations were made solely on chick embryonic tissue, we suggest that published ultrastructural studies are consistent with the possibility that RPE of other species, including human, may lack true desmosomes.

Junctional competence in clones of mammary epithelial cells, and modulation by conditioned medium

Colony-forming epithelial cells exfoliated in human milk have been examined by immunofluorescence using antibodies to cytokeratins (tonofilaments), and to high molecular weight desmosomal core proteins. The cells may be classified by their ability to form junctional complexes with their neighbours. Those deficient in desmosomal junctions, called D- cells, grow into colonies of noncontiguous cells without desmosomes, and with a perinuclear network arrangement of cytokeratins. Junction forming, or D+ cells, grow as contiguous cell sheets with abundant desmosomes and well developed bundles of tonofilaments. D- cells may also segregate D+ cells among their progeny yielding mixed clones, and a gradual increase in the overall number of D+ cells during culture. Established D+ cells have surface markers characteristic of mammary epithelium and are presumably derived by exfoliation of luminal cells of the alveoli or ducts which contain desmosomal junctions. D- cells also possess mammary epithelial cell markers, but their origin is unknown. Medium conditioned by the Nil 8 line of hamster cells contains a junction-promoting activity that accelerates the rate, or frequency, of segregation of D+ cells from D- cells, so that milk cells grown in this medium predominantly give closed colonies of D+ cells. Medium conditioned by the MRC5 strain of human embryo lung cells, however, contains a junction-inhibiting activity, which prevents new junction formation and probably destroys existing junctions, so that cells in this medium mostly grow as open colonies of cells with D- phenotype. It is hoped that studies with this experimental system will assist in the better understanding of normal and abnormal regulation of desmosomal junctions and their role in tissue integrity.

Dye coupling between amacrine cells in carp retina

Amacrine cells in isolated retinas of the carp (Cyprinus carpio) were intracellularly recorded and marked with a fluorescent dye, Lucifer yellow. On occasion, dye coupling was found to occur between amacrine cells when the dye was iontophoretically injected into an amacrine cell, generating one of the transient or sustained types of photoresponse. Dye-coupled cells in the vicinity of the marked cell were very similar to the marked cell in soma shape and dendritic stratification. Dye diffusion is assumed to take place at gap junctions between dendrities of amacrine cells which belong to a population of similar type cells in morphology and possibly in function.

Epithelial structure revealed by chemical dissection and unembedded electron microscopy

Cytoskeletal structures obtained after extraction of Madin-Darby canine kidney epithelial cell monolayers with Triton X-100 were examined in transmission electron micrographs of cell whole mounts and unembedded thick sections. The cytoskeleton, an ordered structure consisting of a peripheral plasma lamina, a complex network of filaments, and chromatin-containing nuclei, was revealed after extraction of intact cells with a nearly physiological buffer containing Triton X-100. The cytoskeleton was further fractionated by extraction with (NH4)2SO4, which left a structure enriched in intermediate filaments and desmosomes around the nuclei. A further digestion with nuclease and elution with (NH4)2SO4 removed the chromatin. The stable structure that remained after this procedure retained much of the epithelial morphology and contained essentially all of the cytokeratin filaments and desmosomes and the chromatin-depleted nuclear matrices. This structural network may serve as a scaffold for epithelial organization. The cytoskeleton and the underlying nuclear matrix intermediate filament scaffold, when examined in both conventional embedded thin sections and in unembedded whole mounts and thick sections, showed the retention of many of the detailed morphological aspects of the intact cells, which suggests a structural continuum linking the nuclear matrix, the intermediate filament network, and the intercellular desmosomal junctions. Most importantly, the protein composition of each of the four fractions obtained by this sequential procedure was essentially unique. Thus, the proteins constituting the soluble fraction, the cytoskeleton, the chromatin fraction, and the underlying nuclear matrix-intermediate filament scaffold are biochemically distinct.

[Ultrastructural characteristics of stromal mechanocytes and their interaction with hematopoietic cells in regenerating bone marrow transplants]

Extramedullar grafts of the bone marrow have been studied electron microscopically 2-8 days after transplantation. The data on ultrastructural peculiarities of the stromal mechanocytes have been obtained at various stages of their differentiation, as well as topographic interrelationships between the mechanocytes and the hemopoietic cells. Digital junctions are described between the stromal mechanocytes (primitive) and the hemopoietic cells (in 3-day-old grafts) which are, probably, the first morphological signs demonstrating restoration of the hemopoietic microenvironment in the bone marrow grafts.

Molecular organization of gap junctions

Highly purified gap junction fractions from heart and liver contain a single major protein component. The proteins isolated from different organs have apparent molecular weights of 26,000-30,000. Peptide mapping and partial sequencing show close homology of the hepatic junctional protein of different species. In contrast, no homologies can be detected when polypeptides from different tissues of the rat were compared by peptide mapping. Preliminary results from partial sequencing, however, show that the amino terminal regions of the liver and heart proteins are related to one another. Sequencing has not yet revealed any such homologies between the lens and the other junction proteins.

Specificity of histiotypic organization and synaptogenesis in reaggregating cell cultures of mouse cerebellum

The fine structure of reaggregating cultures of cells from 6- to 7-day-old mouse cerebellum was studied at intervals between 3 and 21 days in vitro (DIV). The resulting aggregates consisted mainly of small neurons (granule, stellate and basket cells), neuroglial cells and their processes. Large neurons were rarely present. By 7 DIV the previously loosely packed components had tightened into a more compact mass. A peripheral plexiform layer had formed which had many fine axons arranged into fascicles of parallel fibers. Deep to this zone was a cellular region containing clusters of neurons interspersed with small areas of neuropil. Axosomatic synapses appeared on neurons which resembled stellate or basket cells but not on granule cells. Axo-dendritic synapses formed in the neuropil of the cellular zone and, less frequently, in the outer plexiform layer. After 3 weeks glial cell processes had increased in volume at the expense of neurons. When cerebellar cells were cultured with cells from pons and medulla, which are normal sources of mossy fiber input, aggregates formed in which synaptic glomeruli were found. They were not seen in aggregates containing cells from retina and olfactory bulb cultured with cerebellum. Our observations suggest: that natural histogenetic mechanisms persist after dissociation and reaggregation of cerebellar cells resulting in a separation of an outer, 'molecular'-like layer from an inner granule cell layer and that neurons retain specificity of their synaptogenic capabilities both with regard to appropriate cell types and the morphological form that the synapses take.

A descriptive study of the blood vessels of the sciatic nerve in the rat, man and other mammals

The gross blood supply and internal vascular organization of the sciatic nerve in the rat (and other mammals including man) have been studied by injection, dissection, and microradiographic methods, in thick sections stained for alkaline phosphatase and semithin sections, and by electron microscopy. In the rat, an anastomotic vascular connection from the arteria comitans to the medial femoral circumflex artery is illustrated. In all species studied, the microvascular network of the nerve has an unusually large calibre and wide spacing, particularly when compared with that of skeletal muscle. Arterioles and venules are present within fascicles; the arterioles are thin-walled, with only rudimentary internal elastic laminae. Endoneurial capillaries are large, continuous and nonfenestrated, with prominent (often multiple) basal laminae and an unusually complete pericyte investment. Endothelial junctions appear to vary in type; not all are tight. Pinocytosis is common. The extensively interconnected vascular network of the nerve fascicle may act as an adaptable in situ reservoir for blood in the precise maintenance of the endoneurial milieu.

Opening of the blood-brain barrier in Anolis carolinensis. A high voltage electron microscope protein tracer study

The tight junctions between endothelial cells of capillaries in the forebrain of Anolis carolinensis are a common component of the structural basis for the blood-brain barrier in this reptile. The complexity of these junctions, which is apparent in platinum replicas of freeze-fractured brain capillaries, is unchanged by treatments designed to render the blood-brain barrier of these lizards leaky to horseradish peroxidase. An alternative route for extravasation of horseradish peroxidase, following injection of chameleons with 2.7 mg of D-glucose to render their brain capillaries leaky, is a system of transient cytoplasmic vesicles and vesiculo-tubular channels whose lumina may be open to the luminal or abluminal surface (or both) of the capillary endothelial cell. High voltage electron microscopy (HVEM) of 0.25 and 0.5 micron thick plastic sections of experimental brain capillary endothelium confirmed the existence of vesiculo-tubular conduits. These channels display a sigmoid morphology and are situated in the cytoplasm at angles oblique to the luminal and abluminal surfaces of the endothelium. Occasionally, the channels spanned the entire endothelial wall of the capillary, and in such cases, appeared to connect the lumen with the brain extracellular compartment. HVEM images (including stereo pairs) of the vesiculo-tubular channels show them to have a scalloped, irregular profile consistent with their proposed formation by fusion of pinocytotic vesicles. Also, HVEM examinations of experimental capillaries from peroxidase-treated lizards reveal massive quantities of dense reaction product in cytoplasmic vesicles and vesiculo-tubular membrane compartments of the endothelium, and the complex pleomorphism exhibited by these structures. Observations made in the present study suggest that as a consequence of severe hyperglycaemia, transendothelial channels form in the brain capillaries by fusion of pinocytotic vesicles generated by accelerated pinocytosis at the luminal surface of the endothelium, and subsequently serve as open routes for massive floods of tracer into the central nervous system.

Control of myometrial function in preterm versus term labor

In this review I have attempted to briefly describe myogenic, neurogenic, and hormonal mechanisms which control myometrial function during term and preterm labor. The synthesis of myometrial gap junctions and receptors in response to hormonal changes appears to be a key element in the gradual evolution of contractility leading to labor. I suggest that the development of myometrial gap junctions is physiologically regulated and can be manipulated pharmacologically. Thus, this system provides a means of regulating myogenic control of smooth muscle. I have proposed a model, or working hypothesis, which may explain the indirect and direct interactions of various substances involved in the control of synthetic processes and stimulating or inhibiting myometrial contractility. The model predicts that term or preterm labor can be initiated at various steps. A more complete understanding of these interactions is necessary for a rational approach to the control of term or preterm labor.

Gastric parietal cell carcinoma--a newly recognized entity: light microscopic and ultrastructural features

Three cases of gastric parietal cell carcinoma are described. Tumour cells are round to polygonal, with abundant eosinophilic, granular cytoplasm reactive with phosphotungstic acid haematoxylin and Luxol Fast Blue. Ultrastructurally the tumour cells are characterized by abundant mitochondria, tubulovesicles, intracellular canaliculi and intercellular lamina filled with undulated microvilli.

Gap junction structure and cell-to-cell coupling regulation: is there a calmodulin involvement?

In most tissues neighboring cells communicate directly with each other by exchanging ions and small metabolites via cell-to-cell channels located at the intermembrane particles of gap junctions. Evidence indicates that the channels close when the [Ca2+]i or [H+]i increases. The channel occlusion (cell-to-cell uncoupling) is mainly a safety device by which cells can isolate themselves from damaged neighboring cells ("healing-over" process). Despite our knowledge of uncoupling agents, the uncoupling mechanism is still poorly understood. Uncoupling treatments have been shown to cause structural changes in gap junctions, characterized by an increase in tightness and regularity (crystallization) of particle packing and a decrease in particle size. Recently these changes have been shown to be induced by Ca2+ or H+ in isolated lens junctions and by Ca2+ in liver junctions, which suggests a close relationship between structural changes and uncoupling, but preliminary studies indicate that the junctional changes may not be synchronous with uncoupling but may lag behind it. However, recent X-ray diffraction data show that the channels of crystalline gap junctions (typical of uncoupled cells) are indeed closed, because they are inaccessible to sucrose (a gap junction permeant). Thus it seems that crystalline junctions are indeed in a non-permeable state, but the occlusion of the channels may precede the crystallization process. In the lens, junction crystallization is inhibited by a calmodulin (CaM) inhibitor, trifluoperazine (TFP). Is CaM involved in the uncoupling mechanism? To test this hypothesis, TFP and calmidazolium (CDZ), the most specific CaM inhibitor, were used on amphibian embryonic cells electrically uncoupled by CO2. Both TFP and CDZ effectively protect the cells from uncoupling, which suggests that CaM participates in the process. As a hypothesis, we propose that channel occlusion follows a CaM-mediated conformational change in the junctional protein. Particle crystallization may follow the conformational changes and result from a modification in electrostatic repulsion among the particles.

Morphology of endoneurial blood vessels of frog sciatic nerve during vascular perfusion

In order to determine if increased injection pressures can alter the permeability and ultrastructure of blood vessels of the frog blood-nerve barrier, these vessels were examined following perfusion of the iliac artery at rates of 0.21 or 0.82 ml/min. At either perfusion rate, endoneurial blood vessel profiles were clearly evident and the surface area of these vessels amounted to 60% of the surface area of the perineurium. In all vessels a large number of vesicles were present within the endothelial cells. Many were attached by necks to one or the other plasma membrane, but no transcellular channels were evident. At the higher flow rate no changes in vesicles or junctions were seen, but blebs and blisters were evident at the luminal membranes of the endoneurial endothelium. When microperoxidase was perfused at 0.82 ml/min, reaction product frequently flooded the endothelial cells, was found as clumps on the cell surface, and was distributed within the endoneurial space. These changes represent the only ultrastructural evidence of endothelial cell damage and altered permeability in response to increased rate of perfusion.

[Composition and synaptic organization of the internal plexiform layer of the sensory membrane of the eye in turtles]

Synaptic organization of the internal plexiform layer (IPL) of the retina has been studied electron microscopically in two aquatic and two land Chelonia species. Certain signs in common of the IPL structure have been revealed, they are similar with those in other Vertebrata, as well as common peculiarities inherent in Chelonia of all the four species studied. Any species-specific differences are not detected. In the land Chelonia, unlike the aquatic Chelonia, a relative content of the amacrinal cell synapses is higher in serial combinations. The ratio of the amacrinal cell synapses to all bipolar neurocyte synapses is within the limits 3.1-4.1.

Junction formation in aggregated embryonal carcinoma cells

Under the action of supplemental calcium, H6 mouse embryonal carcinoma cell aggregates undergo compaction, a morphological phenomenon similar to mouse embryonic compaction. Formation of various types of cell junctions, especially gap junctions, is associated with compaction of the embryo and we sought to analyze the pattern of junction formation during aggregation and compaction of H6 cells. At 24 hr of aggregation, gap junctions were abundant in both uncompacted and compacted aggregates but quantitative analysis of freeze fracture replicas of these junctions showed a 20-fold increase in the size of the largest gap junctions in compacted aggregates. Such a difference in size could even be detected at 12 hr of aggregation. Tight junctions were not normally formed in 12 hr aggregates but initial stages of tight junction formation could be noticed in 12 hr compacted aggregates. More definitive tight junctions and desmosomes were evident only after 48 hr of aggregation. Thus we have observed that both uncompacted and compacted aggregates can form gap junctions at similar frequencies, suggesting that cell flattening, which contributes to the compacted morphology, is not a requisite for gap junctions. Likewise, generation of the compacted morphology seems to be independent of gap junction formation. This supports the idea that compaction in embryonal carcinoma cells results from calcium-induced cell flattening, probably through the mobilization of cytoskeletal elements. Calcium-dependent features of H6 cell aggregation and compaction enables the independent analysis of separate steps in compaction.

Intercellular junctions of the iris epithelia in Macaca mulatta

The intercellular junctions in the anterior myoepithelium and posterior pigmented epithelium of the rhesus monkey iris were examined using an ultrastructural tracer, conventional electron microscopy, and the freeze-fracture technique. Within the anterior myoepithelium the lateral cell margins were joined by puncta adhaerentia, desmosomes, and gap junctions. The distribution of the puncta adhaerentia and desmosomes was restricted to the apico-lateral region of these cells. Joining the apical surface of the anterior myoepithelium and posterior pigmented epithelium gap junctions, puncta adhaerentia, and desmosomes also were present. Adjacent posterior pigmented epithelial cells were joined by an apico-lateral junctional complex, which consisted of a zonula occludens, zonula adhaerens, and gap junction. These cells also were connected by one or more desmosomes. Intravenously injected horseradish peroxidase, which diffused from the ciliary body stroma, was prevented from reaching the posterior chamber by the presence of the zonulae occludentes between adjacent posterior pigmented epithelial cells. Their presence was confirmed using the double replica method of freeze-fracturing. The zonulae occludentes appeared as a continuous series of branching and anastomosing strands of particles on the P-fracture face, which were complemented on the E-fracture face by a series of shallow grooves. These junctions varied in complexity from one to eight or more strands indicating that they are analogous in both location and degree of permeability to the zonulae occludentes present in the nonpigmented ciliary epithelium.

The Purkinje fiber-myocardial cell region in the goat heart as studied by combined scanning electron microscopy and chemical digestion

The 3-dimensional architecture of the junctional region between Purkinje fibers and ordinary myocardial cells has been closely studied by combined scanning electron microscopy and chemical digestion in the goat heart. It was revealed that the Purkinje fibers forming the terminal arborization of the atrioventricular bundle are followed by transitional cells which are in contact with ordinary myocardial cells.