Freeze-fracture ultrastructure of the perinodal astrocyte and associated glial junctions

Freeze-fracture examination of nodes of Ranvier from adult rat optic nerve demonstrates the presence of astrocytic processes at the majority of nodes of Ranvier. Astrocytic processes often run along the entire length of the nodal gap, although they do not necessarily encircle the entire nodal circumference. The E- and P-fracture faces and the cross-fractured cytoplasm of these astrocytes (termed 'perinodal astrocytes') were examined. The cytoplasm of perinodal astrocytes contains 10-nm filaments. The P-faces of perinodal astrocytic membranes are characterized by orthogonal arrays of intramembranous particles ('assemblies'), with a center-to-center periodicity of approximately equal to 6 nm. Complementary orthogonally arranged pits are observed on the E-faces of the astrocytic membranes. The density of these arrays in perinodal astrocytic membranes is similar to that in parenchymal astrocytic membranes, but is substantially lower than that at pericapillary astrocytic membranes. In addition, gap junctions are present between astrocytes, and between astrocytes and paranodal oligodendroglial layers. These findings indicate that astrocytic processes comprise an important structural component of central nodes of Ranvier, and provide a morphological basis for a possible astrocytic role in nodal function.

Gap junction connexon configuration in rapidly frozen myocardium and isolated intercalated disks

By using two ultrarapid freezing techniques, we have captured the structure of rat and rabbit cardiac gap junctions in a condition closer to that existing in vivo than to that previously achieved. Our results, which include those from fully functional hearts frozen in situ in the living animal, show that the junctions characteristically consist of multiple small hexagonal arrays of connexons. In tissue frozen 10 min after animal death, however, unordered arrays are common. Examination of junction structure at intervals up to 40 min after death reveals a variety of configurations including dispersed and close-packed unordered arrays, and hexagonal arrays. By use of an isolated intercalated disk preparation, we show that the configuration of cardiac gap junctions in vitro cannot be altered by factors normally considered to induce functional uncoupling. These experiments demonstrate that, contrary to the conclusions of some earlier studies (Baldwin, K. M., 1979, J. Cell Biol., 82:66-75; Peracchia, C., and L. L. Peracchia, 1980, J. Cell Biol., 87:708-718), the arrangement of gap junction connexons, in cardiac tissue at least, cannot be used as a reliable guide to the functional state of the junctions.

[Functional status and ultrastructure of the smooth muscle coat of arteries]

The ultrastructure of smooth-muscle sheath of the rabbit ear artery was studied in situ with the blood flow preserved both in contracted and in relaxed states. In the latter state a decrease in the volume of smooth-muscle cells and a relative increase of interfellular space were revealed. These findings suggest a redistribution of fluid in the artery wall during contraction and relaxation which can be important for transport of substances in the vascular wall. In relaxation of smooth-muscle cell such phenomena as desaggregation of thick myosin myofilaments, considerable reduction of the micropinocytose apparatus, shifting of contractile elements to the cell's central zone, partial discomplexation of the endoplasmatic reticulum, were revealed. Possible mechanisms of the ultrastructural changes and their functional role are discussed.

Membrane particles and gap junctions in the retinas of two species of cephalopods, Octopus ocellatus and Sepiella japonica

To study correlation between membrane structure and photoreceptor function, we compared the size and density of intramembrane particles (IMPs) in various membrane compartments of freeze-fractured retinas in a cuttle-fish, Sepiella japonica, and an octopus, Octopus ocellatus. Distribution of gap junctions in the retinas was also examined. Similar results were obtained in the two species. P-faces of both rhabdomeric microvillar membrane and non-rhabdomeric plasma membrane of the apical process were characterized by a random distribution of dense IMPs (ca. 5500-6500/microns2), which showed a unimodal size distribution with a mean diameter of ca. 10 nm. Unlike other invertebrate ocelli, the plasma membrane of the cell body in both the outer and inner segments had significantly denser P-face particles (ca. 7500-8000/microns2) than the rhabdomeric microvillar membrane. The size distribution of IMPs in each part of the membrane was also unimodal, but with a mean diameter of ca. 8 nm. In tangential fractures, each lamella of the myeloid body showed a patchwork of P-faces with irregularly arranged, dense particles and E-faces with orderly patterened granulation. Density and size distribution of the P-face particles in the myeloid membrane resembled those in the rhabdomeric microvillar membrane. The plasma membranes of the supporting cell and the gial cell had relatively sparse P-face particles (ca. 1500-3000/microns2). In addition to the previously reported gap junctions, which connected visual cell inner segments with each other, directly or via collaterals, small gap junctions were found between the visual cell axons and presumed efferent nerve fibres in the plexiform layer. Large-sized gap junctions provided mutual connections for both supporting cells and glial cells. In conclusion, IMPs of 10 nm in mean diameter in the microvillar and non-microvillar parts of the apical process plasma membrane and in the myeloid membrane represent the molecules or their clusters of two photopigments in the cephalopod visual cell, rhodopsin and retinochrome, respectively, and electrical transmission plays a role in visual cell-efferent nerve interactions.

Stellate neurons in rat dorsal cochlear nucleus studies with combined Golgi impregnation and electron microscopy: synaptic connections and mutual coupling by gap junctions

Stellate neurons in the outer two layers of the rat dorsal cochlear nucleus (DCN) were studied by the Golgi-EM method. Stellate cell bodies are usually spherical or ovoidal and range from 9 microns to 14 microns in mean diameter. The smallest cells are situated underneath the ependymal layer and the largest cells in layer 2. Primary dendrites are short, thin and smooth and arise abruptly from the perikaryon, without a tapering main stem. Meandering secondary and tertiary dendrites extend in all directions, carry few pleomorphic spines lacking a spine apparatus and often show artifactual beading. The axons are impregnated only for a short distance (10-45 microns). The nucleus is indented, the nucleolus varies in position, and the chromatin, evenly dispersed in the centre, forms small clumps along the nuclear envelope. The cytoplasm is rich in free polyribosomes and contains scattered cisterns of granular endoplasmic reticulum. Varicosities of thin fibres, containing round synaptic vesicles, form asymmetric synapses on perikarya, dendritic shafts and spines of stellate cells. Such fibres run parallel to the long axis of the DCN or are oriented radially and are interpreted as axons of cochlear granule cells. Two kinds of bouton containing pleomorphic vesicles, one kind electron lucent and the other electron dense, form symmetric synapses on perikarya and dendritic shafts of stellate cells. The lucent boutons occur more frequently than the dense boutons, especially on the distal dendritic branches. The boutons with pleomorphic vesicles presumably represent terminals of local circuit neurons, probably the stellate and cartwheel cells. In addition, stellate cells show numerous dendro-somatic and dendro-dendritic appositions characterized by gap junctions and puncta adhaerentia. Most of the dendrites involved in these appositions resemble stellate cell dendrites and it is concluded that DCN stellate cells are coupled electrotonically with one another. The axons of stellate cells acquire a thin myelin sheath. Since the Golgi impregnation did not stain axons of stellate cells past this point, we were unable to demonstrate the synaptic targets of stellate cells.

How a gap junction maintains its structure

In gap junctions, identical membrane proteins are linked up in pairs (dyads) that bridge the extracellular space between two apposed cell membranes. Typically, several thousand of these dyads are aggregated in the plane of the membranes and form a junctional plaque with a distinct boundary. The question thus arises as to what maintains the dyads in an aggregated state. From a statistical mechanical analysis of the positions of dyads in a freeze-fracture electron micrograph, we report here that the aggregates are not maintained by an attractive force between pairs of dyads, but probably by the minimization of the repulsive force between apposed membranes. On the basis of this analysis we present a model for the structure of mature gap junctions as well as certain aspects of the formation and disassembly of gap junctions.

Connexon rearrangement in cardiac gap junctions: evidence for cytoskeletal control?

Using ultrarapid-freezing techniques and freeze-fracture electron microscopy, we report here a close association between cardiac gap junctions and specialized membrane domains containing regularly-spaced furrows. These specialized furrowed domains are observed only during periods of gap junction re-organisation (i.e., connexon redistribution) and may reflect the presence of underlying cytoskeletal elements controlling the position of connexons in the membrane.

Gap junctions in the heart of the adult Protopterus aethiopicus

In thin sections and in freeze-fracture replicas small and sparse gap junctions appear to be developed on the longitudinal plasma membrane of Protopterus cardiac cells near a macula or fascia adhaerens. By thin-section electron microscopy, they had septalaminar profiles with a length between 0.042 and 0.260 micron. In freeze-fracture images they appear on the P-fracture face as maculate particle aggregations with complementary pits on the E-fracture face. Particles with a central intercellular channel could be observed. The average center-to-center distance between neighbouring particles or pits is 10.05 +/- 1.87 nm (N = 2429). The diameter of the junctional maculae in replicas lies between 0.037 and 0.229 nm. The particle packing density increases in larger maculate aggregations, while particle-free areas emerge which could be related to the degradation or reformation of gap junctions Atypical configurations of gap junctions observed in the myocardium of lower vertebrates are rarely encountered in this primitive vertebrate.

Freeze-fracture of 2-cell mouse embryos. A new method for fracture of very small and scarce biological samples

Membrane morphology and development of intercellular junctions in 2-cell mouse embryos was studied with freeze-fracture electron microscopy. In order to circumvent the dual problem of relative scarcity of samples and their very small size (about 50 micrometers per blastomere), both of which made conventional microtomy-induced freeze-cleavage impractical, a new protocol for handling 2-cell embryos was derived from freeze-fracture procedures established for monolayer cultures. Freshly-collected 2-cell embryos were placed within a 2 mm2 area on plastic Thermonox or Polystyrene coverslips which had been previously coated with 0.002% poly-l-lysine. The embryos were then fixed in 2.5% glutaraldehyde buffered to pH 7.3 with sodium cacodylate and containing 0.01% ruthenium red. The embryos were washed three times in buffer and then glycerinated. The next day, the 2 mm2 area of coverslip containing the embryos (75-100) was cut out with a fine scalpel, inverted and placed on a drop of Elvanol-glycerol mixture and frozen in a slurry of liquid nitrogen-cooled Freon-22. Fracturing was achieved in a Balzers BAF 301 Freeze-Etch Unit by positioning the microtome blade underneath a corner of the inverted coverslip and raising the knife until the coverslip 'popped off'. This procedure left the surface of the Elvanol droplet covered with fractured embryos. The exposed tissue was shadowed with platinum and coated with carbon. This protocol produces large expanses of blastomere plasma membrane as well as cross-fractures of entire blastomeres and 2-cell embryos. The replicas revealed blastomeres endowed with a rich assortment of pleomorphic microvilli.(ABSTRACT TRUNCATED AT 250 WORDS)

Some aspects of desmosomal morphology during differentiation of hamster cheek pouch epithelium

Desmosomes function as intercellular attachment devices. In keratinised epithelia, squames are shed from the surface and these are replaced by less differentiated cells of the basal or parabasal layers, which migrate progressively towards the surface. In order to perform these migratory activities, cellular movement must occur, with the implication that cells must be able to make and break their desmosomal attachments. The present study investigates desmosomal ultrastructure with the objective of suggesting mechanisms by which desmosomes are altered during differentiation. The majority of desmosomes in basal, spinous and granular layers were similar in morphology, with differences being quantitative rather than qualitative. Desmosomes appeared to be more frequent and of smaller dimensions in granular cells when compared with those present on cells from lower strata. Upon keratinisation, desmosomal cytoplasmic structural specialisations disappeared and the intercellular contact layer became more prominent. A number of features which might represent desmosomal formation or synthesis were observed in nucleated cellular layers. These included an apparent budding or fusing of attachment plaques from existing desmosomes. Asymmetrical attachment plaques were prevalent with a markedly reduced number of tonofilaments associated with one plaque or an attachment plaque which was generally or locally reduced in electron density. Other appearances suggested that de novo desmosomal formation occurred as a result of the production of either single or paired attachment plaques, often containing fine filaments extending for a short distance into the cytoplasm. In view of the increased frequency of desmosomes in granular cells, the majority of these desmosomal abnormalities may represent synthetic rather than degradative activities.

Junctions between intraepithelial carcinoma and non-neoplastic tissue of the esophagus. Light and electron microscopic studies

This report describes light and electron microscopic observations in 11 patients with intraepithelial carcinomas concomitant with invasive squamous cell carcinomas of the esophagus. The junctions between the intraepithelial carcinomas and non-neoplastic tissues were examined using an electron microscope. Vertical sections through the basal laminae revealed intraepithelial carcinomas with bulky outgrowths and simple replacement histological patterns. The bulky outgrowths contained many pseudopodial cytoplasmic projections from the tumor cells through the basal laminae, while the simple replacement patterns included rare small breaks in the basal laminae. Horizontal sections parallel to the basal laminae showed that the cells of the poorly differentiated squamous cell carcinoma were readily distinguishable from the non-neoplastic cells in the surface layer of the esophageal epithelia and distinctly smaller and darker than the normal prickle cells. At most of the junctions, mesenchymal cells, degenerated cells, and amorphous material separated the tumor cells from the non-neoplastic epithelial cells. However, tumor cells were occasionally attached directly to normal epithelial cells with well-developed desmosomes. Ductal involvement of the carcinomas was found in the submucosal esophageal gland proper. The tumor cells invaded between the ductal cells and basal laminae, and neoplastic cells were also directly attached to the benign ductal cells by poorly-developed desmosomes. The host-tumor junctions in the intraepithelial carcinomas of the human esophagus consisted of basal laminae with hemidesmosomes and pseudopodial projections, mesenchymal cell accumulations and direct attachments with desmosomes.

Alcoholic liver injury in baboons: transformation of lipocytes to transitional cells

Ultrastructure of the lipocytes (the main perisinusoidal cells) and their alterations in the progression of hepatic fibrosis were studied in liver biopsy specimens of baboons pair-fed with diets containing alcohol, or isocaloric carbohydrate, for up to 112 mo. In control baboons, 97% of the cells in the Disse space were lipocytes characterized by a volume density of lipid droplets occupying greater than 20% of the cell volume and by the presence of microfilament bundles with associated dense bodies and pinocytic vesicles. Intercellular junctions of the adherens type were present between lipocytes and hepatocytes. After alcohol consumption, the number of lipocytes (as assessed by light microscopy) was significantly decreased in fatty livers and at various stages of hepatic fibrosis; this was associated with a decreased hepatic vitamin A content. In baboons fed alcohol, only 48% of cells were lipocytes, whereas 52% were transitional cells defined by a volume of lipid droplets less than 20% of the cell. Like the lipocytes, transitional cells exhibited microfilament bundles, dense bodies, and pinocytic vesicles, and were attached to the hepatocytes by cell junctions. The rough endoplasmic reticulum in transitional cells was conspicuous and had an area significantly greater than that in lipocytes of controls and alcohol-fed animals (69% and 37%, respectively). There was a significant correlation between the percentage of transitional cells as well as the area of their rough endoplasmic reticulum and the degree of hepatic fibrosis. Thus, in baboons fed alcohol, the progression of hepatic fibrosis is associated with transformation of lipocytes to transitional cells characterized by a depletion of lipid droplets and a hypertrophy of the rough endoplasmic reticulum; these transitional cells may play a role in promoting hepatic fibrosis in alcoholic liver injury.

Reversible effects of heptanol on gap junction structure and cell-to-cell electrical coupling

It has been shown that certain alkanols such as heptanol and octanol reversibly uncouple crayfish axons. We have studied the effects of heptanol on the electrical coupling in Xenopus embryo cells and on the gap junction particle distribution of stomach and pancreas epithelia. Cell-to-cell electrical communication in Xenopus embryos reversibly decreases 10-15 min after the beginning of heptanol superfusion to reach values of coupling ratio lower than 0.05. Particle density of rat stomach and pancreas gap junction reversibly increases after exposure to heptanol-Tyrode's solutions. In stomach, gap junction particles from isles with hexagonal patterns after 10 to 15 min; in pancreas crystallization occurs after 20 to 30 min. Optical diffractions are used to average particle spacings in treated and control junctions of pancreas.

Increased capillary permeability of the stria vascularis to HRP, induced by experimental acute hypotension in rats

Acute hypotension was produced in rats by using two experimental techniques: intravenous infusion of a ganglion-blocking agent (Arfonad), and venesection of a femoral vein. Horseradish peroxidase (HRP) was injected intravenously after each procedure, and subsequently observed in the inner ear by light and electron microscopy. In both experimental models, a large amount of tracer spread into the intercellular spaces, but it was halted by tight junctions bordering the stria vascularis. The endothelium exhibited a high distribution density of labelled vesicles, which suggested increased vesicular transport. There was no extravasation of HRP from capillaries in the spiral ligament in spite of the presence of some labelled pinocytotic vesicles. The present study was concerned with the discovery of enhanced capillary permeability of the stria vascularis under acute hypotension, as in the case of acute hypertension (Sakagami et al., 1984).

Ultrastructural lesions in testes from hyperprolactinemic men

Testicular tissue from eight men with prolactinomas and elevated serum prolactin were evaluated by light (LM) and transmission electron microscopy (TEM). A semiquantitative assessment of testicular morphology was employed to provide a morphology index for each tissue specimen. Although in each biopsy specimen germ cell exfoliation was evident, as was abnormal structural change in the seminiferous epithelium, there was no apparent correlation with the overall degree of tissue pathology (morphology index) and the serum level of prolactin. All of the tissue displayed variably thickened seminiferous tubule walls which, when viewed by TEM, were composed of thickened laminae propriae and redundant and involuted basal laminae. Likewise, all tubules contained Sertoli cells with overt cytoplasmic degeneration, principally in the apical (adluminal) region of the cell. This was visualized, in part, as a retraction of the apical cytoplasm from periluminal spermatids and degeneration or absence of Sertoli-germ cell junctional specializations. Sertoli-Sertoli cell junctional complexes appeared structurally intact. Leydig cell ultrastructure was typical of normal cells and contained a variable amount of lipid and smooth endoplasmic reticulum. This also was without positive correlation with the overall degree of tissue pathology or level of serum prolactin. Our results demonstrate the variable degree of testicular pathology associated with hyperprolactinemia in man, and suggest that abnormal tubule walls and altered Sertoli cell ultrastructure are consistent findings in this abnormal endocrine condition.

The blood-brain barrier in hypoxia: ultrastructural aspects and adenylate cyclase activity of brain capillaries

The ultrastructure of brain microvessels, their permeability to serum albumin, the activities of some endothelial enzymes and the effect of histamine were investigated in rats after a prolonged hypobaric-hypoxic treatment. After prolonged hypoxia, the permeation of serum albumin into endothelial cells increased together with the number of pinocytotic vesicles of the endothelium. Intracarotid histamine stimulated this process even further, and its effect was mediated by H2-histamine receptors. After hypoxia the specific activity of capillary alkaline phosphatase and gamma-glutamyl transpeptidase remained unchanged, while that of adenylate cyclase was greatly increased. Histamine did not modify the structure of tight junctions of isolated capillaries of normoxic animals. Both hypoxia- and histamine-induced modification of the brain microvessels were accompanied by an increase of pinocytosis, which may be stimulated by the activation of capillary adenylate cyclase.

Myometrial electrophysiologic activity and gap junctions in the pregnant rabbit

Electromyographic activity of the myometrium, intrauterine pressure, and myometrial gap junctions were examined in rabbits at various stages of pregnancy and during parturition. Electromyographic activity occurred throughout pregnancy and was characterized by electromyographic bursts lasting up to 5 minutes at a frequency of one to four per hour. The gap junction area of plasma membrane showed a significant increase at delivery at a time when electromyographic activity changed to rhythmic bursts of short duration at a frequency of about one per minute. This study shows that the rabbit uterus is active throughout pregnancy; furthermore, the development of large numbers of gap junctions between myometrial cells occurs at a time when uterine activity changes to the expulsive activity required for labor.

Ultrastructural morphometric of gap junctions during differentiation of stratified squamous epithelium

The presence of gap junctions in stratified epithelia has now been extensively documented, but there have been few attempts to quantify them. In the present report, samples of hamster cheek pouch mucosa were processed for electron microscopy and electron micrographs from defined basal, spinous and granular layers were obtained. Using a combination of direct measurement and stereological intersection counting techniques, the relative surface areas of peripheral gap junctions (i.e. those in direct contact with the epithelial plasma membrane) and annular gap junctions (i.e. those present as complete, approximately circular profiles within the epithelial cell cytoplasm) were determined. Following estimation of the plasma membrane surface area of ‘average’ epithelial cells from each of the defined strata, relative values were transformed into absolute data. Data from peripheral and annular junctions were pooled to provide an estimate of total gap junctions area. Relative surface area estimates were similar for peripheral, annular and total gap junctions, in that values were invariably highest in the spinous layer and lowest in the granular layer. Absolute data indicate that there is more than a threefold increase in the area of membrane differentiated into gap junctions in the average spinous cell when compared with the average basal cell. Values for total gap-junctional areas in the average granular cell are reduced somewhat with respect to the average spinous cell and this is effected by a decrease in the area of peripheral gap junctions. We conclude that there is synthesis of gap junctions between basal and spinous cells, which is followed by evidence of degradation between spinous and granular cells. The magnitude of the estimates of area is comparable to those obtained from other stratified and non-stratified epithelia and it would thus appear that gap junctions may play a significant role in cellular control processes in all viable epithelial strata.

Quantitative evaluation of gap junctions in rat brown adipose tissue after cold acclimation

The decrease in the metabolic capacity of rat brown adipose tissue during the late postnatal period can be reversed by cold acclimation of the animals. In order to find out whether a parallel decrease in capability for intercellular communication observed during this period is also reversed by cold acclimation, gap junction size and number per unit area of cell surface have been quantified in freeze-fracture replicas; cell diameters have been measured in semi-thin sections. It was found that the specific number of gap junctions remains unchanged during cold acclimation. However, the mean gap junction size increases by 75% and the ratio of gap junctional area per cell volume, an index for intercellular exchange capacity, is doubled. This result illustrates further the parallelism between metabolic capacity and cell communication in brown fat.

Cytoplasmic surface and intramembrane components of rat heart gap junctional proteins

Gap junctions were purified from rat hearts in the presence of absence of proteolysis inhibitors and examined by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and electron microscopy of thin sections. In absence of proteolysis inhibitors or in presence of ethylenediaminetetraacetic acid or leupeptin, gap junctions contained a single major protein band at relative molecular weight (Mr) 29,500 and minor bands at Mr 44,000-47,000, 17,750, and 16,500 and showed smooth cytoplasmic surfaces in electron micrographs. SDS-PAGE of junctions prepared with phenylmethylsulfonylfluoride (PMSF) showed markedly decreased intensity of the Mr 29,500 band and increased intensity of bands at Mr 44,000, 45,500, and 47,000; electron microscopy of these gap junctions showed presence of a fuzzy layer on their cytoplasmic surfaces. Urea (8 M) could not remove this fuzzy layer. In electron micrographs of rat ventricular myocytes, cytoplasmic surfaces of gap junctions were fuzzy. We conclude that rat heart gap junction protein consists of an intramembrane component (Mr 29,500) that extends into the "gap" and a cytoplasmic surface component (Mr 14,500-17,500) that corresponds to the fuzzy layer and is hydrolyzable by a serine protease.

Pattern of synaptic connections in the pineal organ of the ayu, Plecoglossus altivelis (Teleostei)

Synaptic connections were studied by means of electron microscopy in the sensory pineal organ of the ayu, Plecoglossus altivelis, a highly photosensitive teleost species. Three types of specific contacts were observed in the pineal end-vesicle: symmetrically organized gap junctions between the basal processes of adjacent photoreceptor cells; sensory synapses endowed with synaptic ribbons, formed by basal processes of photoreceptor cells and dendrites of pineal neurons; conventional synapses between pineal neurons, containing both clear and dense-core vesicles at the presynaptic site. Based on these findings, the following interpretations are given: (i) The gap junctions may be involved in an enhancement of electric communication and signal encoding between pineal photoreceptor cells. (ii) The sensory synapses transmit photic signals from the photoreceptor cells to pineal nerve cells. (iii) The conventional synapses are assumed to be involved in a lateral interaction and/or summation of information in the sensory pineal organ. A concept of synaptic relationships among the sensory and neuronal elements in the pineal organ of the ayu is presented.

Aqueous outflow in the beagle. II. Postnatal morphologic development of the iridocorneal angle: corneoscleral trabecular meshwork and angular aqueous plexus

The morphology and postnatal ontogeny of the major pathway for aqueous outflow in the canine eye has been examined. Normal canine chamber angles of the Beagle at one day, one through ten weeks, three, four and six months, and one and two years of age were used. Angle tissue was prepared and examined by standard techniques for light and electron microscopy. At birth the corneoscleral trabecular meshwork largely consisted of spongy mesenchyme. By the end of the first month the corneoscleral trabecular meshwork developed into a series of longitudinally oriented sheets anteriorly covered by the peripheral extremity of the corneal endothelium. The angular aqueous plexus (trabecular veins), which was formed at birth, was continually separated from the corneoscleral meshwork by an endothelial lining. Transcellular channels interconnected the trabecular veins and the corneoscleral trabecular meshwork. The corneoscleral trabecular meshwork and angular aqueous plexus in the canine angle are similar topographically, structurally and ultrastructurally to that described in other nonprimate and primate angles.

[Ultrastructure of intercellular contacts in the histogenesis of the epidermis in the mouse]

Development of intercellular contacts in dermal epithelium of the back has been studied in 9-, 16-, 20-day-old mouse embryos and in 10-day-old animals. At the double layered stage of histogenesis the proper contacts of basal epitheliocytes are poorly developed. Early appearance of gap junctions takes place. These cells are kept in the layer at the expense of more differentiated peridermal cells and a continuous basal membrane. An intensive development of the intercellular junctions is observed in 15-day-old embryos. The spine cells make their contacts only by their processes. In the zone of the contacts desmosomes, connective complexes, invaginations are often seen. By the 20th day of embryogenesis, contact plates are formed, although essential intercellular gaps still remain. By the 10th day of the postnatal period, a column of the epidermal proliferative unit is formed. Melanocytes have both desmosomes and semidesmosomes. Apigmented granular dendrocytes (Langerhans cells) are connected with epidermocytes by means of simple and gap junctions. The data obtained make wider our knowledge on cellular interactions between variously differentiated cells in the tissue composition as an integrated system.