Vitamin-A-induced mucous metaplasia. An in vitro system for modulating tight and gap junction differentiation

Intramembrane events accompanying junction formation in a liver cell line

To study intramembrane events leading to the establishment of intercellular junctions between epithelial cells in vitro, we examined monolayer cultures of a rat liver cell line by an in situ freeze-fracturing technique (Pauli et al., J. Cell Biol., 72:763, 1977). Our observations indicate that an early step of junction formation between liver cells consists of the differentiation of a particle-poor membrane stretch showing a honeycomb pattern of shallow P-face depressions or E-face bulges ("formation band"). This change in membrane organization precedes and accompanies the subsequent aggregation of junctional particles. The latter process results in the formation of irregular particle islands with peripheral branchings which tend to encompass the depressions in the membrane. The linear branchings grow and interconnect in a network of beaded strands, which gradually transform into smooth tight junctional fibrils, as previously described in fetal liver in vivo (Montesano et al., J. Cell Biol., 67:310, 1975), while the particle islands assume the typical configuration of mature gap junctions. Formation bands are particularly prominent between liver cells growth in the presence of hydrocortisone (5 microgram/ml) in the culture medium.

Definitive evidence for the existence of tight junctions in invertebrates

Extensive and unequivocal tight junctions are here reported between the lateral borders of the cellular layer that circumscribes the arachnid (spider) central nervous system. This account details the features of these structures, which form a beltlike reticulum that is more complex than the simple linear tight junctions hitherto found in invertebrate tissues and which bear many of the characteristics of vertebrate zonulae occludentes. We also provide evidence that these junctions form the basis of a permeability barrier to exogenous compounds. In thin sections, the tight junctions are identifiable as punctate points of membrane apposition; they are seen to exclude the stain and appear as election- lucent moniliform strands along the lines of membrane fusion in en face views of uranyl-calcium-treated tissues. In freeze-fracture replicas, the regions of close membrane apposition exhibit P-face (PF) ridges and complementary E-face (EF) furrows that are coincident across face transitions, although slightly offset with respect to one another. The free inward diffusion of both ionic and colloidal lanthanum is inhibited by these punctate tight junctions so that they appear to form the basis of a circumferential blood-brain barrier. These results support the contention that tight junctions exist in the tissues of the invertebrata in spite of earlier suggestions that (a) they are unique to vertebrates and (b) septate junctions are the equivalent invertebrate occluding structure. The component tight junctional 8- to 10-nm-particulate PF ridges are intimately intercalated with, but clearly distinct from, inverted gap junctions possessing the 13-nm EF particles typical of arthropods. Hence, no confusion can occur as to which particles belong to each of the two junctional types, as commonly happens with vertebrate tissues, especially in the analysis of developing junctions. Indeed, their coexistance in this way supports the idea, over which there has been some controversy, that the intramembrane particles making up these two junctional types must be quite distinct entities rather than products of a common precursor.

Formation of gap and tight junctions between reaggregated blastomeres of the killifish, Fundulus

Blastomeres from eggs of the killifish, Fundulus, were mechanically dissociated and reaggregated by pelleting in a simple saline solution. Formation of gap and tight junctions was followed by electron microscopy of freeze-fracture replicas. Five to eight min after pelleting, neither new nor old junctions were observed. After 10-15 min, small gap junctions were found, but these were not associated with distinct formation plaques. Larger gap junctions were observed after 45 min, and the images were consistent with growth by accretion of intramembrane particles. In aggregates, after 20 min or more, tight junctions were much more commonly found than in intact blastulae, and it seemed likely that they were being formed by cells that were not doing so in the intact embryo. Initial stages consisted of short strands that appeared to grow in length. Also, more elaborate junctions were seen than occur in situ. Particle-free membrane often occurred near incomplete junctions, and large junctions like those in situ separated particle-rich from particle-free membrane. In this system, the formation of both gap and tight junctions occurs with shorter latency, and is more precisely timed, than heretofore described.

Effects of estrogen and androgen on the ultrastructure of secretory granules and intercellular junctions in regressed canine prostate

Epithelial cells in the prostate of the castrated or hypophysectomized dog were studied by thin-section and freeze-fracture electron microscopy to determine in vivo responses to estradiol-17 beta 17-cyclopentylpropionate (ECP) and testosterone cyclopentylpropionate (TCP). Particular attention was given to changes in specific organelles and intercellular junctions that might reflect hormone action. The few secretory granules that remain in the regressed epithelium (vestigial granules) serve as markers of prior androgen responsiveness. Pharmacologic doses of ECP caused regressed glandular cells to acquire a novel phenotype. Characteristic features of these estrogen-modified glandular (EMG) cells are newly formed secretory granules and tonofilament bundles that coexist with vestigial granules, thus demonstrating bipotentiality of response. Glandular cell-tight junctions appear unaltered by the endocrine manipulations. Although EMG cells have squamous cell features, tight junctions remain intact. Desmosomes in the canine prostate are dimorphic and are classified 70F and 100F according to the width of the filaments that converge on the dense plaques. In intact dogs, 100F desmosomes are associated with basal-reserve cells, whereas only the 70F variety is found between glandular cells. TCP treatment does not alter this distribution. Following ECP administration, both 70F and 100F desmosomes are present between EMG cells. The coexistence of newly formed secretory granules and tonofilaments of 100F desmosomes in the same EMG cell represents estrogen-induced bidirectional differentiation. Our findings indicate that androgens and estrogens are individually capable of controlling the expression of secretory granules and desmosomes. In intact animals, male and female sex hormones may act in concert to direct epithelial cell differentiation of the prostate.

GAp junctions in skin tumors of molluscum contagiosum

Thin section, tracer, and freeze-fracture experiments demonstrate that the infection caused by molluscum contagiosum virus implies alterations of cellular contacts in the epidermis, never before observed in cutaneous viroses. These changes are basically represented by a distinctive variation in particle packing of gap junctions, a greater proliferation of these junctions and the presence of a high amount of intracytoplasmic gap junctions. Skin tumors caused by molluscum contagiosum virus appear to be a convenient model to study the morphological and topological variations as well as the turnover of gap junctions in the epidermis.

Internal organization of plasma membranes during the acantholytic process. A freeze-fracture study

The internal organization of plasma membranes was studied in seven subjects with pemphigus vulgaris and in one subject with familial pemphigus using the freeze-fracture technique. The results obtained seem to suggest that the acantholytic process is produced in a similar fashion whatever the pathogenic agent and that desmosomes represent the "target" membrane specializations in the acantholytic phenomena. The reduction and eventual disappearance of desmosomal particle aggregations is seemingly accompanied by an increase in gap junctions.

Occluding junctions in a cultured transporting epithelium: structural and functional heterogeneity

MDCK cells (epithelioid of renal origin) form monolayers which are structurally and functionally similar to transporting epithelia. One of these similarities is the ability to form occluding junctions and act as permeability barriers. This article studies the junctions of MDCK monolayers formed on a permeable and transparent support (a disk of nylon cloth coated with collagen) by combining two different approaches: (i) Scanning of the electric field: the disk is mounted as a flat sheet between two Lucite chambers and pulses of 20--50 microA cm-2 are passed across. The apical surface of the monolayer is then scanned with a microelectrode to detect those points where the current is flowing. This shows that the occluding junctions of this preparation are not homogeneous, but contain long segments of high resistance, intercalated with sites of high conductance. (ii) Freeze fracture electron microscopy: the junctions are composed of regions of eight to ten strands intercalated with others where the strands are reduced to one or two ridges. The sites of high conductance may correspond to those segments where the number of junctional strands is reduced to 1 or 2. It is concluded that the occluding junctions of MDCK monolayers are functionally and morphologically heterogeneous, with "tight" regions intermixed with "leaky" ones.

The bicellular and reflexive membrane junctions of renomedullary interstitial cells: functional implications of reflexive gap junctions

Renomedullary interstitial cells are loosely organized within the interstitial space surrounding collecting ducts, limbs of Henle, and capillaries of the rat renal medulla. These cells possess long processes, which interact with each other and with cell bodies to form bicellular tight, intermediate, and gap junctions. In addition, both cell bodies and cell processes possess "reflexive" gap and intermediate junctions. Possible functions of renomedullary interstitial cell membrane junctions are discussed. Particular attention is given to a consideration of the functional significance of "reflexive" gap junctions.

[Reversible impairment of spermatogenesis induced by aromatic retinoid in guinea pigs (author's transl)]

Forty-one male guinea pigs were treated orally with high doses of aromatic retinoid (Ro 10-9359) 25 mg/kg/day in oleum arachidis for 6 weeks. Twenty-three animals were investigated at the end of this treatment. The remaining 18 animals were investigated after an additional retinoid-free interval of 6 weeks. A control group of 13 animals received oleum arachidis only. The histological investigation of the testes of the 23 animals studied immediately after the 6-week application of retinoid revealed reduced diameters of the seminiferous tubules, disorganization and desquamation of the seminiferous epithelium with decreased spermatogenetic activity and lack of mature elements. On the other hand, the 18 animals which were investigated 6 weeks after completion of retinoid treatment did not show any alterations, as compared to the control animals. Our results show that high systemic doses of aromatic retinoid clearly induce impairment of the spermatogenesis; however, all changes were reversible within 6 weeks after withdrawal of the drug.

Thin section and freeze-fracture studies of hexachlorophene induced alterations in the rat liver with special regard to intercellular junctions

The ultrastructural changes in the liver cells of male Wistar rats induced by the oral and intraperitoneal application of hexachlorophene (HCP) were studied by using the freeze-fracture technique and by producing ultrathin sections. The rats were given the HCP once at a dose of 30 mg/kg body weight and were sacrificed 60 hours after treatment. The enlargement of bile canaliculi, the disorganization of tight junctions and the increase in number of gap junctions were the most prominent alterations observed in the liver cells. Furthermore, an increase in the smooth and rough endoplasmic reticulum and the microbodies were found. Pathological alterations in the mitochondria were also seen. In general, the effects of the intraperitoneal application on the liver were more pronounced than the effects of the oral application.

Ultrastructural characteristics of carcinogen-induced dysplastic changes in tracheal epithelium

Squamous metaplasias with various degrees of atypia, carcinomas in situ, and microinvasive carcinomas produced in rat tracheal transplants by a 4-week exposure to demethylbenz(a)anthracene were studied 12-96 weeks after cessation of carcinogen exposure. In addition to the classical nuclear and cytoplasmic indicators of atypism, the squamous metaplasias with slight and moderate atypia were characterized by the presence of glycogen in the spinous layer, atypical composite keratohyalin granules in the granular layer, and in most cases an orthokeratinized horny layer. The squamous metaplasias with severe atypia showed an increased secretory activity and a decreased number of desmosomes. Carcinomas in situ were characterized by numerous intracellular and extracellular microcysts filled with secretory material identified by histochemical techniques as containing neutral glycoproteins and sialomucins. Microinvasive carcinoma showed similar characteristics, with an increase in polylobulated nuclei and appearance of multilaminar ergastoplasm. An outstanding feature in all these lesions was the presence of dark fusiform epithelial cells. They were observed in the middle layers in the squamous metaplasias with severe atypia and appeared in all layers of the carcinomas in situ.

Intercellular junctions in the adrenal medulla: a comparative freeze-fracture study

Observations of freeze-fractured specimens revealed that intercellular junctions in adrenal medulla are different in nature and number according to species. Only gap junctions of diverse size exhibiting characteristic loop-like configurations were found in hamster chromaffin cells. In addition to such gap junctions, polymorphic focal tight junctions occasionally combined with particle clusters or small gap junctions were found in guinea-pig. So far, no intercellular junctions were found in rat. Discussion is focused on the possible function of these junctions, in keeping with their presumably high lability.

Tight junctions of dissociated and reaggregated embryonic lung cells

Treatment of embryonic lung tissue with trypsin resulted in clustering of intramembrane particles (IMP) and gradual disassembly of tight junctions. In dissociated single cells kept in trypsin-free medium, IMP are randomly distributed but degradation of tight junctions continue. Vesicles containing tight junction elements were observed within the cytoplasm. It is therefore assumed that tight junctions may be degraded in two ways: breakdown of elements to IMP, and endocytosis. In cells reaggregated by rotation tight junctions reassembled only in hystotypic aggregates. Cycloheximide which interferes with histotypic reaggregation prevents the reassembly of tight junctions.

Junctions between interstitial cells of the renal medulla: a freeze-fracture study

Cell junctions between interstitial cells of the renal medulla were studied in freeze-fracture replicas of kidneys from rat, rabbit, hamster and the tree-shrew Tupaia belangeri. In all species studied a composite type of intercellular junction was found comprising elements of tight junctions and irregular gap junctions of highly variable size and shape. The number of these junctions increased towards the tip of the papilla. Our findings suggest that the composite junctions observed play a role in the maintainance of the ladder-like arrangement of the interstitial cells in the inner zone. The existence of irregular gap junctions raises the possibility that the functions of the interstitial cells are coordinated, especially during alterations of the functional state of the kidney.

Hypertrophic smooth muscle. III. Increase in number and size of gap junctions

The smooth muscle cells of the circular musculature of the guinea pig ileum are connected by gap junctions (nexuses) which occupy about 0.21% of the cell surface. When the muscle hypertrophies in the portions of the ileum oral to an experimental stenosis, the muscle cells increase in size and number. Gap junctions become markedly larger than in control muscles and occupy 0.49% of the cell surface. While the cells double their surface area, the number of nexuses per unit surface remains unchanged (47--48 per 1000 microns2). The packing density of intramembrane particles (or pits) in the nexuses of hypertrophic muscle cells is 6700 . microns-2, which is slightly less than in control muscle cells (7200 . microns-2). A characteristic grouping of the particles (or the pits) within the nexus is often observed. Nexuses between two processes originating from the same cell are common. Nexuses do not occur in the longitudinal muscle.

Reformation of gap and tight junctions in regenerating liver after cholestasis

Morphometric analysis of the alterations in interhepatocyte junctions induced by bile duct ligation revealed that after 48 h, during which time the serum bilirubin increased 6 to 8 fold, the membrane area occupied by gap junctions on the apico-lateral and medio-lateral sides decreased from 3.6% in controls to 0.02% in the ligated group. The strands of the zonulae occludentes were reduced in number and showed increased discontinuities. Within 45 min of recanalization of the common bile duct, clusters of particles appeared within and adjacent to the tight junctional areas or in the lateral hepatocyte membrane. Subsequently, the particle aggregations localized in the apico-lateral membrane areas increased in number and size becoming finally indistinguishable from those of controls within 96 h after the onset of recanalization. The zonulae occludentes also rearranged and reestablished their original structure during this period. The serum bilirubin fell to normal within 24 h of recanalization. It is concluded that metabolic and ultrastructural restitution associated with the recanalization of the ligated bile duct have no strict temporal correlation to one another. These studies provide further evidence that alterations in gap and tight junctions induced by pathological processes, e.g. during bile duct ligation, are completely reversible when regeneration occurs.

Intercellular junctions and the development of the blood-brain barrier in Manduca sexta

In early embryonic development of the tobacco horn moth no blood-brain barrier is present, as shown by the unimpeded entry of exogenous tracers into the nervous system. However, later on, just before hatching, lanthanum and horseradish peroxidase (HRP) are unable to move inwardly beyond the level of the perineurium, which is the morphological site of the blood--brain barrier in the adult moth, as well as in other insects. Freeze-fracture studies indicate that in the early embryo, 10 nm particles are scattered about in the perineurial membrane PF, either as separate entities or as short linear arrays. By hatching or just before, however, the 10 nm particles have become aligned into lengthy linear aggregates as PF ridges with EF grooves. These would appear to be the simple, arthropod-form of tight junction, and are presumed to be the basis of the perineurial blood-brain barrier. At about the same time, gap junctional elements appear both between adjacent perineurial cells and between glial cells. In both cell types, the gap junctions form from free 13 nm EF particles which gradually become aligned or clumped into strands and aggregates which ultimately coalesce to form first irregular masses and then the macular plaques typical of mature gap junctions. Many of the latter stages are coincident with the hatching of a motile larvae, so that the perineurial and glial cells are by this stage coupled via the channels of the gap junctional particles. They are therefore able to undergo both ionic and metabolic exchange and cooperation during larval life, in addition to being able to respond to hormonal substances in an integrated way. During the 5 larval instars more gap junctions form as the perineurial layer grows thicker. These junctions become more regular in outline and their particles more tightly packed; these larval structures are compared with junctions found in the adult which tend to be more extensive but otherwise similar. Since no septate junctions are apparent during Manduca embryonic or larval life when the blood-brain barrier forms, nor in adults, the results of this report support the contention that it is the tight junctions rather than septate ones which form the basis of permeability barriers in this, and probably other, arthropod systems.

Double replica technique applied to choroid plexus from early foetal sheep: completeness and complexity of tight junctions

Choroid plexuses from early and late sheep foetuses were examined by an improved freeze-fracture technique and the use of double-replicas to define the structure of the tight junction network of the epithelial cells. 'Complex' strands which consist of two normal parallel strands separated by a single row of pits or particles are defined and demonstrated in complementary faces. Since this strand variety was encountered in the same proportion in the different development stages investigated, it could not be correlated with changes in permeability. It is more likely that the 'complex' strands are associated with the transfer of gap junction particles within the membrane. The question of the significance of discontinuities in P face ridges was also resolved by the double replica technique: the few discontinuities which were observed could be accounted for by particles in the complementary E faces. Furthermore, approximately the same number of such junction displacements was found in early and late stages which makes it unlikely that this phenomenon could contribute to changes in permeability. Thus it has not been possible so far to relate any structural feature of the tight junction network in developing choroid plexus epithelial cells with the reported changes in permeability of the blood-C.S.F. barrier.

Hormonal modulation of ovarian interstitial cells with particular reference to gap junctions

Thin-section and freeze-fracture studies on the rat ovarian interstitial cells revealed reductions in the size and the number of gap junctions after pituitary ablation. Small gap junctions, however, persist as long as 90 days after hypophysectomy even though regressive cytoplasmic changes are completed 75 d earlier. Administration of exogenous human chorionic gonadotrophin (HCG) results in the restoration of the normal interstitial cell morphology which is accompanied by amplification of junctional membrane. Within 24 h of hormone application, gap junction growth is characterized by the appearance of formation plaques. These studies suggest that the effect of hormone on interstitial cell gap junctions is to modulate the junctional surface area.

The formation of gap and tight junctions between retinal pigment cells in cell cultures

The reformation of the junctional complex of retinal pigment cells was studied after trypsin disaggregation and in vitro reaggregation. Control specimens show a zonula occludens (tight junction) with integrated gap junctions and very large macular gap junctions. Isolation after trypsination results in disaggregation of the large gap junctions and fragmentation of the tight junctions with disaggregation of their integrated gap junctions. After two to four days of incubation the restoration of the zonula occludens is complete. After approximately five days of incubation, large gap junctions are found with a patchy arrangement of particles similar to that seen in vivo.

Effect of retinoids on the differentiation of chick embryo metatarsal skin explants

Twelve retinoids were evaluated in organ culture for activity in modulating epithelial differentiation of metatarsal skin explants from 13-day chick embryos. The epithelium differentiated into a squamous, keratinizing epidermis; but, in the presence of active retinoids, keratinization was inhibited, and a mucous metaplasia developed. The methyl-keto and 1-methoxyethyl cyclopentenyl analogs of retinoic acid were about tenfold more effective than retinoic acid in altering epithelial differentiation. The dichlorophenyl analog exhibited about the same activity as retinoic acid. The following analogs were one-half to one-third as effective as retinoic acid in inhibiting keratinization: the chlorotrimethylphenyl analog of retinoic acid and the 13-cis, 10-fluoro analog of trimethylmethoxyphenyl methyl retinoate. The other 7 retinoids were essentially not active at the concentration tested (1.4--2.0 x 10(-5) M). The activity of synthetic retinoids in altering epithelial differentiation may be related to their ability to affect or treat epithelial lesions provided that modification of the retinoid molecule can enhance its activity and decrease toxicity.

Tight junction development between cultured hepatoma cells: possible stages in assembly and enhancement with dexamethasone

Freeze-fracture and thin-section methods were used to study tight junction formation between confluent H4-II-E hepatoma cells that were plated in monolayer culture in media with and without dexamethasone, a synthetic glucocorticoid. Three presumptive stages in the genesis of tight junctions were suggested by these studies: 1) "formation zones" (smooth P-fracture face ridges deficient in intramembranous particles), apparently matched across a partially reduced extracellular space, develop between adjacent cells; 2) linear strands and aggregates of 9--11 nm particles collect along the ridges of the formation zones. The extracellular space was always reduced when these structures were found matched with pits in gentle E-face depressions; 3) the linear arrays of particles on the ridges associate within the membranes to form the fibrils characteristic of mature tight junctions. The formation zones resemble tight junctions in terms of size, complexity and the patterns of membrane ridges. Although some of the beaded particle specializations may actually be gap junctions, it is unlikely that all can be interpreted in this way. No other membrane structures were detected that could represent developmental stages of tight junctions. Dexamethasone (at 2 x 10(-6)M) apparently stimulated formation of tight junctions. Treated cultures had a greater number of formation zones and mature tight junctions, although no differences in qualitative features of the junctions were noted.

Structure and biochemical composition of desmosomes and tonofilaments isolated from calf muzzle epidermis

Complexes of plasma membrane segments with desmosomes and attached tonofilaments were separated from the stratum spinosum cells of calf muzzle by means of moderately alkaline buffers of low ionic strength and mechanical homogenization. These structures were further fractionated by the use of various treatments including sonication, sucrose gradient centrifugation, and extraction with buffers containing high concentrations of salt, urea, citric acid, or detergents. Subfractions enriched in desmosome-tonofilament-complexes and tonofilament fragments were studied in detail. The desmosome structures such as the midline, the trilaminar membrane profile, and the desmosomal plaque appeared well preserved and were notably resistant to the various treatments employed. Fractions containing desmosome-tonofilament complexes were invariably dominated by the nonmembranous proteins of the tonofilaments which appeared as five major polypeptide bands (apparent molecular weights: 48,000; 51,000; 58,000; 60,000; 68,000) present in molar ratios of approx. 2:1:1:2:2. Four of these polypeptide bands showed electrophoretic mobilities similar to those of prekeratin polypeptides from bovine hoof. However, the largest polypeptide (68,000 mol wt) migrated significantly less in polyacrylamide gels than the largest component of the hoof prekeratin (approximately 63,000 mol wt). In addition, a series of minor bands, including carbohydrate-containing proteins, were identified and concluded to represent constituents of the desmosomal membrane. The analysis of protein-bound carbohydrates (total 270 microgram/mg phospholipid in desmosome-enriched subfractions) showed the presence of relatively high amounts of glucosamine, mannose, galactose, and sialic acids. These data as well as the lipid composition (e.g., high ratio of cholesterol to phospholipids, relatively high contents of sphingomyelin and gangliosides, and fatty acid pattern) indicate that the desmosomal membrane is complex in protein and lipid composition and has a typical plasma membrane character. The similarity of the desmosome-associated tonofilaments to prekeratin filaments and other forms of intermediate-sized filaments is discussed.

Development of Sertoli cell junctions in vitro--a freeze-fracture study

Seminiferous tubules of 1-day-old rats were maintained in organ culture for up to 40 days. Five classes of intercellular junctions between Sertoli cells were observed by the freeze-fracture method as the tissue aged: (a) typical gap junctions; (b) focal tight junctions; (c) macular tight junctions; (d) meandering tight junctions; and (e) extensive tight junctions. The relative proportions of these types of Sertoli cell junctions were quantitated as the organ cultures progressed. The junctional structures observed and classified in organ culture were identical to those seen in vivo, but the timing of their appearance and/or disappearance, as well as their relative proportions, was different from that observed in the developing animal. Extensive tight junctions, with numerous parallel strands, were observed in the 40-day cultures; however, their oblique orientation with respect to the myoid layer was in contrast to the parallel orientation observed in vivo.

Ca++-dependent disassembly and reassembly of occluding junctions in guinea pig pancreatic acinar cells. Effect of drugs

Incubation of guinea pig pancreatic lobules in Ca++-free Krebs-Ringer bicarbonate solution (KRB) containing 0.5 mM ethylene glycol-bis(beta-aminoethyl ether)N,N,N',N'-tetraacetate (EGTA) results in the progressive fragmentation of the occluding zonulae (ZO) with formation of multiple discrete junctions (fasciae occludentes) localized in the lateral and lumenal plasmalemma. After 1--2 h of such incubation, most ZO appear completely disassembled. This results in the disappearance of the heterogeneity in density of intramembrane particles on the P-fracture faces of the basolateral and lumenal plasmalemma. If Ca++ ions are reintroduced into the incubation fluid at this point, continous zonulae reform around the apices of the cells; in contrast, the density of intramembrane particles (imp) at the lumenal plasmalemma remains the same as in the basolateral region, at least for 3 h after Ca++ reintroduction. When added to the incubation fluid, cycloheximide (at a dose known to inhibit protein synthesis greater than 95%) and cytochalasin B (at doses which disrupt microfilaments and modify the cell shape) had no effect on the organization of ZO, on their disassembly in Ca++-free, EGTA medium, or on their Ca++-dependent reformation. Likewise, the organization and disassembly of ZO were unaffected by colchicine; however, after treatment with the latter drug the reassembly was defective, with formation of strand networks on the lateral surface and incomplete segregation of the lumenal region. Antimycin A, on the other hand, when added to the Ca++-EGTA medium, induced a large proliferation of long, infrequently anastomosed junctional strands, usually arranged to form ribbons, festoons, and other bizarre arrays. The possible relationship of these in vitro findings to the in vivo biogenesis and turnover of occluding junctions is discussed. It is suggested that the impairment of reassembly of zonulae by colchicine might be correlated with the disorder induced by the drug on the general organization of pancreatic exocrine cells. Moreover, antimycin A could act by promoting the aggregation of a pool of free junctional strand components (or precursors) that might exist normally in pancreatic exocrine cells.

Studies on the ultrastructure and permeability of the hemotrichorial placenta. I. Intercellular junctions of layer I and tracer administration into the maternal compartment

The distribution of lanthanum chloride and horseradish peroxidase within the full-term chorioallantoic placenta of the rat was investigated 1 to 20 min after administration of these tracers into the maternal blood circulation. Both tracers rapidly penetrate trophoblastic layer I and diffuse into the interspace between layers I and II. They are localized in extensive infoldings and caveolae of the outer surface of layer II. The syncytial character of layer II is confirmed at this development stage of the placenta. There is no vesicular uptake or penetration beyond layer II until 20 min after tracer administration. Our results indicate that trophoblastic layer II is the main barrier in the chorioallantoic rat placenta preventing the permeation of macromolecules from maternal to fetal compartments. With freeze-fracturing, particulate strutures of variable arrangement and size are found between adjacent cells of layer I. With goniometric analysis small gaps between the appositions of the membrane are observed. These structures are interpreted as tight and/or gap junctions during stages of assembly or disassembly.

Loss and reappearance of gap junctions in regenerating liver

Changes in intercellular junctional morphology associated with rat liver regeneration were examined in a freeze-fracture study. After a two-thirds partial hepatectomy, both gap junctions and zonulae occludentes were drastically altered. Between 0 and 20 h after partial hepatectomy, the junctions appeared virtually unchanged. 28 h after partial hepatectomy, however, the large gap junctions usually located close to the bile canaliculi and the small gap junctions enmeshed within the strands of the zonulae occudentes completely disappeared. Although the zonulae occludentes bordering the bile canaliculi apparently remained intact, numerous strands could now be found oriented perpendicular to the canaliculi. In some instances, the membrane outside the canaliculi was extensively filled with isolated junctional strands, often forming very complex configurations. About 40 h after partial hepatectomy, very many small gap junctions reappeared in close association with the zonulae occludentes. Subsequently, gap junctions increased in size and decreased in number until about 48 h after partial hepatectomy when gap junctions were indistinguishable in size and number from those of control animals. The zonulae occludentes were again predominantly located around the canalicular margins. These studies provide further evidence for the growth of gap junctions by the accretion of particles and of small gap junctions to form large maculae.

Alterations of intercellular junctions in acinic cell carcinoma of the canine pancreas

Intercellular junctions in spontaneous canine pancreatic acinic cell adenocarcinomas were compared to those in control canine pancreas. The neoplastic cells displayed proliferation and fragmentation of tight junctions and reduction in size and number of gap junctions. Marked decrease in desmosomal density was observed only in the poorly differentiated carcinoma. In the well differentiated carcinomas a few of the desmosomes were characteristic of those found in squamous cells. No quantatitive or qualitative differences in cell junctions were noted between primary and metastatic tumor.

Polarized monolayers formed by epithelial cells on a permeable and translucent support

An epithelial cell line (MDCK) was used to prepare monolayers which, in vitro, develop properties of transporting epithelia. Monolayers were formed by plating cells at high densities (10(6) cells/cm2) on collagen-coated nylon cloth disks to saturate the area available for attachment, thus avoiding the need for cell division. An electrical resistance developed within 4-6 h after plating and achieved a steady-state value of 104 +/- 1.8 omega-cm2 after 24 h. Mature monolayers were morphologically and functionally polarized. They contained junctional complexes composed of desmosomes and tight junctions with properties similar to those of "leaky" epithelia. Monolayers were capable of maintaining a spontaneous electrical potential sensitive to amiloride, produced a net water flux from the apical to basal side, and discriminated between Na+ and Cl- ions. The MDCK permeability barrier behaves as a "thin" membrane with negatively charged sites. It has: (a) a linear conductance/concentration relationship; (b) an asymmetric instantaneous current/voltage relationship; (c) a reduced ability to discriminate between Na+ and Cl- caused by lowering the pH; and (d) a characteristic pattern of ionic selectivity which suggests that the negatively charged sites are highly hydrates and of medium field strength. Measurements of Na+ permeability of electrical and tracer methods ruled out exchange diffusion as a mechanism for ion permeation and the lack of current saturation in the I/deltapsi curves does not support the involvement of carriers. The discrimination between Na+ and Cl- was severely but reversibly decreased at low pH, suggesting that Na+-specific channels which exclude Cl- contain acidic groups dissociated at neutral pH. Bound Ca++ ions are involved in maintaining the integrity of the junctions in MDCK monolayers as was shown by a reversible drop of resistance and opening of the junctions in Ca++-free medium containing EGTA. Several other epithelial cell lines are capable of developing a significant resistance under the conditions used to obtain MDCK monolayers.

Morphologic changes of the junctional complex of the hepatocytes in rat liver after bile duct ligation

Using freeze-fracture techniques, we have examined the changes in the morphology of the tight junctional network around the canalicular lumen of the hepatocytes in rat liver after experimental bile-duct ligation. The more or less regular belt of parallel strands formed by the tight junctions around the canalicular lumen of normal hepatocytes is changed after extrahepatic obstruction. A more irregular network is formed withe a reduced number of strands which also extend more in an abluminal direction with formation of irregular loops. Striking changes are seen at the gap junctions: the small patches normally situated within the tight junctional network become less numerous; at some canaliculi they are even absent; also the larger gap junctional areas normally present in deeer abluminal extensions of the lateral cell membrane become hard to find or are even absent. This altered tight junctional pattern suggests an increased permeability so that the pathway of intercellular escape of biliary constituents towards the blood stream in cholestasis becomes as plausible as transhepatocytic regurgitation. The disappearance of the gap junctions would result in a lack of intercellular communication and uncoupling of liver cells, which may lead to a more individual behaviour of adjecent hepatocytes, explaining the heterogeneity in canalicular changes in cholestasis.

Cytoplasmic filaments and gap junctions in epithelial cells and myofibroblasts during wound healing

During the healing of an experimental skin wound, epidermal cells and granulation tissue fibroblasts (myofibroblasts) develop an extensive cytoplasmic contactile apparatus. Concurrently, the proportion of epidermal cell surface occupied by gap junctions increases when compared to normal skin, and newly formed gap junctions appear between myofibroblasts; this suggests that epidermal cell migration and granulation tissue contraction are synchronized phenomena.

Exocrine pancreas under experimental conditions. IV. Alterations of intercellular junctions between acinar cells following pancreatic duct ligation

Exocrine pancreatic tissue was investigated during various intervals after pancreatic duct ligation. Concomitant to the increase of the intraluminal pressure, alterations are found in the arrangement of the luminal membrane surface and the zonulae occludentes of the acinar cells. The zonulae occludentes exhibit a disarrangement of their strands. The number of strands diminishes and small interruptions as well as large discontinuities of the strands are observed in most regions of the acinar cells. Furthermore, gap junctions are found very infrequently between acinar cells. The ultrastructural alterations of the zonulae occludentes suggest a gradual adaptation to the increasing unilateral pressure. However, a leakage of the paracellular permeability barrier occurs which contributes to the known shunt between the compartments of the pancreatic juice and the interstitial space following pancreatic duct obstruction. The present investigation is a further example of alterations in the paracellular permeability and intercellular communication of epithelial cells under pathological conditions.

Effect of retinyl acetate on sulfated glycosaminoglycan biosynthesis in dermal and epidermal cells in vitro

The effects of retinyl acetate on the biosynthesis of sulfated glycosaminoglycans in dermal and epidermal cells isolated from newborn mice was investigated. Three compartments were analysed for [35S]-glycosaminoglycans; the culture medium, the cellular matrix, and the cells. The individual levels of chondroitin 4-sulfate, dermatan sulfate and heparin and/or heparan sulfate in these compartments as a function of retinyl acetate was also analysed. The addition of retinyl acetate resulted in a dose dependent increase 35SO4 incorporation in the cellular and matrix compartments of the dermis in vitro. At the optimum concentration of 1.8 X 10(-6) M, this increase was 50%. The levels of35SO4 incorporated into medium glycosaminoglycans were relatively constant. There were also changes in the levels of the individual sulfated glycosaminoglycans. The glycosaminoglycan profile was modified differently in each of the three compartments analysed. In the epidermal cells, retinyl acetate at an optimum concentration of 1.8 X 10(-6) M resulted in a dose dependent increase in 35SO4 incorporation in the cellular, matrix and medium compartments. There was no apparent change in the glycosaminoglycan profile, with heparin and/or heparan sulfate being the major sulfated glycosaminoglycan.