Transepithelial electrical resistance and tight junctions of human gingival keratinocytes

Human gingival keratinocytes (HGKs) were studied by means of freeze-fracture technique, conventional electron microscopy and the transepithelial electrical resistance for the investigation of intercellular contacts. For the purpose of comparison, MDCK cells and HaCat cells were also included. An unexpected finding was the presence of tight junctions in the HGKs. In vivo the tight junctions, which were of low complexity and P-face-associated, were co-distributed with desmosomes; in one case, the strands ran directly through desmosomal plaques. Where tight junctions and desmosomes occurred together, no gap junctions were seen. In contrast, where no tight junctions were present, gap junctions and desmosomes were co-localized. However, the unfavourable fracture planes through the tissue did not allow a clearcut allocation of gap junction/tight junction occurrence to certain strata. In vitro, HGKs also expressed tight junctions which formed networks of low complexity and high P-face association. Whereas desmosomes were highly expressed, gap junctions were not observed in cultured keratinocytes. Transepithelial electrical resistances (TEER) of cultured HGKs were higher than the values in low resistance-MDCK cells and HaCat cells but considerably lower than the values in high resistance MDCK cells, supporting the fundamental correlation between tight junction morphology and TEER. The results with this cell culture model of the human gingiva provide some valuable information about in vitro differentation and concommittent changes in cellular contacts of human gingival keratinocytes.

The pecten oculi of the chicken: a model system for vascular differentiation and barrier maturation

The pecten oculi is a convolute of blood vessels in the vitreous body of the avian eye. This structure is well known for more than a century, but its functions are still a matter of controversies. One of these functions must be the formation of a blood-retina barrier because there is no diffusion barrier for blood-borne compounds available between the pecten and the retina. Surprisingly, the blood-retina barrier characteristics of this organ have not been studied so far, although the pecten oculi may constitute a fascinating model of vascular differentiation and barrier maturation: Pectinate endothelial cells grow by angiogenesis from the ophthalmotemporal artery into the pecten primordium and consecutively gain barrier properties. The pectinate pigmented cells arise during development from retinal pigment epithelial cells and subsequently lose barrier properties. These inverse transdifferentiation processes may be triggered by the peculiar microenvironment in the vitreous body. In addition, the question is discussed whether the avascularity of the avian retina may be due to the specific metabolic activity of the pecten.

N-cadherin expression in endothelial cells during early angiogenesis in the eye and brain of the chicken: relation to blood-retina and blood-brain barrier development

The factors responsible for the induction and maintenance of blood-brain barrier properties are still undefined. The process of blood-brain barrier formation is thought to take place in a two-stage manner: the initial commitment of vascular sprouts by neuroectodermal cells may be followed by the stabilization of barrier properties. In the present study, we investigated the expression pattern of neural (N)-cadherin during early angiogenesis in the brain and the pecten oculi of the chicken. The pecten has been introduced previously as a model for the investigation of the formation and maturation of barrier properties in the central nervous system. Whereas perineural and choroid vessels remained immunonegative for N-cadherin, vascular sprouts invading both the brain and the pecten primordium acquired anti-N-cadherin immunoreactivity. Confocal laser scanning and immunoelectron microscopy indicated that the antigen was located at the ablumenal endothelial membrane in contact with subendothelial cells. With the onset of barrier differentiation as determined by junctional restriction of the tight junction protein occludin, N-cadherin labelling rapidly decreased. Specific intraneuroectodermal upregulation and decline of endothelial N-cadherin was confirmed by in situ hybridization and suggests that N-cadherin expression by cerebral and pecteneal endothelial cells represents an initial and transient signal which may be involved in the commitment of early blood vessels to express blood-brain and blood-retina barrier properties.

Distribution of mitochondria within Müller cells--II. Post-natal development of the rabbit retinal periphery in vivo and in vitro: dependence on oxygen supply

The occurrence and localization of mitochondria within glial (Müller) cells and neurons of the peripheral (avascular) rabbit retina was studied electron microscopically and by immunocytochemical demonstration of the mitochondrial enzyme GABA transaminase (GABA-T). Post-natal development in vivo was compared with development of organ cultures from neonatal rabbit retinae, grown over 2 weeks in vitro. The adult pattern of mitochondrial localization (restriction to the sclerad end of the cells) was observed from the beginning of enzyme expression at early post-natal stages. However, when neonatal retinal pieces were grown in vitro with their vitread surface exposed to the air, their Müller cells contained mitochondria along most of their length. When functionally developed retinae from postnatal day 14 were explanted in vitro, they retained their sclerad mitochondrial distribution for almost 24 h but thereafter the inner portions of their cytoplasm became occupied by mitochondria within a few hours. This was achieved mainly by mitochondrial migration rather than by formation of new mitochondria because it was not prevented by cycloheximide-induced inhibition of protein synthesis. These data support the following hypotheses: (1) the mitochondrial distribution in Müller cells is determined by the local cytoplasmic O2 pressure (pO2), (2) existing mitochondria move towards cytoplasmic regions of sufficient pO2 by rather rapid migration and (3) the start of this migration is delayed by almost 24 h due to the action of as yet unknown control mechanisms. In contrast, the mitochondrial content of retinal ganglion and amacrine cells in the vitread retinal layers was virtually independent of the source and level of oxygen supply.

Distribution of mitochondria within Müller cells--I. Correlation with retinal vascularization in different mammalian species

The distribution of mitochondria within retinal glial (Müller) cells and neurons was studied by electron microscopy, by confocal microscopy of a mitochondrial dye and by immunocytochemical demonstration of the mitochondrial enzyme GABA transaminase (GABA-T). We studied sections and enzymatically dissociated cells from adult vascularized (human, pig and rat) and avascular or pseudangiotic (guinea-pig and rabbit) mammalian retinae. The following main observations were made. (1) Müller cells in adult euangiotic (totally vascularized) retinae contain mitochondria throughout their length. (2) Müller cells from the periphery of avascular retinae display mitochondria only within the sclerad-most end of Müller cell processes. (3) Müller cells from the vascularized retinal rim around the optic nerve head in guinea-pigs contain mitochondria throughout their length. (4) Müller cells from the peripapillar myelinated region ('medullary rays') of the pseudangiotic rabbit retina contain mitochondria up to their soma. In living dissociated Müller cells from guinea-pig retina, there was no indication of low intracellular pH where the mitochondria were clustered. These data support the hypothesis that Müller cells display mitochondria only at locations of their cytoplasm where the local O2 pressure (pO2) exceeds a certain threshold. In contrast, retinal ganglion cells of guinea-pig and rabbit retinae display many mitochondria although the local pO2 in the inner (vitread) retinal layers has been reported to be extremely low. It is probable that the alignment of mitochondria and the expression of mitochondrial enzymes are regulated by different mechanisms in various types of retinal neurons and glial cells.

The tight junctions of the leptomeningeal blood-cerebrospinal fluid barrier during development

The outer blood-cerebrospinal fluid barrier is formed by leptomeningeal cells of the arachnoidea. The structures underlying this barrier are tight junctions. In contrast to the tight junctions of endo- and epithelial cells, which have been investigated by means of ultrastructural as well as by molecular methodology, equivalent studies on meningeal cells are lacking. In the present study, therefore, the ultrathin section and freeze-fracture morphology of cranial leptomeningeal cells of carp, frog, chicken and rat was investigated by quantitative morphometry. In addition, the developmental features of the meningeal barrier in chicken and rat were compared. The parameters determined were the complexity of the tight junctions, the density of strands and branchpoints and the degree of association of tight junction particles with the protoplasmatic and exoplasmatic leaflets of cellular membranes. The complexity of tight junctions was highest in chicken and lowest in frog meningeal cells, whereas intermediate values were reached in the carp and the rat. E- and P-face associations were similar in carp and frog, whereas in chicken, the P-face association and in rat the E-face association dominated. During development, tight junction complexity continuously increased up to adult stages in the chicken, whereas in the rat, the adult value was already reached at postnatal day 2. At early embryonic stages, particle insertion into tight junctions was incomplete but occurred equally into both membranous leaflets; if completed at E19 in the chicken, a redistribution of particles toward a higher P-face association was observed. In the rat, tight junction particles were redistributed toward a higher E-face association. These results are discussed in the context of blood-brain barrier induction, maintenance and regulation.

Specific interaction of food proteins with apical membranes of the human intestinal cell lines Caco-2 and T84

A comparison between the intestinal epithelial cell lines Caco-2 and T84 was made to assess the influence of enterocytic differentiation on food protein binding capacities of the brush border membrane. Cell morphology and expression of brush border-associated enzymes were studied as differentiation markers. Food protein binding to isolated brush border membranes was measured with a dot blot chemiluminescence assay. Early at confluence, Caco-2 cells exhibited a more differentiated state compared to T84 cells. Brush border membranes of both cell lines bound gliadin peptides, beta-lactoglobulin and ovalbumin specifically. Binding capacities increased from gliadin peptides to ovalbumin to beta-lactoglobulin. There was correlation of membrane binding capacity with degree of cell differentiation. Due to their similarity to small intestinal epithelial cells, the colon carcinoma cell lines Caco-2 and T84 represent models for studying food protein-enterocytic brush border membrane interactions in relation to varying degrees of cell differentiation.

Vascular endothelial growth factor induces endothelial fenestrations in vitro

Vascular endothelial growth factor (VEGF) is an important regulator of vasculogenesis, angiogenesis, and vascular permeability. In contrast to its transient expression during the formation of new blood vessels, VEGF and its receptors are continuously and highly expressed in some adult tissues, such as the kidney glomerulus and choroid plexus. This suggests that VEGF produced by the epithelial cells of these tissues might be involved in the induction or maintenance of fenestrations in adjacent endothelial cells expressing the VEGF receptors. Here we describe a defined in vitro culture system where fenestrae formation was induced in adrenal cortex capillary endothelial cells by VEGF, but not by fibroblast growth factor. A strong induction of endothelial fenestrations was observed in cocultures of endothelial cells with choroid plexus epithelial cells, or mammary epithelial cells stably transfected with cDNAs for VEGF 120 or 164, but not with untransfected cells. These results demonstrate that, in these cocultures, VEGF is sufficient to induce fenestrations in vitro. Identical results were achieved when the epithelial cells were replaced by an epithelial-derived basal lamina-type extracellular matrix, but not with collagen alone. In this defined system, VEGF-mediated induction of fenestrae was always accompanied by an increase in the number of fused diaphragmed caveolae-like vesicles. Caveolae, but not fenestrae, were labeled with a caveolin-1-specific antibody both in vivo and in vitro. VEGF stimulation led to VEGF receptor tyrosine phosphorylation, but no change in the distribution, phosphorylation, or protein level of caveolin-1 was observed. We conclude that VEGF in the presence of a basal lamina-type extracellular matrix specifically induces fenestrations in endothelial cells. This defined in vitro system will allow further study of the signaling mechanisms involved in fenestrae formation, modification of caveolae, and vascular permeability.

On the role of Müller glia cells in histogenesis: only retinal spheroids, but not tectal, telencephalic and cerebellar spheroids develop histotypical patterns

The establishment of cell and fibre layers and the specification of different cell types are crucial processes during development of the central nervous system. Here we investigated the developmental architecture of radial glia cells in these processes using so-called spheroids that arise from dissociated chicken embryonic neural cells in rotation culture. We were able to produce retinal, tectal, and telencephalic spheroids from E6 embryos and cerebellar spheroids from E10 embryos. Cell and fibre differentiation can be observed in all types of spheroids, however, it is most abundant in retinal spheroids. Moreover, only in retinal spheroids a histotypic organization can be detected. Using immunohistochemistry and electron microscopy, we assign this -at least partially- to the capacity of Müller cells to form radial scaffolds, since we observe a congruency between these radial scaffolds and the presence of rosettes formed by photoreceptor precursors and Müller cells. Tectal, telencephalic and cerebellar spheroids do not show organized radial glia scaffolds, instead, the radial glia cells are randomly arranged and the spheroids do not show histotypical organization. The application of the specific gliotoxin 6-aminonicotinamide to growing retinal spheroids leads to a significant decrease in the number and size of the rosettes. Concomitantly, the degree of histotypical organization is also drastically reduced. This organizing capacity of Müller cells in vitro now strongly suggests the presence of a comparable function also in vivo. Moreover, since non-retinal radial glia cells are not able to re-organize an histotypic organization in vitro, Müller cells seem to be qualitatively different from other radial glia cells. In future studies we want to untangle these differences.

Development of the neonatal rabbit retina in organ culture. 1. Comparison with histogenesis in vivo, and the effect of a gliotoxin (alpha-aminoadipic acid)

Organ cultures from neonatal rabbit retinae grew well over periods of up to 2 weeks in vitro. Proliferation in vitro declined in parallel with the decline seen in vivo, although the rate of proliferation in the explants was slightly reduced. The proliferation of progenitor cells in vitro produced the same cell types produced postnatally in vivo. Postnatally generated cell clones, labeled by means of a retroviral vector, consisted mainly of rods and Müller cells. The layers of the retinae developed as in vivo; an outer plexiform layer occurred after the first 2 days in vitro. Ultrastructurally, ribbon synapses (outer and inner plexiform layer) and conventional synapses (inner plexiform layer) were observed. The photoreceptor cells grew well-developed inner segments and cilia but no mature outer segments. The cultured retinae contained a well-developed, regular lattice of Müller cells expressing vimentin as in vivo. The neuron-to-Müller cell-ratios were essentially the same as in vivo, viz. about 15 to 16 neurons, among them about 10 to 11 (rod) photoreceptor cells per Müller cell. When the glia cell-specific toxin alpha-aminoadipic acid (alpha AAA) was applied, the pattern of vimentin-positive Müller cells became irregular, or even locally missing. In such cases, the tissue became disorganized as indicated by a local disappearance of the regular layering, and development of many rosettes. It is concluded that an intact lattice of Müller cells is necessary for the migration of young neurons, and for correct formation of retinal layers.

Maturation of the blood-retina barrier in the developing pecten oculi of the chicken

The major interest in the development of the blood-brain barrier and its underlying induction mechanisms is given by the crucial role they play in the maturation of the central nervous system in general. Whilst it is believed that it is the microenvironment in the brain that destines the endothelial cells to become committed to barrier properties, the analysis of the multitude of factors probably responsible for this commitment is extremely difficult. Therefore, in a previous study, we inaugurated the pecten oculi of the avian eye as a relatively simple in vivo model of the blood-brain barrier [Gerhardt, S. et al, Cell Tissue Res., 285 (1996) 91-100]. In the present study, we demonstrate data on the development of the pecten which allow us to understand better the commitment of barrier properties in endothelial cells in an environment which is considerably less complex than that realized in the brain. The pecten is built up by mainly two cell types, the pigmented glial cells and the endothelial cells. The pigmented cells, which are believed to originate from the retinal pigment epithelium, lose their tight junctions in the microenvironment of the vitreous body, whereas the endothelial cells, which originate from the permeable choroidal vessels, gain tight junctions and other barrier properties in the microenvironment of the vitreous body. On embryonic day 7 (E7), tight and gap junctions between epithelial-like glial cells line the vitreal border of the developing pecten. By E16, these junctions disappear, and the endothelial cells gradually acquire barrier characteristics (continuous and P-face associated tight junctions, no extravasation of lanthanum nitrate, and the exclusive expression of the glucose transporter isoform 1 and the barrier specific antigen HT7 in their luminal and abluminal membranes). The results are discussed considering the switch from an epithelial (glial) to an endothelial barrier.

Interactions of triethyltin-chloride (TET) with the energy metabolism of cultured rat brain astrocytes: studies by multinuclear magnetic resonance spectroscopy

The effect of triethyltin-chloride (TET), a highly neurotoxic compound, on the cellular metabolism of rat brain astrocytes in vitro was examined by nuclear magnetic resonance (NMR) spectroscopy. 5-week-old cultures were exposed to TET (0.2-40 microM) either for (1) acute (3h), (2) 24 h, or (3) chronic treatment (8 d). Cells were labeled with 1-(13)C-glucose, cell extracts were prepared and 31P, 1H, and 13C spectra were analyzed. Cytotoxic effects of TET were assessed by vital dye uptake assay using neutral red (NR) and by exclusion of trypan blue (TB). Cells were examined ultrastructurally by electron microscopy. The data show that the major target of TET at concentrations already causing morphological effects on cultured astrocytes is not the energy metabolism, but that TET rather alters the intracellular concentrations of organic osmolytes, such as myo-inositol, taurine and hypotaurine, which are part of the control of ion and volume regulation and osmotic balance in astrocytes.

Development of blood-brain barrier tight junctions in the rat cortex

The structural equivalent of the blood-brain barrier are the complex tight junctions (TJs) between endothelial cells of brain capillaries. In this study, we have quantitatively investigated by the freeze-fracture technique the modulation of the fine structure of TJs in blood-brain barrier endothelial cells during development of the rat cerebral cortex. The complexity of the TJ network as defined by fractal dimension, the integrity of TJ strands and the degree of TJ particle association to the protoplasmic leaflet of the membrane bilayer in percent of total TJ length were evaluated at embryonic days (E) 13, 15, 18, postnatal day (P) 1 and adult. We observed that the overall complexity of the TJ network and P-face association of TJ particles are significantly increased between E18 and P1. The increase in both of these TJ parameters in combination with the completed particle insertion starting from E18 is likely to reflect the process of transition to the mature state of the blood-brain barrier, which is characterized by high complexity of TJs and predominance of P-face association of TJ particles and correlated tightly with previous physiological measurements, e.g. transendothelial electrical resistance. Two populations of TJs differing in TJ particle density were distinguishable at E15 and E18, which indicates a non-linear asynchronous mechanism of TJ assembly. At E13, particle-free membrane specializations arranged in a TJ-like pattern strongly resembled TJ specific grooves and ridges. Similar results were obtained from cultures of brain endothelial cells in the presence of low calcium conditions, which suggests the involvement of the cadherin/catenin complex in TJ regulation. The particle-free 'TJ precursors' strongly indicate an established TJ associated cytoskeletal network before the TJ particles are present in their intra-junctional location.

Impregnation of collagen corneal shields with liposomes: uptake and release of hydrophilic and lipophilic marker substances

PURPOSE

Liposomes and collagen corneal shields (CCS) have been used as ophthalmic drug delivery devices. With regard to a possibly combined application, we studied the effects of surface charge and bilayer fluidity of liposomes on their uptake and release by CCS.

METHODS

12-hours-CCS were soaked in large unilamellar liposomes, which had been labelled with 4,5-carboxyfluorescein (CF) and N-(lissamine rhodamine B sulfonyl)-diacyl-phosphatidylethanolamine (PE-RhB) in the aqueous space and in the liposome bilayer, respectively. Released fluorophores were determined fluorometrically in the elution buffer at intervals from 1 to 240 min after immersion.

RESULTS

The CF concentration in the CCS soaked in a CF solution was two to seven times higher than immersion in the liposome suspensions. Among those, the negatively charged, cholesterol-containing preparation led to the highest CF concentration in the CCS. The PE-RhB concentration was highest after soaking the CCS in neutral, cholesterol-free liposomes. All types of liposomes were found inside the CCS by freeze fracture electron microscopy. The release kinetics data indicate a first order release. More than 90% of CF was released by the CCS within the first 30 min. This was equal after soaking the CCS in the CF solution or in liposomes. With DOPC-liposomes, the maximal release was already attained after 10 min. In general, the differences in the release kinetics of both hydrophilic and lipophilic markers, obtained by the various liposome types were small.

CONCLUSIONS

Our results indicate that surface charge and bilayer fluidity are of minor importance for the interaction with collagen corneal shields. However, since the release kinetics of a liposome-encapsulated hydrophilic or lipophilic substance are similar to the release of a non-encapsulated drug, the combination of liposomes with collagen shields may be useful mainly with respect to the encapsulation of drugs which do not penetrate the ocular surface as well as to prolong corneal contact time of the liposomes.

The pecten oculi of the chicken as a new in vivo model of the blood-brain barrier

The pecten oculi of White Leghorn chicken was investigated in terms of possible blood-brain barrier properties by means of light and electron microscopy. The morphology, histochemistry and immunocytochemistry of this intraocular blood vessel convolute was examined. The permeability of the blood vessel for the electron-dense tracer lanthanum-nitrate was also determined. The endothelia of the pectinal capillaries were found to be continuous, possessing elaborate tight junctions with a high P-face association in freeze-fracture images and a low incidence of vesicles. Two barrier-specific proteins, i.e. the HT7-antigen and the glucose transporter isoform GluT-1 were specifically and exclusively expressed by the endothelial cells. Endothelial cells also showed no paracellular or transcytotic extravasation of lanthanum-nitrate. We conclude that the endothelium of the pecten oculi is the site of the blood-retina barrier. Furthermore, we demonstrate that the unique morphological characteristics of the pecten make it advantageous as a new in vivo model for the investigation of the blood-brain barrier.

Subcellular distribution of glucose transporter (GLUT-1) during development of the blood-brain barrier in rats

Electron microscopy was used to quantify the subcellular distribution of the GLUT-1 isoform of the glucose transporter in developing microvessels of the brain of embryonic rats from E (embryonic stage) 13 to E19 and in adult rats. Gold-conjugated secondary antibodies were used to localize, on ultrathin sections of brain, a rabbit polyclonal antiserum (anti-GLUT-1) raised against a synthetic peptide encoding 13 amino acids of the C-terminus of the human glucose transporter. Staining was weak at E13 but increased in density during development into adulthood. The increase represented an increase in the absolute amount of transporter per vessel profile, with a concomitant decrease in vessel size with the narrowing of the wall. At early stages, the percentages of total particles per profile of lumenal membrane, ablumenal membrane, and cytoplasm were approximately equivalent. The ratio of lumenal to ablumenal particle density then shifted from below 1 at E13 to above 2 at E19 and to 4 in the adult. In contrast, vessels of the choroid plexus were devoid of labeling, but the choroid plexus epithelium stained as early as E15. In the brain, no astrocytes, neurons, or pericytes were stained at any stage examined. Developmental upregulation of the GLUT-1 glucose transporter therefore seems to occur at the blood-brain barrier, and the modulation of the subcellular distribution of the transporter can be correlated with other observed changes in the microvessels as they develop the blood-brain barrier phenotype.

Cellular reactions at the lesion site after crushing of the rat optic nerve

Rat optic nerves were subjected to crush injury to study the local tissue reactions leading to wound healing and tissue repair. We used antibodies against glial fibrillary acidic protein (GFAP), vimentin, the S1OO protein (S1OOP), lysozyme, and ED1 as markers for astroglial cells and microglia/macrophages at the light and electron microscopic level during the 3 weeks following the crush. The crush injury produced a vast area of tissue damage including the disruption of the blood-brain barrier (BBB). In the first days after crushing, astrocytes were absent from the lesion site. S1OOP-positive astrocytes reappeared in the lesion center as early as 6 days after crushing. These astrocytes reestablished former topological structures such as perivascular and subpial glia limitans. At the edges of the lesion site reactive astrocytes enclosed and embedded axonal and myelin debris. Preceding the astroglial repopulation, a massive infiltration of microglia/macrophages (phagocytes) into the lesion center took place. ED1-positive/lysozyme- positive cells of round shape were seen in the lesion center at 2 days after crushing, and their number peaked around 1 week after crushing. They efficiently cleared the debris from the lesion site and mostly disappeared after 3 weeks. With immuno-electron microscopy we found the ED1 antigen related to the membranes of phagosomes. The microglia/macrophages observed in the nerve segments distal of the lesion (Wallerian degeneration site) were different from those in the lesion center: 1) they appeared later, about 6 days after crushing; 2) they were ED1 positive, but lysozyme negative and showed a branched morphology; and 3) they persisted in the distal nerve segment but showed little phagocytosis. We suggest that these cells are mostly activated microglia.

Müller glial cells of the tree shrew retina

The tree shrew is one of the few mammalian species whose retinae are strongly cone dominated, which is usually the case in reptilian and avian retinae. Müller cells of the tree shrew (Tupaia belangeri) retina were studied by transmission electron microscopy of tissue sections and freeze-fracture replicas, by immunolabeling of the intermediate filament protein vimentin in radial paraffin sections and in whole retinae, as well as by intracellular dye injection in slices of retinae. In addition, enzymatically isolated cells were stained by Pappenheim's panoptic staining method. The cells showed an ultrastructure that is similar to other mammalian Müller cells with two exceptions: Due to the extensive lateral fins of cone inner segments, the apical microvilli of Müller cells are arranged in peculiar palisades, and the basket-like Müller cell sheaths around neuronal somata in both nuclear layers consist of unusual multilayered membrane lamellae. Unlike Müller cells in other mammalian species studied thus far, but similar to reptilian and avian Müller cells, those of tree shrews commonly have two or more vitread processes rather than one main trunk. Müller cell densities range between some 13,000 mm-2 in the periphery and about 20,000 mm-2 in the retinal center. Neuron:(Müller)glial cell ratios were estimated to be 7.9:1 in the center and 6.2:1 in the periphery. For each Müller cell, about 1.5 (cone) photoreceptor cells, four or five interneurons of the inner nuclear layer, and about one cell of the ganglion cell layer were counted. This is a much lower number of neurons per Müller cell than in most other mammals studied.

Distinct roles of the receptor tyrosine kinases Tie-1 and Tie-2 in blood vessel formation

Tie-1 and Tie-2 define a new class of receptor tyrosine kinases that are specifically expressed in developing vascular endothelial cells. To study the functions of Tie-1 and Tie-2 during vascular endothelial cell growth and differentiation in vivo, targeted mutations of the genes in mice were introduced by homologous recombination. Embryos deficient in Tie-1 failed to establish structural integrity of vascular endothelial cells, resulting in oedema and subsequently localized haemorrhage. However, analyses of embryos deficient in Tie-2 showed that it is important in angiogenesis, particularly for vascular network formation in endothelial cells. This result contrasts with previous reports on Tie-2 function in vasculogenesis and/or endothelial cell survival. Our in vivo analyses indicate that the structurally related receptor tyrosine kinases Tie-1 and Tie-2 have important but distinct roles in the formation of blood vessels.

Effects of enhanced extracellular ammonia concentration on cultured mammalian retinal glial (Müller) cells

Müller (glial) cells of the neonatal rabbit retina were cultured as confluent monolayers and exposed to enhanced concentrations of ammonia (0.25, 0.5, 1, 3, 7, and 10 mM) in medium for various periods (30 min to 10 d). This caused, in a time- and dose-dependent manner, similar changes in the Müller cells as had previously been described in cultured astrocytes. The most conspicuous events were 1) an increasing size of cell nuclei, 2) an accumulation of phagocytotic vacuoles, and 3) a rearrangement of intermediate filaments. 4) A considerable number of cells died when higher ammonia concentrations were applied for more than 1 h. Simultaneous application of dibutyryl-cyclic adenosine monophosphate (dBcAMP) prevented almost completely both the increase in cell nucleus size and the changes of intermediate filaments, but only partly the early cell death of a subpopulation of cells, and the accumulation of phagocytotic vacuoles. Further changes evoked by enhanced ammonia concentration were 5) an accumulation of lipofuscin-like material ("fatty degeneration") revealed by lipophilic stain, 6) reduced immunoreactivity for cathepsin D, and increased immunoreactivity for 7) glial fibrillary acidic protein, 8) glutamine synthetase, and 9) bcl-2 protooncogene protein. These findings are discussed in respect to the possible underlying pathophysiological mechanisms.

Structure--function relationships in gap junctions

Gap junctions are metabolic and electrotonic pathways between cells and provide direct cooperation within and between cellular nets. They are among the cellular structures most frequently investigated. This chapter primarily addresses aspects of the assembly of the gap junction channel, considering the insertion of the protein into the membrane, the importance of phosphorylation of the gap junction proteins for coupling modulation, and the formation of whole channels from two hemichannels. Interactions of gap junctions with the subplasmalemmal cytoplasm on the one side and with tight junctions on the other side are closely considered. Furthermore, reviewing the significance and alterations of gap junctions during development and oncogenesis, respectively, including the role of adhesion molecules, takes up a major part of the chapter. Finally, the literature on gap junctions in the central nervous system, especially between astrocytes in the brain cortex and horizontal cells in the retina, is summarized and new aspects on their structure-function relationship included.

Quantification of tight junction complexity by means of fractal analysis

The concept of fractal geometry provides an elegant tool for the quantitative and objective structural description of various objects, the fractal analysis. Fractal analysis quantifies the structural complexity of objects by a characteristic singular value, the fractal dimension (FD). It can be estimated, e.g. by the box-counting method and provides a highly integrated measure in the range 1 < FD < 2 for curves extending within a plane. In this study, fractal analysis is used for the first time to evaluate the complexity of the tight junction network between adjoining cells. Bovine brain endothelial cells were cultured under various experimental conditions and the tight junctions were drawn to scale as visualized by the freeze fracture technique. These drawings were analyzed by fractal analysis, and by two other methods commonly used in this field, viz. the strand counting (SC) and complexity index (CI) methods. In contrast to the latter methods, the FD shows no directional preference and therefore no assumptions on the dynamic properties of the network's complexity are required. Thus, FD is demonstrated to provide the most sensitive, reliable and complete measure of tight junction complexity. In combination with SC and CI, additional information can be achieved concerning the directionality of the altered arrangement of tight junctional strands. Our analysis allows for the following conclusions. (1) Defined experimental influences can modify the complexity of tight junctions that are formed between endothelial cells in vitro, and (2) these structural modifications of the tight junctions are mainly due to an altered strand branching pattern.

Astrocytes alter their polarity in organotypic slice cultures of rat visual cortex

The ultrastructure of astrocytes in an organotypic slice culture of the rat visual cortex was investigated using ultrathin sections and freeze-fracture replicas. After a culture period of 9-15 days, a glial scaffold formed that separated the bulk of the slice neuropil from the medium and the underlying plasma clot. However, the glial cells and processes did not build a dense barrier but allowed the outgrowth of neurites. A basal lamina covering the medium-oriented surface of the astrocytes was not found. In freeze-fracture replicas, orthogonal arrays of particles (OAP) were characteristic components of astrocytic membranes. The OAP density in membranes bordering the medium was 35 +/- 13 OAP/microns 2, corresponding to 2.5% of this membrane area; the OAP density in membranes within the slice neuropil was 22 +/- 12 OAP/microns 2, corresponding to 1.4% of this membrane area. Although the difference was significant, it was greatly reduced when comparing OAP densities in endfoot and non-endfoot membranes in vivo. Another node of polarity was recognized in astrocytes of the organotypic slice culture. In membranes of astrocytes bordering upon the medium, the density of non-OAP intramembranous particles (IMP) was clearly higher (1130 +/- 136 IMP/microns 2) than in membranes of astrocytes in the center of the slice (700 +/- 172 IMP/microns 2). This pronounced IMP-related polarity was observed neither in vivo nor in cultured astrocytes. The present study suggests, together with data from the literature, that the distribution of astrocytic OAP across the cell surface is influenced by the existence of a basal lamina and neuronal activity, and that astrocytes possess a more remarkable plasticity of membrane structure than previously suspected.

Modulation of tight junction structure in blood-brain barrier endothelial cells. Effects of tissue culture, second messengers and cocultured astrocytes

Tight junctions between endothelial cells of brain capillaries are the most important structural elements of the blood-brain barrier. Cultured brain endothelial cells are known to loose tight junction-dependent blood-brain barrier characteristics such as macromolecular impermeability and high electrical resistance. We have directly analyzed the structure and function of tight junctions in primary cultures of bovine brain endothelial cells using quantitative freeze-fracture electron microscopy, and ion and inulin permeability. The complexity of tight junctions, defined as the number of branch points per unit length of tight junctional strands, decreased 5 hours after culture but thereafter remained almost constant. In contrast, the association of tight junction particles with the cytoplasmic leaflet of the endothelial membrane bilayer (P-face) decreased continuously with a major drop between 16 hours and 24 hours. The complexity of tight junctions could be increased by elevation of intracellular cAMP levels while phorbol esters had the opposite effect. On the other hand, the P-face association of tight junction particles was enhanced by elevation of cAMP levels and by coculture of endothelial cells with astrocytes or exposure to astrocyte-conditioned medium. The latter effect on P-face association was induced by astrocytes but not fibroblasts. Elevation of cAMP levels together with astrocyte-conditioned medium synergistically increased transendothelial electrical resistance and decreased inulin permeability of primary cultures, thus confirming the effects on tight junction structure and barrier function. P-face association of tight junction particles in brain endothelial cells may therefore be a critical feature of blood-brain barrier function that can be specifically modulated by astrocytes and cAMP levels. Our results suggest an important functional role for the cytoplasmic anchorage of tight junction particles for brain endothelial barrier function in particular and probably paracellular permeability in general.

Fibroblast anchorage to microtextured surfaces

The contact between tissue and the implanted biomaterial is influenced by the micromorphology of the implant surface as well as biomechanical reactions. This effect is mediated by subcellular morphological structures and can affect the anchorage of the material inside the body of the host. The aim of the present study was to ascertain by transmission electron microscopy how human gingival fibroblasts interact with surface events. A special replica technique was used to produce a line pattern of 1 micron pitch with a depth of 1 micron. It was demonstrated, by transmission electron microscopy, that cells seeded on this surface extended cellular processes into the grooves, leading to an intensive contact and probably to mechanical interlocking. The typical morphological structures at several points indicated the presence of focal adhesion sites.