Cytochemical characteristics of astrocytic plasma membranes specialized with numerous orthogonal arrays

Water permeability of aquaporin-4 channel depends on bilayer composition, thickness, and elasticity

Aquaporin-4 (AQP4) is the primary water channel in the mammalian brain, particularly abundant in astrocytes, whose plasma membranes normally contain high concentrations of cholesterol. Here we test the hypothesis that the water permeabilities of two naturally occurring isoforms (AQP4-M1 and AQP4-M23) depend on bilayer mechanical/structural properties modulated by cholesterol and phospholipid composition. Osmotic stress measurements were performed with proteoliposomes containing AQP4 and three different lipid mixtures: 1), phosphatidylcholine (PC) and phosphatidylglycerol (PG); 2), PC, PG, with 40 mol % cholesterol; and 3), sphingomyelin (SM), PG, with 40 mol % cholesterol. The unit permeabilities of AQP4-M1 were 3.3 ± 0.4 × 10(-13) cm(3)/s (mean ± SE), 1.2 ± 0.1 × 10(-13) cm(3)/s, and 0.4 ± 0.1 × 10(-13) cm(3)/s in PC:PG, PC:PG:cholesterol, and SM:PG:cholesterol, respectively. The unit permeabilities of AQP4-M23 were 2.1 ± 0.2 × 10(-13) cm(3)/s, 0.8 ± 0.1 × 10(-13) cm(3)/s, and 0.3 ± 0.1 × 10(-13) cm(3)/s in PC:PG, PC:PG:cholesterol, and SM:PG:cholesterol, respectively. Thus, for each isoform the unit permeabilities strongly depended on bilayer composition and systematically decreased with increasing bilayer compressibility modulus and bilayer thickness. These observations suggest that altering lipid environment provides a means of regulating water channel permeability. Such permeability changes could have physiological consequences, because AQP4 water permeability would be reduced by its sequestration into SM:cholesterol-enriched raft microdomains. Conversely, under ischemic conditions astrocyte membrane cholesterol content decreases, which could increase AQP4 permeability.

Cloning, transport properties, and differential localization of two splice variants of GLT-1 in the rat CNS: implications for CNS glutamate homeostasis

At least two splice variants of GLT-1 are expressed by rat brain astrocytes, albeit in different membrane domains. There is at present only limited data available as to the spatial relationship of such variants relative to the location of synapses and their functional properties. We have characterized the transport properties of GLT-1v in a heterologous expression system and conclude that its transport properties are similar to those of the originally described form of GLT-1, namely GLT-1alpha. We demonstrate that GLT-1alpha is localized to glial processes, some of which are interposed between multiple synapse types, including GABAergic synapses, whereas GLT-1v is expressed by astrocytic processes, at sites not interposed between synapses. Both splice variants can be expressed by a single astrocyte, but such expression is not uniform over the surface of the astrocytes. Neither splice variant of GLT-1 is evident in brain neurons, but both are abundantly expressed in some retinal neurons. We conclude that GLT-1v may not be involved in shaping the kinetics of synaptic signaling in the brain, but may be critical in preventing spillover of glutamate between adjacent synapses, thereby regulating intersynaptic glutamatergic and GABAergic transmission. Furthermore, GLT-1v may be crucial in ensuring that low levels of glutamate are maintained at extrasynaptic locations, especially in pathological conditions such as ischemia, motor neurone disease, and epilepsy.

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.

Three-dimensional morphology of astrocytes and oligodendrocytes in the intact mouse optic nerve

The three-dimensional morphology of astrocytes and oligodendrocytes was analysed in the isolated intact mature mouse optic nerve, by correlating laser scanning confocal microscopy and camera lucida drawings of single cells, dye-filled with lysinated rhodamine dextran or horseradish peroxidase, respectively. These techniques enabled the entire process field of single dye-filled cells to be visualized in all planes and resolved the fine details of glial morphology. Morphometric analysis showed that the processes of all astrocytes had branches ending at the pial surface, on blood vessels, and freely in the nerve; branches ending in the nerve were described to end at nodes of Ranvier in the accompanying paper. Astrocytes were classified into a single morphological population in which each cell subserved multiple functions. The results of this study do not support the contention that astrocytes can be subdivided into two morphological and functional subtypes, namely type-1 and type-2, which have process ending either at the glia limitans or at nodes, respectively. Three-dimensional analysis of oligodendrocyte units, defined as the oligodendrocyte, its processes and the axons it ensheaths, showed the provision of single myelin segments for an average of 19 nearby axons (range 12-35) with a mean internodal length of 138 microns (range 50-350 microns). Mouse optic nerve oligodendrocytes were a homogeneous population and were markedly similar to those in the rat optic nerve. The results of our analysis of oligodendrocyte morphology are consistent with the view that the number and internodal length of myelin sheaths supported by a single oligodendrocyte are related to the diameter of the ensheathed axons.

Astroglial membrane structure is affected by agents that raise cyclic AMP and by phosphatidylcholine phospholipase C

The role of signal transduction mechanisms in the production of the characteristic orthogonal arrays of particle assemblies in the astroglial plasma membrane was investigated in vitro by freeze-fracture electron microscopy. Agents which raise cellular cAMP levels and subsequently activate protein kinase A, such as forskolin (50 microM), isoproterenol (10 microM) and 8-bromo-cAMP (1 mM), increased the density, the number of assemblies per unit area of cleaved cell membrane, and the frequency of astrocytes with assemblies. Agents that lead to the activation of protein kinase C, such as phorbol 12,13-myristate acetate (at 50 nM) and choline-dependent phospholipase C (at 0.01-0.1 U ml-1), did not affect the assembly concentration. Thus, protein kinase A but not protein kinase C appears to be involved in the production of assemblies or their insertion into the astroglial plasma membrane. Although choline-dependent phospholipase C did not affect the astroglial assemblies, it caused the non-assembly, background particles to aggregate. A choline-dependent phospholipase C from a different source (B. cereus) was also active though at a higher concentration. Phospholipases of different specificities, such as phospholipase A2, phospholipase D or inositol-dependent phospholipase C were inactive over a wide range of concentrations. Two other astroglia derived cells, Müller cells and cells of the C6 glioma cell line, were also similarly affected by choline-dependent phospholipase C, while six other cells types including neurons, endothelial cells and fibroblasts were unaffected. It appears that phosphatidylcholine plays a significant role in determining the membrane structure of astrocytes. In a search for a means of isolating the assemblies, the binding of three lectins: ConA, WGA and PNA, conjugated to gold, was tested by label-fracture to ascertain whether the assemblies have an external oligosaccharide component. None of the lectins bound specifically to assemblies.

Effects of a benzodiazepine on the muscle membrane architecture of the rat diaphragm. A freeze-fracture study

1. Midazolam increases, stimulation-independently, the amount of intermembraneous particles on the sarcolemma of the muscle fibres of the diaphragm. 2. Midazolam does not affect the amount of orthogonal arrays of particles on the sarcolemma. 3. Possible mechanisms for the action of midazolam are discussed.

Cytology and organization of reactive astroglia in human cerebellar cortex with severe loss of granule cells: a study on the ataxic form of Creutzfeldt-Jakob disease

In order to investigate the cellular basis of human astrogliosis, we have selected the cerebellar cortex because it provides a relatively simple and geometrical organization of both neuronal and glial populations. A pathological system with severe and progressive loss of granule cells was studied: the ataxic form of Creutzfeldt-Jakob disease, where the tissue geometry is minimally disturbed. The quantitative study revealed a drastic reduction in the numerical density of granule cells in the Creutzfeldt-Jakob disease cerebellum, and a significant increase in the numerical density of astrocytes. Karyometric analysis showed that the nuclear area was significantly greater in reactive astroglial cells than in normal astroglia. Glial fibrillary acidic protein immunocytochemistry revealed astroglial hypertrophy, but the geometry and spatial domains of astroglial subtypes were strictly preserved. Vimentin expression was detected in Bergmann glia and in certain astrocytes of the granular layer. Ultrastructural analysis showed that reactive astroglia had large nuclei, with expanded interchromatinic regions which contained clusters of interchromatin granules and nuclear bodies, and prominent reticulate nucleoli. In the cytoplasm, hypertrophied bundles of intermediate filaments were observed, some of them associated with the nuclear envelope. Numerous adhering and gap junctions were also found among reactive astroglial cells. Perivascular glial processes showed a terminal web of intermediate filaments and a conspicuous plasmalemmal undercoat. Interendothelial tight junctions were preserved. Our results suggest that the severe loss of granule cells induces a highly ordered astroglial response which tends to preserve the geometry of the astroglial scaffold, the domains of each astroglial subtype, the neuronal microenvironmental conditions and the efficiency of the blood brain barrier, in order to promote neuron survival.

Astrocytic orthogonal arrays of intramembranous particle assemblies are modulated by brain endothelial cells in vitro

Solo astroglial cultures have randomly distributed, intramembranous, orthogonal arrays of particles (assemblies) which are only revealed by freeze-fracture electron microscopy. Co-culturing astrocytes with brain endothelial cells brought about localized, tightly packed assembly aggregates and greatly increased the overall assembly density. Cytosol homogenates of freeze-thawed brain endothelial cells caused a transient increase in astroglial assembly numbers. These results, taken together with the fact that astrocytes in vivo have the highest concentration of perivascular sites, suggest that brain endothelial cells influence the distribution and concentration of astrogial assemblies both in vivo and in vitro through cellular interactions. Meningeal cells and fibroblasts also augmented the astroglial assembly densities in co-culture, while neuronal cells (cerebellar granule cells and PC12 cells primed with nerve growth factor) and other control cell types did not affect assembly number in co-culture with astrocytes. Moreover, brain endothelial cells did not induce any formation of assemblies in the membranes of two transformed astroglial cell lines.

Orthogonal arrays of intramembranous particles in the Müller cell and astrocyte endfoot membrane of rabbit retina. Postnatal development and adulthood

The freeze-fracture technique was applied to the retina of early postnatal and adult rabbits to investigate the distribution and density of orthogonal arrays of intramembrane particles (OAP) within the vitread endfoot membranes. In adult animals, two distinct types of endfoot membranes were observed within the central myelinated retina but not in the retinal periphery. One type of endfoot membranes contained low density of individual 'background' particles, and a more or less stripe-like pattern of OAP; this type was found only within the myelinated centre and is concluded to represent membranes of retinal astrocytes. The other type of endfoot membranes was rather tightly packed with individual 'background' particles, and contained OAP which formed rows only at the margins of footplates; this kind of membrane was found throughout the whole retina, and represents Müller cell endfeet. The density of OAP in both types of endfoot membranes was higher within the central myelinated retina than in the Müller cell endfeet of the retinal periphery. In early postnatal retinae, a discrimination between the two types of endfeet was impossible. At the day of birth, only very few OAP were observed, and the majority of footplates were free of OAP. Within the next 10 days, both density and size of OAP were found to increase but fail to reach adult levels. Quantitative data are presented with the hope of providing a basis for future correlation with functional maturation of rabbit retinal glia.

Orthogonal arrays of particles in astroglial cells: quantitative analysis of their density, size, and correlation with intramembranous particles

Astroglial cells were investigated by means of freeze-fracture in normal rat and mouse brain, cell culture and human gliomas. Membranes of these cells were quantitatively analyzed for their intramembranous particles (IMPs) and orthogonal arrays of particles (OAPs). Measurement of the size of OAPs and IMPs has permitted the search for a correlation between the 7-nm IMPs, which are distributed randomly in the membrane, and the subunits of OAPs (OAP-Su, also 7 nm in diameter). Using cultured astroglial cells treated with basic fibroblast growth factor (bFGF), arginine vasopressin, or sorbitol, good evidence for a relationship between the density of 7-nm IMPs and the size of OAPs can be demonstrated. These findings led to a hypothetical model of OAP modulation. A preliminary report has been published elsewhere (Neuhaus and Wolburg, 1989).

Developmental alterations in membrane organization of rat subpial astrocytes

Subpial astrocytic processes were examined in developing rats, mainly with complementary replicas, to see how orthogonal arrays of particles (OAs) are formed and become numerous in membranes covered by basal lamina. Only a few (4.2%) endfeet in the membranes contacting the basal lamina (subpial membranes) had acquired OAs by the 19-day foetal stage. The number of endfeet provided with OAs increased drastically in the prenatal period, continued to increase at birth (P0), and somewhat more slowly in the early postnatal period (P0-P3), reaching 100% at P10. There were neuronal processes as well abutting on the basal lamina at the pial surface but they were easy to distinguish from astrocytic endfeet because of their larger intramembrane particles (IMPs), which are sparsely distributed and in patch-like aggregations. The distribution density of OAs in differentiated astrocytic endfeet also increased very gradually with age until P0, a little faster in the early postnatal period, and drastically from P10 to adult. Ordinary globular IMPs increased in number with age and continued to increase in the lateral membrane where OAs were still very few, though less rapidly in the subpial membrane as OAs became numerous. With maturation, larger IMPs became conspicuous in the lateral membrane but not in the subpial, suggesting that larger IMPs were predominantly used to form OAs. We have proposed the idea that relatively large IMPs line up to form single linear arrays (SLs), appearing as grooves on the E face, and that occasionally some SLs line up in multiple rows [multiple linear arrays (MLs)] and that SLs or MLs fuse with one another to become rod-like strands, then divide into squares to become OAs. SLs and MLs appeared ontogenetically earlier than OAs, and continued to appear in membranes provided with OAs. In areas where membranes were bent, transition of these three structures was observable and the proportion of OAs increased with age. Further, in such areas, alignment of OAs was different according to membrane curvature: concentric in and around protrusions, perpendicular to the edge of invaginations. This unique association of OA alignment with membrane curvature suggests that OAs contribute to some membrane stability in the area covered by the basal lamina and provide the membrane with special resistance to bending.

Substructure of cisternal organelles of neuronal perikarya in immature rat brains revealed by quick-freeze and deep-etch techniques

Membrane-bounded organelles possessing cisternae, i.e., rough endoplasmic reticulum and Golgi apparatus, in immature rat central neurons were examined by quick-freeze and deep-etch techniques to see how their intracisternal structures are organized and how ribosomes are associated with the membrane of the endoplasmic reticulum. Cisternae of endoplasmic reticulum, 60-100 nm wide, were bridged with randomly-distributed strands (trabecular strands, 12.5 nm in mean diameter). Luminal surfaces of cisternae of the endoplasmic reticulum were decorated with various-sized globular particles, some as small as intramembrane particles, and others as large as granules formed by soluble proteins seen in the cytoplasm. A closer examination revealed much thinner strands (3.3 nm in mean diameter). Such thin strands were short, usually winding toward the luminal surface, and sometimes touching the luminal surface with one end. Ribosomes appeared to be embedded into the entire thickness of cross-fractured membranes of endoplasmic reticulum, that is, their internal portions appeared to be situated at almost the same level as the cisternal luminal surface. From the internal portion of ribosomes, single thin strands occasionally protruded into the lumen, suggesting that these thin strands were newly synthesized polypeptides. A horizontal separation within ribosomes appeared to occur at the same level as the hydrophobic middle of the membrane of the endoplasmic reticulum. Interiors of the Golgi apparatus cisternae, which were much narrower than cisternae of endoplasmic reticulum, were similarly bridged with trabecular strands, but the Golgi trabecular strands were thinner and more frequent. Their cisternal lumina were also dotted with globular particles. No identifiable profiles corresponding to the thin strands in the endoplasmic reticulum were observed. Golgi cisternae showed a heterogeneous distribution of membrane granularity; the membrane in narrow cisternal space was granule-rich, while that in expanded space was granule-poor, suggesting a functional compartmentalization of the Golgi cisternae.

Orthogonal arrays of particles in plasma membranes of Müller cells in the guinea pig retina

Plasma membranes of guinea pig Müller cells were examined with a freeze-fracture technique to see how orthogonal arrays are distributed in the avascular retina. Examination of the portion approximately intermediate between the optic disc and equator of the eyeball showed that all end-feet of Müller cells were provided with arrays. Orthogonal arrays were concentrated on vitreal end-foot membranes, i.e., membranes that were covered by the basal lamina and contacted the vitreous body, called vitreal membranes here. The arrays were rarely observed in the portions of end-feet that did not contact the vitreous body, called lateral membranes. The distribution density of arrays in the vitreal membranes was 122.5 +/- 45.3/microns2, which was over 10 times higher than that (9.6 +/- 9.6/microns2) in the lateral membranes. The arrays became numerous and extended in shape at the periphery of the vitreal membrane, characteristically aligned in rows at the border where vitreal met lateral membrane, but never intruded into the domain of lateral membrane. Some arrays were composed of loosely attached particles and/or rod-like profiles. Sometimes rod-like profiles, 9-13 nm wide and 20-50 nm long, called linear structures here, were isolated, and sometimes they appeared in rows. Ordinary intramembrane particles (IMPs) were significantly smaller and less numerous in vitreal than in lateral membranes. IMPs larger than 9 nm in diameter were significantly fewer in the vitreal membranes, which suggests that they have been consumed to form the arrays. Although the distribution of orthogonal arrays is similar to that of K+ channels (Newman: J. Neurosci., 7:2423-2432, 1987), we consider the array an unlikely candidate for the ion channel, because its subunit particles do not protrude onto either the inner or outer surface of the membrane (Gotow and Hashimoto: J. Neurocytol., 17:399-413, 1988). Judging from their unique alignment in rows where the membrane is bent and vitreal and lateral membranes meet, the arrays may contribute to some membrane stability, resisting the physical tension at the interface with mesenchymal tissue.

Müller (glial) cells but not astrocytes in the retina of the goldfish possess orthogonal arrays of particles

Müller cells as the main glial component of the retina were investigated in the goldfish by means of ultrathin sections and freeze-fracture replicas. In the optic nerve head, they were directly compared with astrocytes. Whereas astrocytic endfeet bordering the vitreous body can easily be identified by their dense bundles of intermediate filaments, scarce membranous organelles, paravitreous caveolae, and lateral desmosomes, Müller cell endfeet reveal a looser arrangement of intermediate filaments, a characteristic pattern of triangularly shaped endoplasmic reticulum, large and pale mitochondria, and, if at all, very few desmosome-like junctions. The paravitreous membranes at the cytoplasmic face are covered by a fuzzy coat, which is less marked in astrocytic endfeet. Caveolae are lacking. Considering the freeze-fracture architecture of the membranes of both glial cell types, the Müller cells reveal orthogonal arrays of particles (OAP), which were predominantly located opposite to the inner limiting membrane; their density (109 +/- 33 OAP/microns 2) decreases abruptly with the loss of the contact between membrane and vitreous body. In contrast, astrocytes of the optic nerve head in the retina do not show any OAP in their membranes at all and are interconnected by tight junctions and desmosomes. The hypothesis suggesting that OAP might be correlated with K+ channels involved in the spatial buffering of the extracellular space is reconsidered with comparative reference to recent electrophysiological data. Further, the heterogeneity of Müller cell and astrocyte membrane equipment with OAP in the goldfish is briefly discussed.

Deep-etch structure of astrocytes at the superficial glia limitans, with special emphasis on the internal and external organization of their plasma membranes

The cytoskeletal system in rat subpial astrocytes and the relationship between astrocytic plasma membrane and basal lamina or cytoplasmic components were examined with a quick-freeze deep-etch technique, mainly using chemically fixed tissues. Attention was focused on the way intramembrane particles (IMPs), particularly orthogonal arrays, are organized in the membranes and related to extramembrane components. The basal lamina was composed of a sheet-like network of strands (4-9 nm thick), some, which we have called 'trabecular' strands, extending through the lamina lucida to touch the astrocytic membrane at irregular intervals. The trabecular strands usually formed a bulbous structure where they touched the membrane, but sometimes appeared to intrude directly into the external lipid layer. The orthogonal arrays did not extend to the outer true surface, and no special structure was detectable in association with them. Small spherical protrusions (7-9 nm in diameter), related to neither the trabecular strands nor the arrays, were observed in the outer surface. Judging from their size and distribution, these are probably tops of tall globular IMPs. In the inner or cytoplasmic true surface, protrusions were relatively numerous; some were large, 15-20 nm in diameter, while others were small (8-10 nm). Some of the small protrusions were identified as transmembrane components. Although protrusions were more conspicuous in the inner than in the outer surface, none of them provided images related or similar to the orthogonal arrays. Some protrusions in the inner surface were connected with thin (4-5 nm) or thick (approximately 10 nm) filaments constituting the underlying network. The thin filaments were also anchored to the intermediate filaments which lay parallel with the astrocytic membranes. In the cytoplasm, the intermediate filaments were firmly packed to form bundles. Because the orthogonal arrays are probably embedded within the astrocytic membrane, they may not serve as a transmembrane channel but rather contribute to some stabilizing function for the membrane.

Relationship between orthogonal arrays of particles and tight junctions as demonstrated in cells of the ventricular wall of the rat brain

Ependymal cells in the ventricular wall and in several circumventricular organs of the rat were compared by means of freeze-fracturing. In principle, tight junctions and orthogonal arrays of particles (OAP) do not coexist in the cells bordering the ventricular wall: (1) Ordinary ependymal cells of the rat possess OAP and are devoid of tight junctions. (2) Epithelial cells of the rat choroid plexus are connected by tight junctions; OAP are lacking here. In some cases, however, tight junctions and OAP coexist in the same cell. In the boundary zone between choroid plexus and ependyma of the rat, the density of OAP is very low, whereas the tight junctions are well developed. In the subfornical and the subcommissural organ (SCO) of the rat both structures are poorly developed; in the SCO they occur segregated in different membranous areas. An overview of the literature confirms that tight junctions and OAP mostly exclude each other. The possibility that in astrocytes and ependymal cells tight junctions may have been replaced by OAP during phylogeny is briefly discussed.

Molecular specialization of astrocyte processes at nodes of Ranvier in rat optic nerve

The HNK-1 and L2 monoclonal antibodies are thought to recognize identical or closely associated carbohydrate epitopes on a family of neural plasma membrane glycoproteins, including myelin-associated glycoprotein, the neural cell adhesion molecule, and the L1 and J1 glycoproteins, all of which have been postulated to play a part in mediating cell-cell interactions in the nervous system. We have used these two antibodies in immunofluorescence and immunogold-electron microscopic studies of semithin and ultrathin frozen sections of adult rat optic nerve, respectively, and we show that they bind mainly to astrocyte processes around nodes of Ranvier. Most other elements of the nerve, including astrocyte cell bodies and large astrocytic processes, are not labeled by the antibodies. To our knowledge, this is the first demonstration that perinodal astrocyte processes are biochemically specialized. We provide evidence that one of the HNK-1+/L2+ molecules concentrated around perinodal astrocyte processes is the J1 glycoprotein; our findings, taken together with previously reported observations, suggest that the other known HNK-1+/L2+ molecules are not concentrated on these processes. Since anti-J1 antibodies previously have been shown to inhibit neuron to astrocyte adhesion in vitro, we hypothesize that J1 may play an important part in the axon-glial interactions that presumably are involved in the assembly and/or maintenance of nodes of Ranvier.

Specialization of astrocytic membrane at glia limitans in rat optic nerve: freeze-fracture observations

The ultrastructure of astrocytic foot processes at the glia limitans in rat optic nerve was studied by freeze-fracture. Two classes of astrocytic end-foot processes were observed. Most astrocytic foot processes in this region show a high density of orthogonal arrays of P-face particles (assemblies) that are randomly oriented. However, astrocytic foot processes with highly organized columns of assemblies are also observed. These columns are 1-4 assemblies wide (approximately 0.1 micron), with the center-to-center distance between the columns being relatively constant (approximately 0.17 micron). Columns of at least 4 micron length have been observed. The regional specialization of the astrocyte membrane and the differentiation of subpial astrocytes into two structural classes may have important functional implications.

Distribution patterns of orthogonal arrays and alkaline phosphatase in plasma membranes of satellite cells in rat spinal ganglia

The plasmalemmal structure of satellite cells in the lumbar spinal ganglia of rat was examined by freeze-fracture and by a cytochemical method for the demonstration of alkaline phosphatase activity. Plasma membranes of satellite cells are the only ones in the ganglia to contain, in addition to globular intramembrane particles, orthogonal arrays of particles 6-7 nm in diameter. The arrays are most concentrated in the portions of the membranes contacting the basal lamina, or outer membranes; they decrease considerably in number in lateral membranes, and are rare in the membrane regions adjacent to the neuronal perikaryon, or inner membranes. Such gradual decrease in array density in satellite cells suggests regional differences of plasma membrane properties within the same cell. Alkaline phosphatase, which was chosen as a cytochemical marker for membrane activity because of its relation to transport function, localizes to inner and lateral membranes, and not to outer membranes of satellite cells. The absence of correlation between localization of orthogonal arrays and such enzymatic activity suggests that the membranes provided with many arrays possess some characteristics different from other membranes that may exclude transport activity. The possible significance of orthogonal arrays and their close association with the basal lamina are discussed.

Freeze-fracture cytochemistry of sympathetic ganglia. Distribution of filipin and tomatin induced membrane deformations in neurons and satellite cells

Application of filipin to sympathetic ganglia results in membrane deformations in both the neurons and the satellite cells. The plasma membranes of the principal ganglion cells show a non-homogeneous distribution of filipin induced deformations with fewer deformations in the perikaryal plasma membrane than in the nerve fiber membrane. The filipin induced membrane lesions are correlated to the number of IMPs of the neuronal membrane i.e. a high density of intramembrane particles (IMP) gives fewer deformations and vice versa. The membrane of the satellite cells contain a higher density of probe induced lesions than the neuronal membrane. The filipin induced deformations in the satellite cells are not correlated to the number of IMPs or to the number of orthogonal arrays of small particles (OAP). Specialized membrane areas such as the gap junction is always devoided of filipin induced lesions. A similar distribution of membrane lesions was found when tomatin was used instead of filipin. These results indicate a possible difference in lipid content between various parts of the neurons and between the neuronal and satellite cell plasma membrane in guinea pig sympathetic ganglia.

Plasma membrane organization of astrocytes in elasmobranchs with special reference to the brain barrier system

The structural machinery contributing to the blood-brain barrier in elasmobranchs has been examined mainly using freeze-fracture techniques. Capillary endothelial cells, which show local aggregations of pinocytotic vesicles and infrequent fenestrations, are connected by poorly developed tight junctions. Astrocytic processes investing the capillary are linked by well-developed tight junctions between lateral membranes immediately beneath the perivascular space. The tight junctions consist of continuous strands of multiple layers coursing circumferentially around the astrocytic processes parallel to one another as well as to the perivascular space. The presence of intramembrane particles (IMPs) within E-face grooves may result in discontinuities in IMP rows on the P-face. Thus, in compensation for the capillary endothelium, perivascular astrocytes constitute the morphological site of the blood-brain barrier in elasmobranchs. Continuous strands of tight junctions are also detected between astrocytic processes forming the glia limitans at the brain surface. These may act as a barrier between meningeal connective tissue and brain parenchyma. Astrocytic membranes have numerous IMPs of 8-9 nm in diameter on their P-faces. These IMPs are uniformly distributed so that astrocytic membranes are easily distinguished from neuronal membranes even in the neuropil. Ependymal cells also have numerous IMPs in all their membrane domains. Orthogonal arrays are not detected in either astrocytic or ependymal plasma membranes.