Orthogonal arrays of particles in plasma membranes of the gastric parietal cell

Structure and functions of aquaporin-4-based orthogonal arrays of particles

Orthogonal arrays or assemblies of intramembranous particles (OAPs) are structures in the membrane of diverse cells which were initially discovered by means of the freeze-fracturing technique. This technique, developed in the 1960s, was important for the acceptance of the fluid mosaic model of the biological membrane. OAPs were first described in liver cells, and then in parietal cells of the stomach, and most importantly, in the astrocytes of the brain. Since the discovery of the structure of OAPs and the identification of OAPs as the morphological equivalent of the water channel protein aquaporin-4 (AQP4) in the 1990s, a plethora of morphological work on OAPs in different cells was published. Now, we feel a need to balance new and old data on OAPs and AQP4 to elucidate the interrelationship of both structures and molecules. In this review, the identity of OAPs as AQP4-based structures in a diversity of cells will be described. At the same time, arguments are offered that under pathological or experimental circumstances, AQP4 can also be expressed in a non-OAP form. Thus, we attempt to project classical work on OAPs onto the molecular biology of AQP4. In particular, astrocytes and glioma cells will play the major part in this review, not only due to our own work but also due to the fact that most studies on structure and function of AQP4 were done in the nervous system.

Aquaporin expression in normal and pathological skeletal muscles: a brief review with focus on AQP4

Freeze-fracture electron microscopy enabled us to observe the molecular architecture of the biological membranes. We were studying the myofiber plasma membranes of health and disease by using this technique and were interested in the special assembly called orthogonal arrays (OAs). OAs were present in normal myofiber plasma membranes and were especially numerous in fast twitch type 2 myofibers; while OAs were lost from sarcolemmal plasma membranes of severely affected muscles with dystrophinopathy and dysferlinopathy but not with caveolinopathy. In the mid nineties of the last century, the OAs turned out to be a water channel named aquaporin 4 (AQP4). Since this discovery, several groups of investigators have been studying AQP4 expression in diseased muscles. This review summarizes the papers which describe the expression of OAs, AQP4, and other AQPs at the sarcolemma of healthy and diseased muscle and discusses the possible role of AQPs, especially that of AQP4, in normal and pathological skeletal muscles.

Membrane organization and function of M1 and M23 isoforms of aquaporin-4 in epithelial cells

Aquaporin-4 (AQP4) water channels exist as heterotetramers of M1 and M23 splice variants and appear to be present in orthogonal arrays of intramembraneous particles (OAPs) visualized by freeze-fracture microscopy. We report that AQP4 forms OAPs in rat gastric parietal cells but not in parietal cells from the mouse or kangaroo rat. Furthermore, the organization of principal cell OAPs in Brattleboro rat kidney is perturbed by vasopressin (arginine vasopressin). Membranes of LLC-PK1cells expressing M23-AQP4 showed large, abundant OAPs, but none were detectable in cells expressing M1-AQP4. Measurements of osmotic swelling of transfected LLC-PK1cells using videomicroscopy, gave osmotic water permeability coefficient ( Pf) values (in cm/s) of 0.018 (M1-AQP4), 0.019 (M23-AQP4), and 0.003 (control). Quantitative immunoblot and immunofluorescence showed an eightfold greater expression of M1- over M23-AQP4 in the cell lines, suggesting that single-channel pf(cm3/s) is much greater for the M23 variant. Somatic fusion of M1- and M23-AQP4 cells ( Pf= 0.028 cm/s) yielded OAPs that were fewer and smaller than in M23 cells alone, and M1-to-M23 expression ratios (∼1:4) normalized to AQP4 in M1 or M23 cells indicated a reduced single-channel pffor the M23 variant. Expression of an M23-AQP4-Ser111Emutant produced ∼1.5-fold greater single-channel pfand OAPs that were up to 2.5-fold larger than wild-type M23-AQP4 OAPs, suggesting that a putative PKA phosphorylation site Ser111is involved in OAP formation. We conclude that the higher-order organization of AQP4 in OAPs increases single-channel osmotic water permeability by one order of magnitude and that differential cellular expression levels of the two isoforms could regulate this organization.

Effects of dexamethasone on the differentiation of membrane structure in cultured astrocytes

Astrocytic processes investing vascular structures or forming the surface of mammalian brain have large numbers of orthogonally packed aggregates of intramembrane particles, termed "assemblies." Similar particle aggregates are expressed by astrocytes derived from neonatal rat forebrain in secondary culture, but they are much more uniformly distributed across the membranes of the cultured cells. Dexamethasone, a potent glucocorticoid, affects the differentiation of astrocyte membrane structure in two patterns, depending on the rate of proliferation in the culture. When confluent secondary cultures of astrocytes are exposed to 5 microM dexamethasone, the densities of assemblies increase, and in some cells approach the values present in the glial limitans in vivo. However, when rapidly proliferating astrocytes are exposed to dexamethasone during the first week of secondary culture, most of the astrocytes fail to express any assemblies. The rate of astrocyte proliferation is slowed, and a lower cell density is reached during the first 2 weeks of secondary culture in dexamethasone. The suppression of assemblies is transient: as the cultures approach confluence, the proportion of cells expressing assemblies increases to nearly control levels, and the density of assemblies increases to greater than control values in some astrocytes. Certain of the effects of dexamethasone on cultured astrocytes may have relevance for understanding the mechanism(s) of its action in treating cerebral edema.

A novel epitope expressed on the surface of developing and mature astrocytes

Plasmalemmal fractions from cultured astrocytes have been used as the immunogen in generating a monoclonal antibody, termed 8C10, which binds to the surface of cultured astrocytes of the rat. 8C10 immunoreactivity is present on the membrane surface of cultured type 1 astrocytes, type 2 astrocytes, oligodendrocytes, meningeal cells, and 02A progenitor cells, and it persists after aldehyde fixation. The antibody also stains aldehyde-fixed central nervous system, in a pattern which suggests that the plasma membranes of fine astrocytic processes in adult neuropil express the epitope. Astrocytic perikarya and processes in white matter are also stained, but there is no immunoreactivity present in neuronal processes or perikarya. Astrocytic processes in developing cerebellar cortex are stained at postnatal ages when some of these processes are guiding the migration of neuronal perikarya.

Serum influences the differentiation of membrane structure in cultured astrocytes

The membranes of mammalian astrocytic processes apposed to blood vessels or forming the surface of the brain contain high concentrations of a characteristic intramembrane particle aggregate, termed "assemblies." In order to identify developmental processes which contribute to this remarkable regional specialization of membrane structure, we have devised culture conditions which support the differentiation of assemblies in secondary cultures of astrocytes derived from neonatal rat forebrain. We report here that different lots of fetal calf serum vary dramatically in their capacity to support the differentiation of assemblies. Fetal calf serum thus appears to exert two distinct influences on astrocyte development: it promotes the differentiation of type 2 astrocytes from bipotential precursor cells, as shown by others, and it influences the density of assemblies in type 1, flat, GFAP-immunoreactive astrocytes in our secondary cultures. Horse serum and defined media also support the appearance of assemblies in flat, GFAP-immunoreactive astrocytes. The separate effects of serum supplementation upon cell lineage and membrane differentiation have to be carefully considered in studies designed to examine factors influencing astrocytic development in vitro.

Orthogonal arrays of intramembrane particles in the supporting cells of the guinea-pig vestibular sensory epithelium

Membrane specializations of the contact region between afferent nerve endings and supporting cells of the sensory epithelia of guinea-pig vestibular endorgans were examined by thin-section and freeze-fracture electron microscopy. The calyx-type nerve endings (C-endings) are separated from supporting cells (SC) by a 25-30 nm space. At irregular intervals along the upper lateral surface of supporting cells, the intercellular space narrows markedly to form special close contacts between the C-ending and SC plasma membranes. Freeze-fracture replicas reveal membrane specializations--orthogonal arrays of particulate units--in the region where the close intercellular contacts were found in sections. Orthogonal arrays consisting of from 5 to 20 units were observed on the cytoplasmic (P) fracture face of the lateral SC plasma membrane. These particulate units from a 12 x 12-nm square, and each unit is composed of four 6-nm subunits. Possible roles of the orthogonal arrays are discussed.

Structural alterations in the membrane of the slow muscle fiber of Rana temporaria after denervation

Mixed fiber bundles as well as separated slow and fast fibers from normal and denervated muscles of Rana temporaria were freeze-fractured. The membranes of both fiber types are distinguished in this species by the presence of fairly regularly distributed particle aggregates or arrays of different shapes and sizes; the number per unit area of the membrane is six times higher in fast than in slow fibers. The intramembrane particle (IMP) density is higher in slow than in fast fibers. After denervation, the fast fiber membrane structure does not change whereas the slow fiber membrane acquires the characteristics of the fast fiber, i.e., an increase in the density of particle arrays and a decrease in IMP density. These changes in the slow fiber membrane are compared to the altered physiological properties of this fiber type after denervation.

Orthogonal arrays of particles in the gastric parietal cell of the rat: differences between superficial and basal cells in the gland and after pentagastrin or metiamide treatment

The basolateral plasma membrane of gastric parietal cells is characterized by the presence of orthogonal arrays of particles revealed by the freeze-fracture technique. These arrays were quantitatively assessed in freeze-fracture replicas of the gastric mucosa at two different levels of the gastric gland and after pentagastrin and metiamide treatment. The arrays were small and scarce in parietal cells located in the upper part of the gland, while they were markedly more abundant in parietal cells situated at the base of the gland. In both superficial and basal cells, the concentration of the arrays was significantly decreased after pentagastrin or metiamide treatment. This decrease was not due to an increase in the surface area of the basal plasma membrane. These results indicate that the concentration of the orthogonal arrays is a distinctive feature between superficial and basal parietal cells and that the arrays can be modulated by parietal cell function.

Disturbance of plasmalemmal astrocytic assemblies in focal and selective cerebral ischemia

Selective cerebral ischemia was induced in the caudate nucleus of seven normothermic anesthetized cats through transorbital clamping of the anterolateral penetrating lenticulostriated arteries. The plasmalemma of astrocytic foot processes has been studied with the freeze-fracture technique and conventional electron microscopy 10, 15 and 30 min after ischemia. After 15 min of circulatory arrest, assemblies of intramembrane particles (IMPs) disappear in some areas of astroglial perivascular plasmalemma in the ischemic caudate nucleus. Interastrocytic gap junctions do not change significantly. 30 min after ischemia, the pericapillary astroglial end foot is expanded and organelles are greatly perturbed (cytotoxic edema). Although the function of astrocytic intramembrane particle assemblies is unclear, it is postulated that the disappearance of this membrane specialization may play a role in the pathophysiology of cytotoxic astroglial edema.

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.

Orthogonal arrays in normal and injured respiratory airway epithelium

Orthogonal arrays are found on plasma membranes of glial cells, in the central nervous system, on muscle plasma membranes at neuromuscular junctions, and on a variety of epithelial cells. These structures have been correlated with ion flux. With the aid of freeze fracture technique, orthogonal particle arrays were found on plasma membranes on airway epithelial cells of rats and hamsters. They have been found in abundance at the base of secretory cells throughout normal airway epithelium. These structures were found to increase in number during regeneration in response to injury and they were found in great numbers on plasma membranes of all airway cells in response to acute and chronic NO2 exposure. The lateral and basal plasma membranes of the respiratory epithelium are a new source for studying orthogonal arrays. The normal number and distribution of these arrays can be perturbed in response to mechanical and chemical injury.

Cytochemical characteristics of astrocytic plasma membranes specialized with numerous orthogonal arrays

Astrocytic membranes contacting the basal lamina are found to be less affected by filipin than subjacent lateral membranes. An abrupt change in density of lesions induced by filipin creates a border between subpial and lateral membranes at the glia limitans. This means that orthogonal array-crowded membranes may contain relatively less cholesterol than other astrocytic membrane domains. Another possible explanation for filipin resistance is also considered in relation to aggregated intramembrane particles of orthogonal arrays and/or membrane-associated filamentous elements including the basal lamina. The polygonal particle junction between astrocytic processes located just below the subpial membrane is strongly resistant to the action of filipin. Both membrane-associated enzymes, i.e. alkaline phosphatase (AlkPase) and Na+,K+-ATPase are commonly detected only in perivascular astrocytic membranes, and not in subpial membranes, suggesting a regional differentiation in function of astrocytic membranes. There are variations in the reactive deposits particularly of those for Na+,K+-ATPase. It is apparent that the distribution polarity of orthogonal arrays is not connected with that of either AlkPase or Na+,K+-ATPase. Judging from the relative resistance to filipin, however, astrocytes throughout the C.N.S., having domains specialized with orthogonal arrays, may possess a unique stabilizing mechanism for their own membranes contacting the basal lamina.

Membrane structure in cultured astrocytes

Astrocytes cultured from the brains of neonatal rat pups acquire at least two specializations of intramembrane particle distribution: 'assemblies' and gap junctions. The number and appearance of assemblies in the cultured astrocytes is not markedly influenced by the presence or absence of a collagen substrate, and the range of concentrations of assemblies in astrocytic membranes is fairly stable from 7 through 28 days in culture. The assemblies are not concentrated in apposition to the substrate, even though the astrocytic membranes containing the highest concentration of assemblies in vivo are apposed to basal lamina. Quantitative analysis shows that assemblies are not uniformly distributed over the plasmalemma of a single cell, raising the possibility that the nature of cells around an astrocytic process may influence its membrane composition in vitro.

Plasma membrane specializations in the cells of the kidney distal segment of the lamprey, Lampetra japonica (von Martens)

The unique and highly specialized structural features of the plasma membrane in the cells of the kidney distal segment of the lamprey, Lampetra japonica, were studied by electron microscopy. The cells of the distal segment are largely filled by a continuous network of cytoplasmic tubules which are derived from the basolateral plasma membrane. Thin sections and freeze-fracture replicas of the membrane of the cytoplasmic tubules show spirally wound parallel rows of particles. The rows are approximately 17 nm apart and are wound at a pitch of approximately 45 degrees with respect to the major axis of the tubules. Another type of membrane specialization was found in the freeze-cleaved surface of the basolateral plasma membrane. It consists of large square aggregations of membrane particles containing 100-400 cuboidal subunits. The distribution of these particles in this cell, as well as in other systems in which they have been noted, suggests a polarization of membrane activity.

Evidence for orthogonal arrays of particles in the plasma membranes of olfactory and vomeronasal sensory neurons of vertebrates

Plasma membranes of sensory neurons from the olfactory and vomeronasal neuroepithelia of the male rat and olfactory neuroepithelium of the tiger salamander (Ambystoma tigrinum) have been examined, using the freeze-fracture technique, for the presence and morphology of orthogonal arrays of particles (OAP). Numerous OAP were scattered on the P-face of plasma membranes of the dendrites and cell bodies from rat vomeronasal sensory neurons. The OAP were 720 +/- 200 nm2 in area and they consisted of 4 to 20 particles whose centre-to-centre distance was about 7 nm. On the E-face, complementary orthogonal arrays of pits were observed. No OAP were detected in the olfactory sensory neurons of the rat. In the dendritic and perikaryal plasma membranes of the tiger salamander olfactory sensory neurons, OAP 2230 +/- 970 nm2 in area were observed on the P-face. The OAP consisted of 12 to 36 particles. The centre-to-centre distance of the particles was about 7 nm. In the olfactory receptor cell plasma membranes of this species, OAP formed complexes of 2 to 28 individual OAP, the longitudinal axes of which were usually arranged in parallel. Complementary complexes of orthogonal arrays of pits were observed on the E-face.

Morphology of cerebral endothelium and astrocytes as determinants of the neuronal microenvironment

The morphological features of the blood-brain barrier to macro-molecules under normal and perturbed conditions are reviewed in the context of some recent investigations. The electric charge on molecules of horseradish peroxidase (HRP) affect its pinocytosis and intracellular fate which pertains to problems of distinguishing endocytosis from vesicular transport across endothelium. When the barrier is opened, the number of pits, vesicles and tubules increases. Such cerebral endothelium resembles normal endothelium of certain fish where numerous membrane invaginations do not signify vesicular or tubular transport. However, such transport has not been entirely ruled out in reactive endothelium. Another route of exudation during barrier opening may be via patent endothelial junctions, especially during intravascular infusion of hyperosmotic solutions. The permeability of the tight junctions, however, is not reflected unequivocally by its intramembranous structure. Although astrocytes do not provide a barrier to the extracellular flow of solutes, their ubiquity may enable them to modify the composition of perineuronal fluid. Their orthogonal arrays of intramembranous particles may be involved. The number of assemblies increases in astrocytes reacting to trauma and to the extracellular accumulation of lactate and CO2. The assemblies might thus participate in the transport of catabolites to and from extracellular fluid.

Particle assemblies in astrocytic plasma membranes are rearranged by various agents in vitro and cold injury in vivo

Distinct aggregates of small intramembranous particles and assemblies characterize the P-face of freeze-fractured astrocytic membranes. To test the lability of the assemblies, astrocytes were treated in vitro with different chemical agents and in vivo by cold injury. The assemblies appeared either to contain or be associated with protein because exposure to medium containing cycloheximide, an inhibitor of protein synthesis, led to a sharp decrease in assemblies, down to 1% of the control levels within three hours. To ascertain whether the assemblies were tethered to the cytoskeleton, the cells were treated in vitro with disruptors of microtubules (colchicine) or microfilaments (cytochalasins); the assemblies became consistently rearranged. Protein denaturants, urea and guanidine HCl, brought about a selective aggregation of assembly with assembly. The lectin, concanavalin A, did not alter the distribution of the assemblies within the plane of the membrane fracture. Surface replicas of in vitro, non-fractured, astrocytes revealed surface particles which did not resemble assemblies. In vivo, the plasma membranes of astrocytes were altered within minutes of cold injury to the brain surface. In the centre of the lesions, damaged astrocytes had assemblies that were clumped like those of in vitro astrocytes exposed to denaturants. In the periphery of the lesions, however, the assemblies did not aggregate but increased in number. These results provide indirect evidence that assemblies may consist of protein, that the recognizable particle constituent of the assembly is confined to the interior of the membrane and is not present on the uncleaved cell surface, and that assemblies are connected with the cytoskeleton. Therefore, certain changes in the environment of the astrocyte caused by injury in vivo or addition of chemical agents in vitro alter the distribution of assemblies in the astrocytic plasma membrane either by a direct effect on the assemblies or indirectly by an alteration of the cytoplasmic proteins.

Changes in number and distribution of orthogonal arrays during postnatal muscle development

Quantitative freeze-fracture electron microscopy was utilized to study the changes in number and distribution of orthogonal arrays (aggregates of 6-7 nm particles) of the sarcolemmas of the fast twitch extensor digitorum longus (EDL) and slow twitch soleus muscles during the first few weeks of postnatal development in the rat. In the adult rat, orthogonal arrays are present in high densities in the fast twitch type II fibers but only in low densities in slow twitch type I fibers. In this study, the changing histochemical profiles of fiber types in the EDL and soleus muscles were also determined for the first month of postnatal development and correlated with the changing number and distribution of orthogonal arrays during the same time frame. At day 3 postpartum, EDL and soleus fibers possessed few orthogonal arrays. The developing EDL fibers rapidly acquired additional orthogonal arrays until the approximate adult number and distribution were attained at postpartum day 25. In contrast, the slow twitch soleus fibers rapidly acquired orthogonal arrays and type IIA fibers until day 35 when both were in excess of adult values. Subsequently, the number of arrays and type IIA fibers declined to normal adult ranges. We suggest that the patterns of development of orthogonal arrays and fiber types are different in the EDL and soleus because the types of innervating motor units are different in the two muscles. The EDL is innervated almost entirely by fast motor units throughout early development and maturity. The soleus, however, is initially innervated by a more heterogeneous population of motor neurons. Thus, during the period of polyneuronal innervation which occurs normally during the first weeks of postnatal development, many individual soleus fibers may possess simultaneous innervation by axons from different motor neuron types. These dual influences may be responsible for the irregular pattern of development of orthogonal arrays and type IIA fibers in developing soleus fibers. Later, as the adult pattern of monosynaptic innervation is developed, expected adult values of orthogonal arrays and fiber types are attained.

Regional organization of astrocytic membranes in cerebellar cortex

The internal organization of plasmalemmal membrane, as revealed by freeze-fracture techniques, varies dramatically and predictably over the surface of astrocytes in mouse cerebellar cortex. Assemblies of uniform, small intramembrane particles packed in orthogonal order into square or rectangular aggregates are specialized distribution of intramembrane particles which, in the cerebellar cortex, are found only in astrocytes. The concentration of assemblies is greatest in astrocytic membrane juxtaposed to vascular structures or facing the cerebrospinal fluid at the glial limitans. Many fewer are present in regions of astrocytic membrane apposed to neural structures and virtually none are present on the astrocyte cell body. Corresponding structures have not yet been found in thin-sectioned preparations. While the distribution of assemblies in membranes facing blood and cerebrospinal fluid compartments suggests that they may have a role in transport of some material into or out of those compartments, their function is unknown. A second, distinct specialization of intramembrane structure appears to represent a junction between apposed astrocytic processes. We have provisionally described this as a 'polygonal particle junction', since it appears as large, irregular particles densely packed without obvious order in co-extensive regions of two astrocytic membranes. This junction is regularly present just below the cerebellar surface in the processes of the glial limitans as well as between large, more proximal radial Bergmann fibers, and also occurs occasionally throughout the molecular layer. With tannic acid mordant after aldehyde-osmium fixation or rapid freezing and freeze-substitution, it is possible to demonstrate subtle electron-dense specializations of the astrocytic membranes and extracellular matrix in thin-sections which correspond to the sites of polygonal particle junctions. The function of this astrocytic specialization is also unknown. Cerebellar astrocytes manifest numerous gap junctions as well, whose structure in freeze-fractured and thin-sectioned preparations is quite distinct from that of assemblies or of polygonal particle junctions.

Subcellular distribution of ionic components in gastric mucosa of the guinea pig

The topographical distribution of cations, anions and polyanions in the guinea-pig stomach has been studied by ultrastructural cytochemical methods. After fixation with the pyroantimonate-osmium tetroxide solution, variable-sized precipitates were localized in the basolateral extracellular space bordering parietal cells or chief cells but not in that bordering mucus-secreting cells. The basal lamina of all gastric cells disclosed a continuous layer of heavy antimonate deposits. Parietal cells disclosed uniformly fine deposits also on the apical plasmalemma both at the main lumen and in the intracellular canaliculi, and revealed, as well, coarse precipitates in the mitochondria. Fixation with a silver acetate-osmium tetroxide solution yielded nitric acid-resistant, silver deposits confined to the luminal surface of the apical plasmalemma in the main lumen and intracellular canaliculi, the lateral intercellular space, the outer surface of the basal plasmalemma and the basal lamina of the parietal cell. Staining with dialyzed iron demonstrated a glycocalyx rich in acid mucosubstance on the basolateral plasmalemma but not on the apical plasmalemma of parietal cells. In contrast, acid glycoconjugate was visualized on the apical plasmalemma of isthmus cells, mucous neck cells and the transitional cell between isthmus and mucous neck cells but little or no acidic glycoconjugate was demonstrated on the basolateral plasmalemma of these cells. The entire plasmalemma of gastroendocrine cells, unlike other epithelial cells, stained uniformly for acidic glycoconjugate. The dialyzed iron and high iron diamine methods stained the outer compartment of mitochondria in parietal cells intensely and that in other gastric cells lightly. These reagents stained the basal lamina of all gastric cells as did ruthenium red. The several characteristic cytochemical properties of parietal cells presumably relate to the unique secretory activity of these cells and are consistent with the view of the intracellular canaliculi of the parietal cell as the main route for hydrogen and chloride in secretion.

The distribution of orthogonal arrays and their relationship to intercellular junctions in neuroglia of the freeze-fractured hypothalamo-neurohypophysial system

Using freeze-fracture techniques, we have investigated membrane specializations of the glia associated with the hypothalamo-neurohypophysial system of the rat. In the paraventricular (PVN) and supraoptic (SON) nuclei, astrocytes in areas of high neuronal density (i.e., magnocellular regions) display orthogonal arrays of 6--7 nm particles solely near gap junctions, while astrocytes in areas of lower neuronal density (i.e., parvocellular regions), contain additional arrays in membranes not displaying gap junctions. Arrays are especially numerous on astrocytic perivascular end-feet in both nuclei and in the laminations of the pial-glial limitans ventral to the SON. Ependymal cells near the PVN show arrays both on their lateral surfaces (displaying gap junctions) and on their apical surfaces (facing the CSF). Tight junctions are not noted on astrocytes or ependymal cells, but are noted on both the somas and myelin lamellae of oligodendroglia. Both of these latter membranes occasionally contain gap junctions as well; however, orthogonal arrays are never noted on oligodendroglia. The plasma membranes of pituicytes in the neurohypophysis display gap junctions, complex junctions, and tight junctions. Orthogonal arrays are noted near the first two of these, but not near the last. Arrays in the neutral lobe appear most dense on membranes adjacent to subpial or perivascular spaces. Pituicyte membranes containing orthogonal arrays appear infrequently near the neural stalk, increasing towards the distal end of the neural lobe. The distribution of orthogonal arrays in this system, as well as in other systems in which they have been noted, suggests a polarization of membrane activity.

Freeze fracture studies of muscle plasma membrane in human muscular dystrophy

Freeze fracture analysis of intramembranous particle density in skeletal muscle plasma dystrophy from 7 patients with Duchenne muscular dystrophy (DMD), 5 patients with facioscapulohumeral muscular dystrophy (FSH) and 5 patients with myotonic dystrophy (MyD) were carried out. Marked depletion of intramembranous particles including orthogonal arrays was noted in DMD while only orthogonal arrays were significantly decreased in FSH. No abnormalities were noted in MyD.

Isoproterenol-induced intramembrane particle aggregation and water flux in toad epidermis

Stimulation of toad skin with isproterenol resulted in a dramatic increase in water flow, and in the appearance of aggregates of intramembrane particles in the apical membrane of granular cells of the replacement layer, just beneath the stratum corneum. This membrane structural modification appears to be a general prerequisite for the change in water permeability of vasopressin-sensitive epithelia.

Assemblies of particles in the cell membranes of developing, mature and reactive astrocytes

Orthogonal arrays of small intramembranous particles characterize freeze-fractured astrocytic plasma membranes. The normal variation of assemblies in plasma membranes of subpial astrocytic processes in mature and developing rats was established and compared with assemblies in plasma membranes of reactive astrocytes. In mature rats, subpial astrocytic processes had the greatest number of assemblies. As the parenchyma was approached, this number decreased in each successive layer of astrocytic processes. The advent of assemblies within the plasma membrane is a new criterion of astrocytic differentiation. The foot processes of subpial astrocytes in foetal rats began to acquire assemblies between day 19 and 20 and continued to mature postnatally by a constant addition and rearrangement of assemblies. In contrast to the paucity of assemblies in deeper laminae of the normal brain, reactive astrocytes comprising the lower lamellae in glial scars had an increased number of assemblies while the most striking feature of the superficial, astrocytic processes within the scar was a rearrangement of assemblies. Although the function of these intramembranous particles in still unknown, it appears from our measurements that they are localized primarily in the outermost astrocytic foot process and to a progressively lesser degree in the underlying astrocytic lamellae.