Fine structural studies on ependymal paracellular and capillary transcellular permeability in the subcommissural organ of the guinea pig

Global Clock Coordination by the Brain Clock in the Suprachiasmatic Nucleus Through Relay and Amplification of Diffusible and Neural Signaling

The brain clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus receives direct retinal input, thereby providing the entire body with an internal representation of external solar time. The pathways by which this small nucleus signals so broadly involve co-occurring nervous and diffusible output signals, although the latter are less understood. Portal pathways, such as the well-known pituitary portal pathway, provide a mechanism, whereby signals of neural origin can reach local, specialized targets without suffering dilution in the systemic blood supply. Newly discovered vascular pathways involve direct connections between each of the sensory circumventricular nuclei at its point of attachment to the brain. These nuclei line the brain's ventricles, and their leaky blood vessels and large perivascular spaces represent a route, whereby secretions from the SCN can be relayed and then amplified, providing a pathway to achieve global coordination of circadian clock signaling. This review provides a narrative that incorporates our understanding of SCN neural and diffusible output signals, with particular emphasis on the contribution of brain fluidic compartments and the fluids therein.

The ependymocytes of the bovine subcommissural organ are functionally coupled through gap junctions

The subcommissural organ (SCO) is a circunventricular organ secreting glycoproteins into the ventricle. It is richly innervated by (1) serotonergic fibers originated in raphe nuclei, that would exert an inhibitory control, and (2) peptidergic fibers of unknown function. Due to the scarce number of the latter, their functional significance might largely depends on whether the cells of the SCO are functionally coupled through gap junctions. This investigation was designed to answer this question. The bovine SCO, either freshly isolated or maintained in organ culture, was processed for immunoblot and immunocytochemistry, using an anti-connexin43 antibody, and dye coupling studies. It was found that the cells of the SCO in situ are functionally coupled through gap junctions made at least of connexin43, but in cultured explants are not. The possibility that coupling of the SCO may be controlled by the neural input and undergoes circadian variations is discussed.

The subcommissural organ

The subcommissural organ (SCO) is a phylogenetically ancient and conserved structure. During ontogeny, it is one of the first brain structures to differentiate. In many species, including the human, it reaches its full development during embryonic life. The SCO is a glandular structure formed by ependymal and hypendymal cells highly specialized in the secretion of proteins. It is located at the entrance of the aqueduct of Sylvius. The ependymal cells secrete into the ventricle core-glycosylated proteins of high molecular mass. The bulk of this secretion is formed by glycoproteins that would derive from two different precursors of 540 and 320 kDa and that, upon release into the ventricle aggregate, form a threadlike structure known as Reissner's fiber (RF). By addition of newly released glycoproteins to its proximal end, RF grows caudally and extends along the aqueduct, fourth ventricle, and the whole length of the central canal of the spinal cord. RF material continuously arrives at the dilated caudal end of the central canal, known as the terminal ventricle or ampulla. When reaching the ampulla, the RF material undergoes chemical modifications, disaggregates, and then escapes through openings in the dorsal wall of the ampulla to finally reach local blood vessels. The SCO also appears to secrete a cerebrospinal fluid (CSF)-soluble material that is different from the RF material that circulates in the ventricular and subarachnoidal CSF. Cell processes of the ependymal and hypendymal cells, containing a secretory material, terminate at the subarachnoidal space and on the very special blood capillaries supplying the SCO. The SCO is sequestered within a double-barrier system, a blood-brain barrier, and a CSF-SCO barrier. The function of the SCO is unknown. Some evidence suggests that the SCO may participate in different processes such as the clearance of certain compounds from the CSF, the circulation of CSF, and morphogenetic mechanisms.

Endothelial cells of the meningeal capillaries in the rainbow trout: a freeze-fracture study

Meningeal capillaries are unfenestrated. They are made up of endothelial cells that have a pinocytotic caveolae density of 41+/- 11/micron (2) and 89 +/- 21/ micron(2) the abluminal and luminal sides respectively. The total density of intramembrane particles is not significantly different between the luminal and the abluminal membranes;however, the coefficients of partition are significantly different (P < 0.001).One or two strands of tight junctions occur between adjacent cells but no gap junctions nor desmosomes exist. The density of nuclear pores is less than 3.2/micron(2). An abundance of intermediate filaments and free vesicles, some of which are seriated, characterize the cytoplasm. The functional significance of these findings is 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.

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.

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.

Filipin resistance in intermediate junction membranes of guinea pig ependyma: possible relationship to filamentous underlying

Plasma membranes in intermediate junctions of ependymal cells are found to show considerable resistance to the antibiotic filipin, suggesting low cholesterol in these membranes. Further, ependymal cells were treated with cytochalasin B (CB) infused into the cerebral ventricle in vivo, and then incubated with filipin. When treated with CB, intermediate junctions show a decrease in their underlying density, mainly composed of microfilaments, and their membranes are found to be more affected by filipin. This reduction of resistance to the antibiotic is clearly demonstrated by thin-section and freeze-fracture as well as quantitative analysis. Nonjunctional lateral membranes, however, show no significant difference in the degree of filipin effect whether treated with CB or not. Although biochemical data on lipid composition have not been available for the intermediate junction membranes, we bring forward a possibility that resistance to filipin in these membranes may come not from less cholesterol but from morphological membrane stability brought about by the filamentous underlying.