Ependymal specializations: II. Ultrastructural aspects of the apical secretion of the toad subcommissural organ

The subcommissural organ and the Reissner fiber: old friends revisited

The subcommissural organ (SCO) is an ancient and conserved brain gland secreting into cerebrospinal fluid (CSF) glycoproteins that form the Reissner fiber (RF). The present investigation was designed to further investigate the dynamic of the biosynthetic process of RF glycoproteins prior and after their release into the CSF, to identify the RF proteome and N-glycome and to clarify the mechanism of assembly of RF glycoproteins. Various methodological approaches were used: biosynthetic labelling injecting ³⁵S-cysteine and ³H-galactose into the CSF, injection of antibodies against galectin-1 into the cerebrospinal fluid, light and electron microscopical methods; isolated bovine RF was used for proteome analyses by mass spectrometry and glycome analysis by xCGE-LIF. The biosynthetic labelling study further supported that a small pool of SCO-spondin molecules rapidly enter the secretory pathways after its synthesis, while most of the SCO-spondin molecules are stored in the rough endoplasmic reticulum for hours or days before entering the secretory pathway and being released to assemble into RF. The proteomic analysis of RF revealed clusterin and galectin-1 as partners of SCO-spondin; the in vivo use of anti-galectin-1 showed that this lectin is essential for the assembly of RF. Galectin-1 is not secreted by the SCO but evidence was obtained that it would be secreted by multiciliated ependymal cells lying close to the SCO. Further, a surprising variety and complexity of glycan structures were identified in the RF N-glycome that further expands the potential functions of RF to a level not previously envisaged. A model of the macromolecular organization of Reissner fiber is proposed.

Continuous delivery of a monoclonal antibody against Reissner's fiber into CSF reveals CSF-soluble material immunorelated to the subcommissural organ in early chick embryos

The subcommissural organ (SCO) is an ependymal differentiation located in the dorsal midline of the caudal diencephalon under the posterior commissure. SCO cells synthesize and release glycoproteins into the cerebrospinal fluid (CSF) forming a threadlike structure known as Reissner's fiber (RF), which runs caudally along the ventricular cavities and the central canal of the spinal cord. Numerous monoclonal antibodies have been raised against bovine RF and the secretory material of the SCO. For this study, we selected the 4F7 monoclonal antibody based on its cross-reactivity with chick embryo SCO glycoproteins in vivo. E4 chick embryos were injected with 4F7 hybridoma cells or with the purified monoclonal antibody into the ventricular cavity of the optic tectum. The hybridoma cells survived, synthesized and released antibody into the CSF for at least 13 days after the injection. E5 embryos injected with 4F7 antibody displayed precipitates in the CSF comprising both the monoclonal antibody and anti-RF-positive material. Such aggregates were never observed in control embryos injected with other monoclonal antibodies used as controls. Western blot analysis of CSF from E4-E6 embryos revealed several immunoreactive bands to anti-RF (AFRU) antibody. We also found AFRU-positive material bound to the apical surface of the choroid plexus primordia in E5 embryos. These and other ultrastructural evidence suggest the existence of soluble SCO-related molecules in the CSF of early chick embryos.

Functional aspects of the subcommissural organ-Reissner's fiber complex with emphasis in the clearance of brain monoamines

Reissner's fiber (RF) extends along the cerebral aqueduct, fourth ventricle, and the entire length of the central canal of the spinal cord. It grows continuously in the caudal direction by addition of newly released glycoproteins by the subcommissural organ (SCO) to its proximal end. Several hypotheses about RF function have been advanced. One of them postulates that RF binds biogenic amines present in the CSF and clears them away. In recent years, this hypothesis has been tested in our laboratory by using several experimental protocols. Firstly, the CSF concentration of monoamines was investigated in RF-deprived rats subjected to immunological neutralization of the SCO-RF complex. Secondly, the capacity of RF to bind monoamines in vivo was studied by injecting radiolabeled serotonin or noradrenaline into the rat CSF, and by perfusing them into the CSF, during one week, using an Alzet's osmotic pump. In vitro binding studies were performed using isolated bovine RF. All the findings obtained indicate that RF binds monoamines present in the ventricular CSF and then transports them along the central canal. In the absence of RF, the CSF concentration of monoamines increases sharply.

Hindbrain floor plate of the rat: ultrastructural changes occurring during development

Most of the molecular and experimental studies on the floor plate (FP) have been performed on the FP region extending along the spinal cord. However, little is known about the hindbrain FP. The FP undergoes regional and temporal changes throughout development, but information with respect to the ultrastructural correlate of such changes is missing. The present investigation was focused on the ultrastructural developmental changes occurring in the FP of the rat hindbrain. The FP cells of the hindbrain secrete a material reacting with antibodies against the secretory glycoproteins of the subcommissural organ (AFRU). This antibody was used to perform an ultrastructural immunocytochemical analysis of the rat FP. From E-12 on, there is a progressive increase in the development of the rough endoplasmic reticulum (RER), so that by E-18, it has reached a high degree of hypertrophy. A unique feature of the hindbrain FP cells is the presence of tubular formations and 140-nm vesicles that appear to originate from RER cisternae. The labelling of these two structures with AFRU and Concanavalin A strongly suggests that they are pre-Golgi compartments containing secretory material. Since these structures are present in the basal process and in the apical cell pole of the FP cells, the possibility that they release their content at these sites, is discussed. It is proposed that a secretory mechanism bypassing the Golgi apparatus (constitutive secretion?) operates in the FP cells. The presence of apoptotic cells within the FP of E-20 embryos and newborns suggests that death, and not re-differentiation, is the fate of the FP cells.

The ultrastructure of the subcommissural organ ependyma of the goat

The cells of pseudostratified columnar ciliated ependyma of the subcommissural organ in the goat were classified into two types on the basis of the distribution of chromatin material and nuclear clefts. Amongst the cell organelles the endoplasmic reticulum was highly developed and formed three types of Nebenkerne systems. The type-I Nebenkerne had spiral concentric lamellae and was associated with round lipid droplets. The type-II Nebenkerne, with widely spaced coils, was expanded towards its central and peripheral parts. The type-III Nebenkerne, composed of a meshwork of lamellae, was modified into a vacuolated form. The concentration of mitochondria was greatly increased towards the basal processes of the ependymal cells. The inclusion bodies included small electron-dense bodies, osmiophilic asteroid droplets, large round to spherical bodies and large round osmiophilic bodies with inner structures.

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.

Ontogenetical study of the chick embryo subcommissural organ by lectin-histofluorescence and electronmicroscopy

The secretory activity of the subcommissural organ (SCO) was studied during embryogenesis of the chick (Gallus gallus) using two lectins labelled with fluorescein isothiocyanate, concanavalin A (Con A) and wheat-germ agglutinin (WGA). While WGA labels the apical or ventricular border of the organ, Con A labels both, the apical and vascular poles of the cells. Glycoproteinaceous secretory products, visualized by Con A appear early, at 5 days, in the ependymal epithelium and expand progressively in a rostrocaudal direction. A correlation is established between histofluorescence and the ultrastructural aspects of the ependymocytes. This throws light on the role of the endoplasmic reticulum in the synthesis, storage and transport of the material produced by the SCO, and points to the existence of two poles of exchange between the secretory cells and the extracellular medium, i.e., the ventricular and the vascular one. WGA reactivity at the apical border is linked up with the formation of Reissner's fibre and shows that the secretory product of the SCO cells undergoes at least partly modifications during its intracytoplasmic transport preceding apical discharge.

Reissner's fiber and the wall of the central canal in the lumbo-sacral region of the bovine spinal cord. Comparative immunocytochemical and ultrastructural study

Reissner's fiber (RF) of the subcommissural organ (SCO), the central canal and its bordering structures, and the filum terminale were investigated in the bovine spinal cord by use of transmission electron microscopy, histochemical methods and light-microscopic immunocytochemistry. The primary antisera were raised against the bovine RF, or the SCO proper. Comparative immunocytochemical studies were also performed on the lumbo-sacral region of the rat, rabbit, dog and pig. At all levels of the bovine spinal cord, RF was strongly immunoreactive with both antisera. From cervical to upper sacral levels of the bovine spinal cord there was an increasing number of ependymal cells immunostainable with both antisera. The free surface of the central canal was covered by a layer of immunoreactive material. At sacral levels small subependymal immunoreactive cells were observed. From all these structures sharing the same immunoreactivity, only RF was stained by the paraldehyde-fuchsin and periodic-acid-Schiff methods. At the ultrastructural level, ependymal cells with numerous protrusions extending into the central canal were seen in the lower lumbar segments, whereas cells displaying signs of secretory activity were principally found in the ependyma of the upper sacral levels. A few cerebrospinal fluid-contacting neurons were observed at all levels of the spinal cord; they were immunostained with an anti-tubulin serum. The lumbo-sacral segments of the dog, rat and rabbit, either fixed by vascular perfusion or in the same manner as the bovine material, did not show any immunoreactive structure other than RF. The possibilities that the immunoreactive ependymal cells might play a secretory or an absorptive role, or be the result of post-mortem events, are discussed.

Reissner's fiber in the sacral cord and filum terminale of the possum Trichosurus vulpecula: a light, scanning, and electron microscopic study

The ending of Reissner's fiber (RF) and structural features associated with gaps or fissures in the rostral part of the filum were investigated using light, scanning, and transmission microscopic techniques in young and mature possums of both sexes. To the best of the author's knowledge the report contains results of the first successful application of the SEM for a study of RF in the spinal cord. Some new observations suggest that while the bulk of RF is formed by the subcommissural organ and moved caudally, additional secretory products may be added by ependymal cells in the sacral and possibly other regions of the spinal cord. Evidence is provided in support of the view that RF may pass through gaps in the ependymal lining in the rostral part of the dorsal wall of the filum terminale and caudal end of the sacral cord to reach the periependymal loose tissue and possibly the subarachnoid space. The region of the gap shows the surface of the ependyma facing the lumen of the filum to be covered with microvilli and cilia, and to be in direct continuity with the external surface of the ependyma covered with basement membrane with glial processes and collagen fibers in close proximity. The present results confirm and extend observations reported by Wislocki et al. ('56).

Intercellular junctions between specialized ependymal cells in the subcommissural organ of the rat

The permeability of intercellular junctions in specialized ependymal cells in the rat subcommissural organ (SCO) has been studied ultrastructurally by freeze-fracturing and tracer experiments with horseradish peroxidase (HRP). In addition to normal smooth membrane, areas which could be classified as a leaky tight junction are found within the ependymal junctional region. This consists of only one or two relatively continuous strands but with interruptions in the apical portion. Some strands are perpendicular to the apical membrane surface and often form hairpin-like bends in the basal portion of the junction. The junctional region also shows areas with no strands but only a rippled membrane structure which may be equivalent to very close appositions without fusion of adjacent ependymal cell membranes. The relative proportions of normal smooth membrane, strands and rippled structure in the junctional region is approximately 3:4:6 including two parts overlapping of the strands and rippled structure. Intraventricularly infused HRP passes through many junctions but is occasionally stopped, leaving unstained intercellular spaces of various lengths between membrane fusions of tight junctions. Even when it is stopped, the intercellular space below the junction is densely stained by the enzyme. Orthogonal arrays of intramembrane particles are found to be distributed on the basal and lateral cell membranes below the junctional region in the SCO ependyma.

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

Morphological investigations on the permeability of intercellular junctions between ependymal cells and between capillary endothelial cells in the subcommissural organ (SCO) of the guinea pig have been carried out using freeze-fracturing and tracer experiments with horseradish peroxidase (HRP). The ependymal junction reveals a moderately developed network of tight junctional strands surrounding the tall ependymal cell. The apical portion of this junctional network tends to form nearly complete strands, whereas the basal portion usually shows irregular, fragmented strands often arranged in hairpin-like structures. The passage of intraventricularly infused HRP is blocked, leaving unstained areas, at the level of membrane fusions. At the same time the lateral intercellular space below the junction is densely stained, probably due to invasion from the basal side through adjacent ordinary ependymal junctions. The SCO capillary endothelium shows a high distribution density of pinocytotic vesicles. Vesicular transport of intravascularly injected HRP is observed, but no HRP penetration occurs through the endothelial junction. The active participation of vesicles in tracer movement is shown in preparations fixed before administration of HRP. Extravasation of this tracer is indicated to some degree in the SCO capillary, but permeability here appears to be comparable to that of ordinary brain capillaries. Accordingly, the SCO ependymal tight junction seems to form an effective barrier not to blood plasma or similar materials but to apically secreted substances, preventing them from spreading back into SCO intercellular spaces.