Recent observations on ependyma and subependymal basement membranes

Fluid transport in the brain

The brain harbors a unique ability to, figuratively speaking, shift its gears. During wakefulness, the brain is geared fully toward processing information and behaving, while homeostatic functions predominate during sleep. The blood-brain barrier establishes a stable environment that is optimal for neuronal function, yet the barrier imposes a physiological problem; transcapillary filtration that forms extracellular fluid in other organs is reduced to a minimum in brain. Consequently, the brain depends on a special fluid [the cerebrospinal fluid (CSF)] that is flushed into brain along the unique perivascular spaces created by astrocytic vascular endfeet. We describe this pathway, coined the term glymphatic system, based on its dependency on astrocytic vascular endfeet and their adluminal expression of aquaporin-4 water channels facing toward CSF-filled perivascular spaces. Glymphatic clearance of potentially harmful metabolic or protein waste products, such as amyloid-β, is primarily active during sleep, when its physiological drivers, the cardiac cycle, respiration, and slow vasomotion, together efficiently propel CSF inflow along periarterial spaces. The brain's extracellular space contains an abundance of proteoglycans and hyaluronan, which provide a low-resistance hydraulic conduit that rapidly can expand and shrink during the sleep-wake cycle. We describe this unique fluid system of the brain, which meets the brain's requisites to maintain homeostasis similar to peripheral organs, considering the blood-brain-barrier and the paths for formation and egress of the CSF.

The glymphatic hypothesis: the theory and the evidence

The glymphatic hypothesis proposes a mechanism for extravascular transport into and out of the brain of hydrophilic solutes unable to cross the blood-brain barrier. It suggests that there is a circulation of fluid carrying solutes inwards via periarterial routes, through the interstitium and outwards via perivenous routes. This review critically analyses the evidence surrounding the mechanisms involved in each of these stages. There is good evidence that both influx and efflux of solutes occur along periarterial routes but no evidence that the principal route of outflow is perivenous. Furthermore, periarterial inflow of fluid is unlikely to be adequate to provide the outflow that would be needed to account for solute efflux. A tenet of the hypothesis is that flow sweeps solutes through the parenchyma. However, the velocity of any possible circulatory flow within the interstitium is too small compared to diffusion to provide effective solute movement. By comparison the earlier classical hypothesis describing extravascular transport proposed fluid entry into the parenchyma across the blood-brain barrier, solute movements within the parenchyma by diffusion, and solute efflux partly by diffusion near brain surfaces and partly carried by flow along "preferred routes" including perivascular spaces, white matter tracts and subependymal spaces. It did not suggest fluid entry via periarterial routes. Evidence is still incomplete concerning the routes and fate of solutes leaving the brain. A large proportion of the solutes eliminated from the parenchyma go to lymph nodes before reaching blood but the proportions delivered directly to lymph or indirectly via CSF which then enters lymph are as yet unclear. In addition, still not understood is why and how the absence of AQP4 which is normally highly expressed on glial endfeet lining periarterial and perivenous routes reduces rates of solute elimination from the parenchyma and of solute delivery to it from remote sites of injection. Neither the glymphatic hypothesis nor the earlier classical hypothesis adequately explain how solutes and fluid move into, through and out of the brain parenchyma. Features of a more complete description are discussed. All aspects of extravascular transport require further study.

Tachycardia and hypertension enhance tracer efflux from the spinal cord

Background

Disruption of cerebrospinal fluid (CSF)/interstitial fluid (ISF) exchange in the spinal cord is likely to contribute to central nervous system (CNS) diseases that involve abnormal fluid accumulation, including spinal cord oedema and syringomyelia. However, the physiological factors that govern fluid transport in the spinal cord are poorly understood. The aims of this study were to determine the effects of cardiac pulsations and respiration on tracer signal increase, indicative of molecular movement following infusion into the spinal cord grey or white matter.

Methods

In Sprague Dawley rats, physiological parameters were manipulated such that the effects of spontaneous breathing (generating alternating positive and negative intrathoracic pressures), mechanical ventilation (positive intrathoracic pressure only), tachycardia (heart atrial pacing), as well as hypertension (pharmacologically induced) were separately studied. Since fluid outflow from the spinal cord cannot be directly measured, we assessed the molecular movement of fluorescent ovalbumin (AFO-647), visualised by an increase in tracer signal, following injection into the cervicothoracic spinal grey or white matter.

Results

Tachycardia and hypertension increased AFO-647 tracer efflux, while the concomitant negative and positive intrathoracic pressures generated during spontaneous breathing did not when compared to the positive-pressure ventilated controls. Following AFO-647 tracer injection into the spinal grey matter, increasing blood pressure and heart rate resulted in increased tracer movement away from the injection site compared to the hypotensive, bradycardic animals (hypertension: p = 0.05, tachycardia: p < 0.0001). Similarly, hypertension and tachycardia produced greater movement of AFO-647 tracer longitudinally along the spinal cord following injection into the spinal white matter (p < 0.0001 and p = 0.002, respectively). Tracer efflux was strongly associated with all blood vessel types.

Conclusions

Arterial pulsations have profound effects on spinal cord interstitial fluid homeostasis, generating greater tracer efflux than intrathoracic pressure changes that occur over the respiratory cycle, demonstrated by increased craniocaudal CSF tracer movement in the spinal cord parenchyma.

Subependymal hyperintense layer on CISS sequence: An MRI study

PURPOSE

The present study aimed to explore the subependymal layers overlying the cerebral ventricles using magnetic resonance imaging.

METHODS

A total of 69 outpatients underwent constructive interference in steady-state (CISS) sequence in thin-sliced, coronal, and sagittal sections.

RESULTS

The subependymal layers were delineated as linear hyperintensities, coursing along the outer margins of the ventricular walls. On coronal images, the hyperintensities surrounding the anterior horn of the lateral ventricle were identified in 97% of patients, while those of the third ventricle were identified in 96% of patients. In the trigone and posterior horn of the lateral ventricle, the hyperintensities were delineated in all patients. On sagittal images, subependymal hyperintensities were identified in all. At the level of the anterior horn and third ventricle, the subependymal hyperintensities were found to communicate with the Virchow-Robin spaces (VRSs) in 68% and 65% of patients, respectively. At the level of the trigone and posterior horn of the lateral ventricle, the VRSs communicated with the subependymal hyperintensities in 83% of patients.

CONCLUSIONS

Subependymal hyperintensity may represent an inflow passage of the VRSs that jointly contribute to efficient transependymal migration of the interstitial fluid into the ventricular cerebrospinal fluid.

Targeting the Choroid Plexuses for Protein Drug Delivery

Delivery of therapeutic agents to the central nervous system is challenged by the barriers in place to regulate brain homeostasis. This is especially true for protein therapeutics. Targeting the barrier formed by the choroid plexuses at the interfaces of the systemic circulation and ventricular system may be a surrogate brain delivery strategy to circumvent the blood-brain barrier. Heterogenous cell populations located at the choroid plexuses provide diverse functions in regulating the exchange of material within the ventricular space. Receptor-mediated transcytosis may be a promising mechanism to deliver protein therapeutics across the tight junctions formed by choroid plexus epithelial cells. However, cerebrospinal fluid flow and other barriers formed by ependymal cells and perivascular spaces should also be considered for evaluation of protein therapeutic disposition. Various preclinical methods have been applied to delineate protein transport across the choroid plexuses, including imaging strategies, ventriculocisternal perfusions, and primary choroid plexus epithelial cell models. When used in combination with simultaneous measures of cerebrospinal fluid dynamics, they can yield important insight into pharmacokinetic properties within the brain. This review aims to provide an overview of the choroid plexuses and ventricular system to address their function as a barrier to pharmaceutical interventions and relevance for central nervous system drug delivery of protein therapeutics. Protein therapeutics targeting the ventricular system may provide new approaches in treating central nervous system diseases.

Fractone Bulbs Derive from Ependymal Cells and Their Laminin Composition Influence the Stem Cell Niche in the Subventricular Zone

Fractones are extracellular matrix structures in the neural stem cell niche of the subventricular zone (SVZ), where they appear as round deposits named bulbs or thin branching lines called stems. Their cellular origin and what determines their localization at this site is poorly studied, and it remains unclear whether they influence neural stem and progenitor cell formation, proliferation, and/or maintenance. To address these questions, we analyzed whole-mount preparations of the lateral ventricle of male and female mice by confocal microscopy using different extracellular matrix and cell markers. We found that bulbs are rarely connected to stems and that they contain laminin α5 and α2 chains, respectively. Fractone bulbs were profusely distributed throughout the SVZ and appeared associated with the center of pinwheels, a critical site for adult neurogenesis. We demonstrate that bulbs appear at the apical membrane of ependymal cells at the end of the first week after birth. The use of transgenic mice lacking laminin α5 gene expression (Lama5) in endothelium and in FoxJ1-expressing ependymal cells revealed ependymal cells as the source of laminin α5-containing fractone bulbs. Deletion of laminin α5 from ependymal cells correlated with a 60% increase in cell proliferation, as determined by phospho-histone H3 staining, and with a selective reduction in the number of slow-dividing cells. These results indicate that fractones are a key component of the SVZ and suggest that laminin α5 modulates the physiology of the neural stem cell niche.SIGNIFICANCE STATEMENT Our work unveils key aspects of fractones, extracellular matrix structures that are present in the SVZ that still lack a comprehensive characterization. We show that fractones extensively interact with neural stem cells, whereas some of them are located precisely at pinwheel centers, which are hotspots for adult neurogenesis. Our results also demonstrate that fractones increase in size during aging and that their interactions with neural stem and progenitor cells become more complex in old mice. Last, we show that fractone bulbs are produced by ependymal cells and that their laminin content regulates neural stem cells.

Ependymal cells: biology and pathology

The literature was reviewed to summarize the current understanding of the role of ciliated ependymal cells in the mammalian brain. Previous reviews were summarized. Publications from the past 10 years highlight interactions between ependymal cells and the subventricular zone and the possible role of restricted ependymal populations in neurogenesis. Ependymal cells provide trophic support and possibly metabolic support for progenitor cells. Channel proteins such as aquaporins may be important for determining water fluxes at the ventricle wall. The junctional and anchoring proteins are now fairly well understood, as are proteins related to cilia function. Defects in ependymal adhesion and cilia function can cause hydrocephalus through several different mechanisms, one possibility being loss of patency of the cerebral aqueduct. Ependymal cells are susceptible to infection by a wide range of common viruses; while they may act as a line of first defense, they eventually succumb to repeated attacks in long-lived organisms. Ciliated ependymal cells are almost certainly important during brain development. However, the widespread absence of ependymal cells from the adult human lateral ventricles suggests that they may have only regionally restricted value in the mature brain of large size.

The ependyma: a protective barrier between brain and cerebrospinal fluid

This review summarizes the current scientific literature concerning the ependymal lining of the cerebral ventricles of the brain with an emphasis on selective barrier function and protective roles for the common ependymal cell. Topics covered include the development, morphology, protein and enzyme expression including reactive changes, and pathology. Some cells lining the neural tube are committed at an early stage to becoming ependymal cells. They serve a secretory function and perhaps act as a cellular/axonal guidance system, particularly during fetal development. In the mature mammalian brain ependymal cells possess the structural and enzymatic characteristics necessary for scavenging and detoxifying a wide variety of substances in the CSF, thus forming a metabolic barrier at the brain-CSF interface.

Ultrastructural investigation of fetal rat brain hemisphere tissue in nonadherent stationary organ culture

Fetal rat brain fragments grown in nonadherent stationary organ culture for 50 days have been investigated ultrastructurally. Synaptogenesis and myelin formation occurred at the same time as the corresponding time-dependent events in the developing brain in vivo. Intermediate junctions were observed between cellular processes lining a central cavity in the fragments and later associated with astrocytes at the surface. Gap junctions and tight junctions were also present. In some fragments cilia were observed in the central cavity. Subependymal basement membrane labyrinths were observed in all fragments after 10 days in culture. The ultrastructural characteristics and the tissue-like structure in general were preserved for at least 50 days in this tissue culture system. The brain fragments may therefore be a valuable supplement to existing culture methods for nervous tissue.

Reaction of rabbit lateral periventricular tissue to shunt tubing implants

The reaction of the periventricular tissue of the lateral ventricle to silicone rubber shunt tubing was studied by scanning and transmission electron microscopy. Nonfunctioning shunt tubing was implanted bilaterally into the frontal horns of rabbits, which were then sacrificed at postoperative intervals of 3 days to 16 weeks. Colchicine was used to study mitotic activity at the 3-day to 4-week postimplantation intervals. Reactive changes that occurred in the periventricular tissue correlated with the degree of contact with the implant and also with the duration of the postoperative period. Ependymal cells underwent progressive attenuation and sloughed completely in the most severely affected areas. Prominent gliosis in the subependyma accompanied the ependymal changes. The ventricular surface directly adjacent to holes in the implant developed ependyma-covered glial evaginations which grew into the implant holes beginning 1 week postimplantation. In the region of the outgrowths, ependymal mitotic activity was significantly increased at 1 and 2 weeks postimplantation. and astroglial mitotic activity was increased at 3 days and 1 week. Proliferation of ependymal and glial cells in the area touching the shunt tubing and mechanical factors contributed to the development of cellular outgrowths which may be a factor in the pathogenesis of shunt obstruction in human hydrocephalus.

Reaggregation of fetal rat brain cells in a stationary culture system. II: Ultrastructural characterization

Ultrastructural characteristics of fetal rat brain cell aggregates in a three-dimensional stationary culture system are described. Transmission electron microscopy showed immature cells which developed into mature astrocytes, oligodendrocytes, and neurons during 20 d in culture. This was accompanied by the development of a neuropil where myelinated axons and synaptic complexes were observed. In addition to confirming earlier ultrastructural investigations on fetal rat brain cell aggregates, the stationary culture system also showed the presence of histiotypic regions within the aggregates. These regions consisted of ependymal cells where cilia were observed on the cell surfaces. Structures resembling subependymal basement membrane labyrinths were also observed. Macrophages seemed to be more numerous in the stationary cultures as compared to other culture systems. The stationary culture system may provide aggregates that are ultrastructurally more complex than those obtained by rotation mediated systems.

The ultrastructure of the ependymoblastoma

The ependymoblastoma is a rare, primitive neuroectodermal tumor morphologically distinct from the ependymoma and the malignant or anaplastic ependymoma. This neoplasm is characterized by uniform neuroepithelial cells, ependymoblastic rosettes, perivascular pseudorosettes and numerous mitotic figures. The fine structure of this neoplasm is characterized by a predominant population of well-differentiated ependymal cells and intermingling mitotic figures. Many cells have an apical surface bearing cilia and microvilli projecting into a lumen and interconnected to adjacent cells by zonulae adhaerentes. The basal surface often forms a basal lamina-lined labyrinth. This tumor also contains a population of cells less differentiated than 3-week embryonal ependymal cells. Intermediate cell forms suggesting multiple lines of differentiation occurring within a single cell are not present in this human ependymoblastoma. The ependymoblast represents a stage in the differentiation of the primitive medulloepithelial cell to the mature ependymocyte.

A suprasellar subarachnoid pouch; aetiological considerations

A child with hydrocephalus treated by a valved shunt was reinvestigated after developing a shunt infection. A pouch was discovered invaginating the floor of the third ventricle and filling slowly with CSF from the region of the interpeduncular cistern. Histology and mechanisms of this pouch formation are discussed.

Ultrastructural studies of subependymal extracellular spaces in adult and neonatal rat brain

Ultrastructural studies of the subependymal neuropil of the lateral ventricular wall in the adult rat reveal two different types of enlarged extracellular spaces which appear to be due to the presence of glycosaminoglycans. Focal irregular enlargements are more numerous and are distinguished by the presence of fibrillar material. The other network is characterized by uniform width, branching processes, and an electron density identical to vascular basement membranes. The origin of the extracellular material is not revealed by these studies, but a relationship is suggested between the focal dilatation and subependymal cells. The distinction of these spaces is emphasized by the study of neonatal brain and by the use of ruthenium red (RR). Focal dilatations are very prominent in the 10 day old rat, but they do not contain fibrillar material. An adult type basement membrane-like network is present at this time. In the adult rat the focal dilatation has great affinity for RR, but the second network is very sparsely stained. The RR affinity is not present in the 10 day old rat; however, by the 26th day, the affinity is almost as great as that of an adult. These findings are considered to enhance the concept of a separate origin, composition, and probable function of these two networks.

Basal membrane labyrinths in the healing stages of chick nutritional encephalopathy

Chicks were fed a diet rich in oxidized oil and deficient in alpha-tocopherol. As soon as nutritional encephalopathy developed the animals were immediately put on to a normal diet, and kept alive for up to 166 days. Examination of the cerebellum showed multiple foci of healed encephalomalacia with EM appearance of basal membrane labyrinths between astroglial cells and around blood vessels. The possible origin of the basement membrane material in this conditions was considered and compared with similar formations in normal animals, in CNS tumour explants, and after portocaval anastomosis as well as after spinal cord lesions.

Subependymal glycosaminoglycan networks in adult and developing rat brain

Histochemical studies of normal adult rat brain indicate two types of glycosaminoglycans in the subependymal region of the lateral ventricle. One network is characterized by an affinity for the cationic dyes alcian blue, aldehyde fuchsin and colloidal iron. These reactions occur at pH 1.0 and at 0.5-0.3 M concentration of MgCl2, which suggests that this material is chondroitin sulfate. The other system is identified by metachromasia with toluidine blue and a loss of PAS staining following sulfation. These findings are consistent with non-sulfated and non-anionic acid mucopolysaccharides. In developing rat brain the differential development of these networks enhances their separate identity. The metachromatic network is present at least by the 10th postnatal day but the polyanionic electrolytes cannot be identified until the 16th to the 22nd days. The possible functional importance of these systems is discussed.