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Despite decades of research, no compelling non-surgical therapies have been developed for foetal hydrocephalus. So far, most efforts have pointed to repairing disturbances in the cerebrospinal fluid (CSF) flow and to avoid further brain damage. There are no reports trying to prevent or diminish abnormalities in brain development which are inseparably associated with hydrocephalus. A key problem in the treatment of hydrocephalus is the blood-brain barrier that restricts the access to the brain for therapeutic compounds or systemically grafted cells. Recent investigations have started to open an avenue for the development of a cell therapy for foetal-onset hydrocephalus. Potential cells to be used for brain grafting include: (1) pluripotential neural stem cells; (2) mesenchymal stem cells; (3) genetically-engineered stem cells; (4) choroid plexus cells and (5) subcommissural organ cells. Expected outcomes are a proper microenvironment for the embryonic neurogenic niche and, consequent normal brain development.

Under normal physiological conditions the cerebrospinal fluid (CSF) is secreted continuously, although this secretion undergoes circadian variations. Mechanisms operating at the vascular side of the choroidal cells involve a sympathetic and a cholinergic innervation, with the former inhibiting and the latter stimulating CSF secretion. There are also regulatory mechanisms operating at the ventricular side of the choroidal cells, where receptors for monoamines such as dopamine, serotonin, and melatonin, and for neuropeptides such as vasopressin, atrial natriuretic hormone, and angiotensin II, have been identified. These compounds, that are normally present in the CSF, participate in the regulation of CSF secretion. Although the mechanisms responsible for the CSF circulation are not fully understood, several factors are known to play a role. There is evidence that the subcommissural organ (SCO)--Reissner's fiber (RF) complex is one of the factors involved in the CSF circulation. In mammals, the predominant route of escape of CSF into blood is through the arachnoid villi. In lower vertebrates, the dilatation of the distal end of the central canal, known as terminal ventricle or ampulla caudalis, represents the main site of CSF escape into blood. Both the function and the ultrastructural arrangement of the ampulla caudalis suggest that it may be the ancestor structure of the mammalian arachnoid villi. RF-glycoproteins reaching the ampulla caudalis might play a role in the formation and maintenance of the route communicating the CSF and blood compartments. The SCO-RF complex may participate, under physiological conditions, in the circulation and reabsorption of CSF. Under pathological conditions, the SCO appears to be involved in the pathogeneses of congenital hydrocephalus. Changes in the SCO have been described in all species developing congenital hydrocephalus. In these reports, the important question whether the changes occurring in the SCO precede hydrocephalus, or are a consequence of the hydrocephalic state, has not been clarified. Recently, evidence has been obtained indicating that a primary defect of the SCO-RF complex may lead to hydrocephalus. Thus, a primary and selective immunoneutralization of the SCO-RF complex during the fetal and early postnatal life leads to absence of RF, aqueductal stenosis, increased CSF concentration of monoamines, and a moderate but sustained hydrocephalus., (Copyright 2001 Wiley-Liss, Inc.)

The subcommissural organ is an ependymal brain gland that secretes glycoproteins to the cerebrospinal fluid (CSF) of the third ventricle. They condense to form a fibre, Reissner's fibre (RF), that runs along the aqueduct and fourth ventricle and the central canal of the spinal cord. A single injection of an antibody against the secretory glycoproteins of RF into a lateral ventricle of adult rats results in animals permanently deprived of RF in the central canal and bearing a "short" RF extending only along the aqueduct and the fourth ventricle. These animals, together with untreated control animals were used to investigate the probable influence of RF in the circulation of CSF in the central canal of the spinal cord. For this purpose, two tracers, (horseradish peroxidase and rabbit immunoglobulin) were injected into the ventricular CSF. The animals were killed 13, 20, 60, 120 and 240 min after the injection, and the amount of the tracers was estimated in tissue sections obtained at proximal, medial and distal levels of the spinal cord. In rats deprived of RF, a significant decrease in the amount of tracers present in the central canal was observed at all experimental intervals, being more evident at 20 min after the injection of the tracers. This suggests that lacking a RF in the central canal decreases the bulk flow of CSF along the central canal. Turbulences of the CSF at the entrance of the central canal of RF-deprived rats might explain the inability of the regenerating RF to progress along the central canal, as well as the reduced flow of CSF in the central canal of these animals.

The circulation of the cerebrospinal fluid along the central canal and its access to the parenchyma of the spinal cord of the rat have been analyzed by injection of horseradish peroxidase (HRP) into the lateral ventricle. Peroxidase was found throughout the central canal 13 min after injection, suggesting a rapid circulation of cerebrospinal fluid along the central canal of the rat spinal cord. It was cleared from the central canal within 2 h, in contrast with the situation in the brain tissue, where it remained in the periventricular areas for 4 h. In the central canal, HRP bound to Reissner's fiber and the luminal surface of the ependymal cells; it penetrated through the intercellular space of the ependymal lining, reached the subependymal neuropil, the basement membrane of local capillaries, and appeared in the lumen of endothelial pinocytotic vesicles. Furthermore, it accumulated in the labyrinths of the basement membrane contacting the basolateral aspect of the ependymal cells. In ependymocytes, HRP was found in single pinocytotic vesicles. The blood vessels supplying the spinal cord were classified into two types. Type-A vessels penetrated the spinal cord laterally and dorsally and displayed the tracer along their external wall as far as the gray matter. Type-B vessels intruded into the spinal cord from the medial ventral sulcus and occupied the anterior commissure of the gray matter, approaching the central canal. They represented the only vessels marked by HRP along their course through the gray matter. HRP spread from the wall of type-B vessels, labeling the labyrinths, the intercellular space of the ependymal lining, and the lumen of the central canal. This suggests a communication between the central canal and the outer cerebrospinal fluid space, at the level of the medial ventral sulcus, via the intercellular spaces, the perivascular basement membrane and its labyrinthine extensions.

Background The subcommissural organ (SCO) differentiates early in ontogeny and remains fully active during the entire life span. It secretes glycoproteins into the cerebrospinal fluid (CSF) flowing through the Sylvius aqueduct (SA); these proteins either assemble to form Reissner fiber (RF), or remain soluble in the CSF and circulate throughout the CSF compartments. Overholser et al. (1954) have demonstrated that offspring littered by rats maintained on a diet deficient in folic acid and/or Vitamin B12 lack a SCO and develop hydrocephalus. This led them to propose that a dysfunction of the SCO during development leads to stenosis of the SA and hydrocephalus. We have investigated several animal models in which the SCO would play a role in the pathogenesis of hydrocephalus. We have now performed a comparative analysis of the changes occurring in the SCO-RF complex of these animal models, with the aim to find a landmark common to all hydrocephalic animals that might help to unfold the mechanism of SCO-dependant hydrocephalus. Materials and methods A comparative study of the SCO-RF complex and SA of five animal models characterized by a dysfunction of the SCO and development of hydrocephalus was carried out. Conventional light and electron microscopy and immunocytochemistry were applied. Results Model 1. Immunological blockage of RF formation by maternal transfer of antibodies against RF-glycoproteins: undamaged and secretory active SCO, missing RF, stenosed SA and moderate hydrocephalus. Model 2. hyh mice with a point mutation of aSNAP gene: undamaged and secretory active SCO, missing RF, obliterated SA and severe hydrocephalus. Model 3. HTx rat: subcommissural portion (two distal thirds) of SCO missing, supracommissural portion (rostral third) of SCO secretory active, RF absent, obliterated SA and severe hydrocephalus. Model 4. Transgenic mice deficient in transcription factor RFX4- v3: subcommissural portion of SCO missing, supracommissural portion of SCO secretory active, RF absent, patent SA and moderate to severe hydrocephalus. Model 5. fyn knockout mice: subcommissural portion of SCO missing, supracommissural portion of SCO secretory active, RF absent, SA with an abnormal shape and severe hydrocephalus. Conclusion 1. The SCO is formed by two zones: the subcommissural and the supracommissural portions. 2. Differentiation of both zones would be controlled by different genes. 3. The subcommissural portion of the SCO is essential for RF formation. Mutant and transgenic animals lacking this portion, although still have a secretory active flow of CSF throughout SA. 6. Absence of RF could cause SA obliteration or a turbulent CSF flow through SA what, in turn, would lead to hydrocephalus. Supported by Fondecyt 1030265 (Chile), the Intramural Program of the NIH, NIEHS (USA), and MEXT (Japan). [ABSTRACT FROM AUTHOR]