Chondroitin sulphate proteoglycan and embryonic brain enlargement in the chick

Abnormal development of transient fetal zones in mild isolated fetal ventriculomegaly

Mild isolated fetal ventriculomegaly (iFVM) is the most common abnormality of the fetal central nervous system. It is characterized by enlargement of one or both of the lateral ventricles (defined as ventricular width greater than 10 mm, but less than 12 mm). Despite its high prevalence, the pathophysiology of iFVM during fetal brain development and the neurobiological substrate beyond ventricular enlargement remain unexplored. In this work, we aimed to establish the relationships between the structural development of transient fetal brain zones/compartments and increased cerebrospinal fluid volume. For this purpose, we used in vivo structural T2-weighted magnetic resonance imaging of 89 fetuses (48 controls and 41 cases with iFVM). Our results indicate abnormal development of transient zones/compartments belonging to both hemispheres (i.e. on the side with and also on the contralateral side without a dilated ventricle) in fetuses with iFVM. Specifically, compared to controls, we observed enlargement of proliferative zones and overgrowth of the cortical plate in iFVM with associated reduction of volumes of central structures, subplate, and fetal white matter. These results indicate that enlarged lateral ventricles might be linked to the development of transient fetal zones and that global brain development should be taken into consideration when evaluating iFVM.

Neurogenesis: A process ontogenically linked to brain cavities and their content, CSF

Neurogenesis is the process underlying the development of the highly evolved central nervous system (CNS) in vertebrates. Neurogenesis takes place by differentiation of specific Neural Precursor Cells in the neurogenic niche. The main objective of this review is to highlight the specific relationship between the brain cavities, and neurogenesis from neural precursors. Brain cavities and their content, Cerebrospinal Fluid (CSF), establish a key relation with the neurogenic "niche" because of the presence in this fluid of neurogenic signals able to control neural precursor cell behaviour, inducing precursor proliferation and neuronal differentiation. This influence seems to be ontogenically preserved, despite the temporal and spatial variations that occur throughout life. In order to better understand this concept, we consider three main life periods in the CSF-Neurogenesis interaction: The "Embryonic" period, which take place at the Neural Tube stage and extends from the isolation of the neural tube at the end of "neurulation" to the beginning of Choroid Plexus activity; the "Fetal" period, which includes the remaining developmental and the early postnatal stages; and the "Adult" period, which continues for the rest of adult life. Each period has specific characteristics in respect of CSF synthesis and composition, and the location, extension and neurogenic activity of the neurogenic niche. However, CSF interaction with the neurogenic niche is a common factor, which should be taken into account to better understand the ontogeny of neuron formation and replacement, as well as its potential role in the success or failure of therapies for the ageing, injured or diseased brain.

Embryonic cerebrospinal fluid formation and regulation

The vertebrate brain is organized, from its embryonic origin and throughout adult life, around a dynamic and complex fluid, the cerebrospinal fluid (CSF). There is growing interest in the composition, dynamics and function of the CSF in brain development research. It has been demonstrated in higher vertebrates that CSF has key functions in delivering diffusible signals and nutrients to the developing brain, contributing to the proliferation, differentiation and survival of neural progenitor cells, and to the patterning of the brain. It has also been shown that the composition and the homeostasis of CSF are tightly regulated following the closure of the anterior neuropore, just before the initiation of primary neurogenesis in the neural tissue surrounding brain cavities, before the formation of functional choroid plexus. In this review we draw together existing literature about the composition and formation of embryonic cerebrospinal fluid in birds and mammals, from the closure of the anterior neuropore to the formation of functional fetal choroid plexus, including mechanisms regulating its composition and homeostasis. The significance of CSF regulation within embryonic brain is also discussed from an evolutionary perspective.

Fluid mechanics as a driver of tissue-scale mechanical signaling in organogenesis

PURPOSE OF REVIEW

Organogenesis is the process during development by which cells self-assemble into complex, multi-scale tissues. Whereas significant focus and research effort has demonstrated the importance of solid mechanics in organogenesis, less attention has been given to the fluid forces that provide mechanical cues over tissue length scales.

RECENT FINDINGS

Fluid motion and pressure is capable of creating spatial gradients of forces acting on cells, thus eliciting distinct and localized signaling patterns essential for proper organ formation. Understanding the multi-scale nature of the mechanics is critically important to decipher how mechanical signals sculpt developing organs.

SUMMARY

This review outlines various mechanisms by which tissues generate, regulate, and sense fluid forces and highlights the impact of these forces and mechanisms in case studies of normal and pathological development.

A proposed role for efflux transporters in the pathogenesis of hydrocephalus

Hydrocephalus is a common brain disorder that is treated only with surgery. The basis for surgical treatment rests on the circulation theory. However, clinical and experimental data to substantiate circulation theory have remained inconclusive. In brain tissue and in the ventricles, we see that osmotic gradients drive water diffusion in water-permeable tissue. As the osmolarity of ventricular CSF increases within the cerebral ventricles, water movement into the ventricles increases and causes hydrocephalus. Macromolecular clearance from the ventricles is a mechanism to establish the normal CSF osmolarity, and therefore ventricular volume. Efflux transporters, (p-glycoprotein), are located along the blood brain barrier and play an important role in the clearance of macromolecules (endobiotics and xenobiotics) from the brain to the blood. There is clinical and experimental data to show that macromolecules are cleared out of the brain in normal and hydrocephalic brains. This article summarizes the existing evidence to support the role of efflux transporters in the pathogenesis of hydrocephalus. The location of p-gp along the pathways of macromolecular clearance and the broad substrate specificity of this abundant transporter to a variety of different macromolecules are reviewed. Involvement of p-gp in the transport of amyloid beta in Alzheimer disease and its relation to normal pressure hydrocephalus is reviewed. Finally, individual variability of p-gp expression might explain the variability in the development of hydrocephalus following intraventricular hemorrhage.

Embryonic cerebrospinal fluid in brain development: neural progenitor control

Due to the effort of several research teams across the world, today we have a solid base of knowledge on the liquid contained in the brain cavities, its composition, and biological roles. Although the cerebrospinal fluid (CSF) is among the most relevant parts of the central nervous system from the physiological point of view, it seems that it is not a permanent and stable entity because its composition and biological properties evolve across life. So, we can talk about different CSFs during the vertebrate life span. In this review, we focus on the CSF in an interesting period, early in vertebrate development before the formation of the choroid plexus. This specific entity is called “embryonic CSF.” Based on the structure of the compartment, CSF composition, origin and circulation, and its interaction with neuroepithelial precursor cells (the target cells) we can conclude that embryonic CSF is different from the CSF in later developmental stages and from the adult CSF. This article presents arguments that support the singularity of the embryonic CSF, mainly focusing on its influence on neural precursor behavior during development and in adult life.

New concepts in the pathogenesis of hydrocephalus

Hydrocephalus is a central nervous system disorder characterized by excessive accumulation of cerebrospinal fluid (CSF) in the ventricles of the brain. Cognitive and physical handicap can occur as a result of hydrocephalus. The disorder can present at any age as a result of a wide variety of different diseases. The pathophysiology of hydrocephalus is unclear. While circulation theory is widely accepted as a hypothesis for the development of hydrocephalus, there is a lack of adequate proof in clinical situations and in experimental settings. However, there is growing evidence that osmotic gradients are responsible for the water content of the ventricles of the brain, similar to their presence in other water permeable organs in the body. Therefore, brain disorders that results in excess macromolecules in the ventricular CSF will change the osmotic gradient and result in hydrocephalus. This review encompasses several key findings that have been noted to be important in the genesis of hydrocephalus, including but not limited to the drainage of CSF through the olfactory pathways and cervical lymphatics, the paravascular pathways and the role of venous system. We propose that as osmotic gradients play an important role in the water transport into the ventricles, the transport of osmotically active macromolecules play a critical role in the genesis of hydrocephalus. Therefore, we can view hydrocephalus as a disorder of macromolecular clearance, rather than circulation. Current evidence points to a paravascular and/or lymphatic clearance of these macromolecules out of the ventricles and the brain into the venous system. There is substantial evidence to support this theory, and further studies may help solidify the merit of this hypothesis.

Cerebrospinal fluid control of neurogenesis induced by retinoic acid during early brain development

Embryonic-cerebrospinal fluid (E-CSF) plays crucial roles in early brain development including the control of neurogenesis. Although FGF2 and lipoproteins present in the E-CSF have previously been shown to be involved in neurogenesis, the main factor triggering this process remains unknown. E-CSF contains all-trans-retinol and retinol-binding protein involved in the synthesis of retinoic acid (RA), a neurogenesis inducer. In early chick embryo brain, only the mesencephalic-rombencephalic isthmus (IsO) is able to synthesize RA. Here we show that in chick embryo brain development: (1) E-CSF helps to control RA synthesis in the IsO by means of the RBP and all-trans-retinol it contains; (2) E-CSF has retinoic acid activity, which suggests it may act as a diffusion pathway for RA; and (3) the influence of E-CSF on embryonic brain neurogenesis is to a large extent due to its involvement in RA synthesis. These data help to understand neurogenesis from neural progenitor cells.

Claudin-5a in developing zebrafish brain barriers: another brick in the wall

Claudins serve essential roles in regulating paracellular permeability properties within occluding junctions. Recent studies have begun to elucidate developmental roles of claudins within immature tissues. This work has uncovered an involvement of several claudins in determining tight junction properties that have an effect on embryonic morphogenesis and physiology. During zebrafish brain morphogenesis, Claudin-5a determines the paracellular permeability of tight junctions within a transient neuroepithelial-ventricular barrier that maintains the hydrostatic fluid pressure required for brain ventricular lumen expansion. However, the roles of Claudins in development may well extend beyond being mere junctional components. Several post-translational modifications of Claudins have been characterized that indicate a direct regulation by developmental signals. This review focuses on the involvement of Claudin-5a in cerebral barrier formation in the zebrafish embryo and includes some speculations about possible modes of regulation.

Intraventricular infusion of hyperosmolar dextran induces hydrocephalus: a novel animal model of hydrocephalus

BACKGROUND

Popular circulation theory of hydrocephalus assumes that the brain is impermeable to cerebrospinal fluid (CSF), and is therefore incapable of absorbing the CSF accumulating within the ventricles. However, the brain parenchyma is permeable to water due to the presence of specific ion channels as well as aquaporin channels. Thus, the movement of water into and out of the ventricles may be determined by the osmotic load of the CSF. If osmotic load determines the aqueous content of CSF in this manner, it is reasonable to hypothesize that hydrocephalus may be precipitated by pathologies and/or insults that produce sustained elevations of osmotic content within the ventricles.

METHODS

We investigated this hypothesis by manipulating the osmotic content of CSF and assaying the development of hydrocephalus in the rat brain. This was achieved by continuously infusing artificial CSF (negative control; group I), fibroblast growth factor (FGF2) solution (positive control; group II) and hyperosmotic dextran solutions (10 KD and 40 KD as experimental solutions: groups III and IV) for 12 days at 0.5 muL/h. The osmolality of the fluid infused was 307, 664, 337 and 328 mOsm/L in Groups I, II, III and IV, respectively. Magnetic resonance imaging (MRI) was used to evaluate the ventricular volumes. Analysis of variance (ANOVA) with pairwise group comparisons was done to assess the differences in ventricular volumes among the four groups.

RESULTS

Group I had no hydrocephalus. Group II, group III and group IV animals exhibited significant enlargement of the ventricles (hydrocephalus) compared to group I. There was no statistically significant difference in the size of the ventricles between groups II, III and IV. None of the animals with hydrocephalus had obstruction of the aqueduct or other parts of CSF pathways on MRI.

CONCLUSION

Infusing hyperosmolar solutions of dextran, or FGF into the ventricles chronically, resulted in ventricular enlargement. These solutions increase the osmotic load in the ventricles. Water influx (through the choroid plexus CSF secretion and/or through the brain) into the ventricles to normalize this osmotic gradient results in hydrocephalus. We need to revise the popular theory of how fluid accumulates in the ventricles at least in some forms of hydrocephalus.

Early embryonic brain development in rats requires the trophic influence of cerebrospinal fluid

Cerebrospinal fluid has shown itself to be an essential brain component during development. This is particularly evident at the earliest stages of development where a lot of research, performed mainly in chick embryos, supports the evidence that cerebrospinal fluid is involved in different mechanisms controlling brain growth and morphogenesis, by exerting a trophic effect on neuroepithelial precursor cells (NPC) involved in controlling the behaviour of these cells. Despite it being known that cerebrospinal fluid in mammals is directly involved in corticogenesis at fetal stages, the influence of cerebrospinal fluid on the activity of NPC at the earliest stages of brain development has not been demonstrated. Here, using "in vitro" organotypic cultures of rat embryo brain neuroepithelium in order to expose NPC to or deprive them of cerebrospinal fluid, we show that the neuroepithelium needs the trophic influence of cerebrospinal fluid to undergo normal rates of cell survival, replication and neurogenesis, suggesting that NPC are not self-sufficient to induce their normal activity. This data shows that cerebrospinal fluid is an essential component in chick and rat early brain development, suggesting that its influence could be constant in higher vertebrates.

Totally tubular: the mystery behind function and origin of the brain ventricular system

A unique feature of the vertebrate brain is the ventricular system, a series of connected cavities which are filled with cerebrospinal fluid (CSF) and surrounded by neuroepithelium. While CSF is critical for both adult brain function and embryonic brain development, neither development nor function of the brain ventricular system is fully understood. In this review, we discuss the mystery of why vertebrate brains have ventricles, and whence they originate. The brain ventricular system develops from the lumen of the neural tube, as the neuroepithelium undergoes morphogenesis. The molecular mechanisms underlying this ontogeny are described. We discuss possible functions of both adult and embryonic brain ventricles, as well as major brain defects that are associated with CSF and brain ventricular abnormalities. We conclude that vertebrates have taken advantage of their neural tube to form the essential brain ventricular system.

Why the embryo still matters: CSF and the neuroepithelium as interdependent regulators of embryonic brain growth, morphogenesis and histiogenesis

The key focus of this review is that both the neuroepithelium and embryonic cerebrospinal fluid (CSF) work in an integrated way to promote embryonic brain growth, morphogenesis and histiogenesis. The CSF generates pressure and also contains many biologically powerful trophic factors; both play key roles in early brain development. Accumulation of fluid via an osmotic gradient creates pressure that promotes rapid expansion of the early brain in a developmental regulated way, since the rates of growth differ between the vesicles and for different species. The neuroepithelium and ventricles both contribute to this growth but by different and coordinated mechanisms. The neuroepithelium grows primarily by cell proliferation and at the same time the ventricle expands via hydrostatic pressure generated by active transport of Na(+) and transport or secretion of proteins and proteoglycans that create an osmotic gradient which contribute to the accumulation of fluid inside the sealed brain cavity. Recent evidence shows that the CSF regulates relevant aspects of neuroepithelial behavior such as cell survival, replication and neurogenesis by means of growth factors and morphogens. Here we try to highlight that early brain development requires the coordinated interplay of the CSF contained in the brain cavity with the surrounding neuroepithelium. The information presented is essential in order to understand the earliest phases of brain development and also how neuronal precursor behavior is regulated.

Dynamics of the epithelium during canalization of the rat ventral prostate

Outgrowth and branching of solid cords are the initial events in postnatal prostatic morphogenesis. These processes involve cell proliferation and their projection into the stroma and precede epithelial canalization. The purpose of the present study was to examine the dynamics of the prostate epithelium during canalization of the rat ventral prostate in the first week of postnatal development using histological, stereological, and ultrastructural analyses. The terminal deoxynucleotidyltransferase [TdT]-mediated deoxy-UTP nick end labeling assay was used to investigate the occurrence of DNA fragmentation. Our results demonstrate that canalization of the prostate epithelium starts as early as on day 1 (24 hr after birth) and progresses thereafter. By the end of the first week (day 6), luminal volume density reached approximately 3% (P < 0.05) of the organ. Canalization was the result of epithelial cell differentiation and apoptosis. The former involved organization of the epithelial cells into a single layer sitting on the basement membrane, polarization, enlargement of secretory organelles and accumulation of secretory vesicles, microvilli formation, and establishment of the adult pattern of cell junctions. The latter was observed to occur mostly to epithelial cells not in contact with the basement membrane. Structures of variable electron density were observed in the developing lumen. In conclusion, different phenomena seem to be involved in the canalization of the rat ventral prostate. However, it was evident from the present results that complex epithelial cell fate decisions take place during this process.

FGF2 plays a key role in embryonic cerebrospinal fluid trophic properties over chick embryo neuroepithelial stem cells

During early stages of brain development, neuroepithelial stem cells undergo intense proliferation as neurogenesis begins. Fibroblast growth factor 2 (FGF2) has been involved in the regulation of these processes, and although it has been suggested that they work in an autocrine-paracrine mode, there is no general agreement on this because the behavior of neuroepithelial cells is not self-sufficient in explants cultured in vitro. In this work, we show that during early stages of development in chick embryos there is another source of FGF2, besides that of the neuroepithelium, which affects the brain primordium, since the cerebrospinal fluid (E-CSF) contains several isoforms of this factor. We also demonstrate, both in vitro and in vivo, that the FGF2 from the E-CSF has an effect on the regulation of neuroepithelial cell behavior, including cell proliferation and neurogenesis. In order to clarify putative sources of FGF2 in embryonic tissues, we detected by in situ hybridization high levels of mRNA expression in notochord, mesonephros and hepatic primordia, and low levels in brain neuroectoderm, corroborated by semiquantitative PCR analysis. Furthermore, we show that the notochord segregates several FGF2 isoforms which modify the behavior of the neuroepithelial cells in vitro. In addition, we show that the FGF2 ligand is present in the embryonic serum; and, by means of labeled FGF2, we prove that this factor passes via the neuroepithelium from the embryonic serum to the E-CSF in vivo. Considering all these results, we propose that, in chick embryos, the behavior of brain neuroepithelial stem cells at the earliest stages of development is influenced by the action of the FGF2 contained within the E-CSF which could have an extraneural origin, thus suggesting a new and complementary way of regulating brain development.

Embryonic cerebrospinal fluid regulates neuroepithelial survival, proliferation, and neurogenesis in chick embryos

Early in development, the behavior of neuroepithelial cells is controlled by several factors, which act in a developmentally regulated manner. Diffusible factors are secreted locally by the neuroepithelium itself, although other nearby structures may also be involved. Evidence suggests a physiological role for the cerebrospinal fluid in the development of the brain. Here, using organotypic cultures of chick embryo neuroepithelial explants from the mesencephalon, we show that the neuroepithelium in vitro is not able to self-induce cell survival, replication, and neurogenesis. We also show that the embryonic cerebrospinal fluid (E-CSF) promotes neuroepithelial stem cell survival and induces proliferation and neurogenesis in mesencephalic explants. These data strongly suggest that E-CSF is involved in the regulation of neuroepithelial cells behavior, supporting the hypothesis that this fluid plays a key role during the early development of the central nervous system.

Analysis of cerebro-spinal fluid protein composition in early developmental stages in chick embryos

Foetal cerebro-spinal fluid (CSF) has a very high protein concentration when compared to adult CSF, and in many species five major protein fractions have been described. However, the protein concentration and composition in CSF during early developmental stages remains largely unknown. Our results show that in the earliest stages (18 to 30 H.H.) of chick development there is a progressive increase in CSF protein concentration until foetal values are attained. In addition, by performing electrophoretic separation and high-sensitivity silver staining, we were able to identify a total of 21 different protein fractions in the chick embryo CSF. In accordance with the developmental pattern of their concentration, these can be classified as follows: A: high-concentration fractions which corresponded with the ones described in foetal CSF by other authors; B: low-concentration fractions which remained stable throughout the period studied; C: low-concentration fractions which show changes during this period. The evolution and molecular weight of the latter group suggest the possibility of an important biological role. Our data demonstrate that all the CSF protein fractions are present in embryonic serum; this could mean that the specific transport mechanisms in neuroepithelial cells described in the foetal period evolve in very early stages of development. In conclusion, this paper offers an accurate study of the protein composition of chick embryonic CSF, which will help the understanding of the influences on neuroepithelial stem cells during development and, as a result, the appropriate conditions for the in vitro study of embryonic/foetal nervous tissue cells.

Chondroitin sulphate proteoglycan is involved in lens vesicle morphogenesis in chick embryos

Proteoglycans have been implicated in the invagination and formation of various embryonal cavitied primordia. In this paper the expression of chondroitin sulphate proteoglycan (CSPG) is analysed in the lens primordium during lens vesicle formation, and demonstrate that this proteoglycan has a specific distribution pattern with regard to invagination and fusion processes in the transformation of placode into lens vesicle. More specifically, CSPG was detected in: (1) the apical surface of lens epithelial cells, where early CSPG expression was observed in the whole of the lens placode whilst in the vesicle phase it was restricted to the posterior epithelium; (2) intense CSPG expression in the basal lamina, which remained constant for the entire period under study; (3) CSPG expression in the intercellular spaces of the lens primordium epithelium, which increased during the invagination of the primordium and which at the vesicle stage was more evident in the posterior epithelium; and (4) CSPG expression on the edges of the lens placode both prior to and during fusion. Treatment with beta- D -xyloside causes significant CSPG depletion in the lens primordium together with severe alterations in the invagination and fusion of the lens vesicle; this leads to the formation of lens primordia which in some cases remain practically flat or show partial invagination defects or fusion disruption. Similar results were obtained by enzyme digestion with chondroitinase AC but not with type II heparinase, which indicates that alterations induced by beta- D -xyloside were due to interference in CSPG synthesis. The findings demonstrate that CSPG is a common component of the lens primordium at the earliest developmental stages during which it undergoes specific modifications. It also includes experimental evidence to show that 'in vivo' CSPG plays an important role in the invagination and fusion processes of the lens primordium.

Biosynthesis and molecular biology of the secretory proteins of the subcommissural organ

Ependymal cells are specialized in the synthesis and release of different factors into the cerebrospinal fluid (CSF). The subcommissural organ (SCO) is one of the most active areas of the ventricular walls secreting into the CSF. This gland is localized in the roof of the third ventricle covering the posterior commissure. Glycoproteins synthesized in SCO cells are released into the ventricular CSF where they aggregate, in a highly ordered fashion, forming an elongated supramacromolecular structure known as the Reissner's fiber (RF). RF grows caudally and extends along the brain aqueduct, the fourth ventricle, and the whole length of the central canal of the spinal cord. The SCO cells synthesize glycoproteins of high molecular weight. A precursor form of 540 kDa is synthesized in bovine and chick SCO cells, and a transcript of 10--14 kb is expressed selectively in the bovine SCO cells. The processing of this molecule generates at least one protein of about 450 kDa (RF-Gly-I), which, after being released, is involved in the formation of RF. Additionally, biochemical data indicate that bovine SCO cells synthesize a second precursor compound of 320 kDa, which is also detected in rat, rabbit, and dog. We postulate that RF is formed by two different complexes, one of which has a very high molecular mass (700 kDa or more) and is made up of at least six polypeptides, with the polypeptide of 450 kDa being its main component. The molecules that form RF in different species have different primary structures but they express common epitopes associated to the existence of cysteine bridges, which are probably crucial for polymerization of RF. Molecular procedures involving the use of anti-RF antibodies have led to the isolation of cDNA clones encoding two proteins known as RF-GLY-I and SCO-spondin. In the last 3 years, five partial cDNA sequences encoding SCO-spondin-like proteins have been obtained (Y08560, Y08561, AJ132107, AJ132106, AJ133488). These clones along with RF-GLY-I and SCO-spondin were computer-assembled generating a cDNA consensus sequence of 14.4 kb. Analyses of the long consensus sequence revealed an extended open reading frame (ORF-1) spanning from base 1,634 to 14,400 that encodes for a putative protein of 4,256 amino acids (approximately 450 kDa). The Mr of the predicted protein is consistent with the observed Mr of the largest protein recognized with anti-RF antibodies in SCO and RF extracts. However, the absence of consensus sequences typically present near the 5J'-end of the translation initiation site suggests the existence of a second open reading frame (ORF-2) extending from base 1 to base 14,400 in frame with the ORF-1 and probably encoding for the largest protein precursor (540 kDa). An antibody raised against a peptide sequence, deduced from the open reading frame encoded by a SCO cDNA, reacted specifically with the bovine and rat SCO-RF complex, thus indicating that the protein encoded by the cloned cDNA is part of RF. Immunoblots of bovine SCO extracts using the anti-peptide serum revealed bands of 540 kDa and 450 kDa, but it did not react with the proteins of 320 and 190 kDa. These data support the existence of two precursors for the bovine RF-glycoproteins (540 and 320 kDa) with the 450-kDa protein being a processed form of the 540-kDa precursor. We postulate that the cloned cDNAs encode for a protein that corresponds to the 540-kDa precursor and that at least part of this sequence is present in the processed form of 450 kDa that is secreted to form the RF.

Disruption of proteoglycans in neural tube fluid by beta-D-xyloside alters brain enlargement in chick embryos

Following neurulation, the anterior end of the neural tube undergoes a dramatic increase in size due mainly to the enlarging of the brain cavity. This cavity is filled with so-called neural tube fluid (NTF), whose positive pressure has been shown to play a key role in brain morphogenesis. This fluid contains a water-soluble matrix, rich in chondroitin sulfate (CS), which has been proposed as an osmotic regulator of NTF pressure genesis. The purpose of the present study is to observe the influence of CS on NTF osmolality and its relation to NTF hydrostatic pressure and brain expansion. NTF was obtained by means of microaspiration from the mesencephalic cavity of chick embryos. The osmolality of NTF between H.H. stages 20 and 29 was measured on the basis of its cryoscopic point. CS synthesis was disrupted by using beta-D-xyloside and the induced variations in brain volume were measured by means of morphometry. We also measured the variations in NTF osmolality, hydrostatic pressure, and the concentration of CS and sodium induced by means of beta-D-xyloside. Our data reveal that, at the earliest stages of development analyzed, variations in NTF osmolality show a characteristic pattern that coincides with the developmental changes in the previously described fluid pressure. Chick embryos treated with beta-D-xyloside, a chemical that disrupts CS synthesis, displayed a notable increase in brain volume but no other apparent developmental alterations. Morphometric analysis revealed that this increase was due to hyperenlargement of the brain cavity. Beta-D-xyloside brings about specific changes in the biochemical composition of NTF, which entails a large increase in CS concentration, mainly in the form of free chains, and in that of sodium. As a result, the fluid's osmolality and brain intraluminal pressure increased, which could account for the increase in size of the brain anlage. These data support the hypothesis that the intraluminal pressure involved in embryonic brain enlargement is directly dependent on NTF osmolality, and that the concentrations of CS and its associated microions could play a key role in the regulation of this process.

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.