Developmental biology of the meninges.

The meninges are membranous layers surrounding the central nervous system. In the head, the meninges lie between the brain and the skull, and interact closely with both during development. The cranial meninges originate from a mesenchymal sheath on the surface of the developing brain, called primary meninx, and undergo differentiation into three layers with distinct histological characteristics: the dura mater, the arachnoid mater, and the pia mater. While genetic regulation of meningeal development is still poorly understood, mouse mutants and other models with meningeal defects have demonstrated the importance of the meninges to normal development of the calvaria and the brain. For the calvaria, the interactions with the meninges are necessary for the progression of calvarial osteogenesis during early development. In later stages, the meninges control the patterning of the skull and the fate of the sutures. For the brain, the meninges regulate diverse processes including cell survival, cell migration, generation of neurons from progenitors, and vascularization. Also, the meninges serve as a stem cell niche for the brain in the postnatal life. Given these important roles of the meninges, further investigation into the molecular mechanisms underlying meningeal development can provide novel insights into the coordinated development of the head.

Breaches of the pial basement membrane and disappearance of the glia limitans during development underlie the cortical lamination defect in the mouse model of muscle-eye-brain disease.

Neuronal overmigration is the underlying cellular mechanism of cerebral cortical malformations in syndromes of congenital muscular dystrophies caused by defects in O-mannosyl glycosylation. Overmigration involves multiple developmental abnormalities in the brain surface basement membrane, Cajal-Retzius cells, and radial glia. We tested the hypothesis that breaches in basement membrane and the underlying glia limitans are the key initial events of the cellular pathomechanisms by carrying out a detailed developmental study with a mouse model of muscle-eye-brain disease, mice deficient in O-mannose beta31,2-N-acetylglucosaminyltransferase 1 (POMGnT1). The pial basement membrane was normal in the knockout mouse at E11.5. It was breached during rapid cerebral cortical expansion at E13.5. Radial glial endfeet, which comprise glia limitans, grew out of the neural boundary. Neurons moved out of the neural boundary through these breaches. The overgrown radial glia and emigrated neurons disrupted the overlying pia mater. The overmigrated neurons did not participate in cortical plate (CP) development; rather they formed a diffuse cell zone (DCZ) outside the original cortical boundary. Together, the DCZ and the CP formed the knockout cerebral cortex, with disappearance of the basement membrane and the glia limitans. These results suggest that disappearance of the basement membrane and the glia limitans at the cerebral cortical surface during development underlies cortical lamination defects in congenital muscular dystrophies and a cellular mechanism of cortical malformation distinct from that of the reeler mouse, double cortex syndrome, and periventricular heterotopia.

A critical function of the pial basement membrane in cortical histogenesis.

Mice with a targeted deletion of the nidogen-binding site of laminin gamma1 were used to study the function of the pial basement membrane in cortical histogenesis. The pial basement membrane in the mutant embryos assembled but was unstable and disintegrated at random segments. In segments with a disrupted basement membrane, radial glia cells were retracted from the pial surface, and radially migrating neurons, including Cajal-Retzius cells and cortical plate neurons, passed the meninges or terminated their migration prematurely. By correlating the disruptions in the pial basal lamina with changes in the morphology of radial glia cells, the aberrant migration of Cajal-Retzius cells, and subsequent dysplasia of cortical plate neurons, the present data establish a causal relationship of proper cortical histogenesis with the presence of an intact pial basement membrane.

Ephrin B1 is expressed on neuroepithelial cells in correlation with neocortical neurogenesis.

To identify molecules involved in neurogenesis, we have raised monoclonal antibodies against embryonic day 12.5 mouse telencephalon. One antibody, monoclonal antibody 25H11, stains predominantly the ventricular zone of the anterior and lateral telencephalon. Purification of the 25H11 antigen, a 47 kDa integral membrane protein, from approximately 2500 mouse telencephali reveals its identity with ephrin B1. Ephrin B1 appears at the onset of neocortical neurogenesis, being first expressed in neuron-generating neuroepithelial cells and rapidly thereafter in virtually all neuroepithelial cells. Expression of ephrin B1 persists through the period of neocortical neurogenesis and is downregulated thereafter. Ephrin B1 is present on the ventricular as well as basolateral plasma membrane of neuroepithelial cells and exhibits an ventricular-high to pial-low gradient across the ventricular zone. Expression of ephrin B1 is also detected on radial glial cells, extending all the way to their pial endfeet, and on neurons in the mantle/intermediate zone but not in the cortical plate. Our results suggest that ephrin B1, presumably via ephrin-Eph receptor signaling, has a role in neurogenesis. Given the ventricular-to-pial gradient of ephrin B1 on the neuroepithelial cell surface and its known role in cell migration in other systems mediated by its repulsive properties, we propose that ephrin B1 may be involved in the migration of newborn neurons out from the ventricular zone toward the neocortex.

The meninges in human development.

The brain and cranial meninges were studied in 61 serially sectioned embryos of stages 8-23. Much earlier stages than those examined by previous authors provided a more comprehensive view of meningeal development. As a result, the possible and probable sources of the cranial and spinal meninges are believed to be: (a) prechordal plate, (b) unsegmented paraxial (parachordal) mesoderm, (c) segmented paraxial (somitic) mesoderm, (d) mesectoderm (neural crest), (e) neurilemmal cells (neural crest), and (f) neural tube. Some of these sources (a, b, d) pertain to the cranial meninges, others (c, d, e) to the spinal coverings. The first of the future dural processes to develop is the tentorium cerebelli, which, at the end of the embryonic period proper, differs considerably in shape and composition from the later fetal and postnatal tentorium. The embryonic dural limiting layer (Duragrenzschicht) probably corresponds to the interface layer of the adult meninges. The appropriate literature was reviewed and summarized.

Different origins and developmental histories of transient neurons in the marginal zone of the fetal and neonatal rat cortex.

Two major classes of early-born neurons are distinguished during early corticogenesis in the rat. The first class is formed by the cortical pioneer neurons, which are born in the ventricular neuroepithelium all over the cortical primordium. They appear at embryonic day (E) 11.5 in the lateral aspect of the telencephalic vesicle and cover its whole surface on E12. These cells, which show intense immunoreactivity for calbindin and calretinin, are characterized by their large size and axonal projection. They remain in the marginal zone after the formation of the cortical plate; they project first into the ventricular zone, and then into the subplate and the internal capsule. Therefore, these cells are the origin of the earliest efferent pathway of the developing cortex. Pioneer neurons are only present in prenatal brains. The second class is formed by subpial granule neurons, which form the subpial granular layer (SGL), previously considered to be found exclusively in the human cortex. SGL neurons are smaller than pioneer neurons. They are generated in a transient compartment of the retrobulbar ventricle between E12 and E14, and we propose the hypothesis that they invade the marginal zone, through tangential subpial migration, at different moments of fetal life. SGL neurons contain calbindin, calretinin, and gamma-aminobutyric acid (GABA), but the GABA-immunoreactive group becomes inconspicuous before birth. The extracellular matrix-like glycoprotein reelin, a molecule crucial for cortical lamination, is prenatally expressed by SGL neurons; postnatally, it is present in both Cajal-Retzius cells and subpial pyriform cells, both derivatives of SGL cells. In the rat, Cajal-Retzius cells are horizontal neurons that remain only until the end of the first postnatal week. They are located in layer I at a critical distance of approximately 20 microm from the pial surface and express reelin and, only occasionally, calretinin. Subpial pyriform cells coexpress reelin and calretinin and remain in layer I longer than Cajal-Retzius cells. Both pioneer neurons and subpial granule neurons are specific to the cortex. They mark the limit between the rudimentary cerebral cortex and olfactory bulb in the rat during early corticogenesis.

Meningeal-cutaneous relationships in anencephaly: evidence for a primary mesenchymal abnormality.

The pathogenesis of cranial and spinal dysraphia has been controversial. Studies of spinal dysraphia have shown that the relationships of the pia and dura to the cutaneous layers were best understood as the result of a primary abnormality of mesenchymal structures, with the nervous system lesions occurring as a result of exposure of the bare spinal cord on the body surface. This study was undertaken to determine if the relationship of the cutaneous layers in anencephaly were similar to those found in spinal dysraphia. We reviewed serial histologic sections of the cranial structures of 10 anencephalic fetuses autopsied at The Johns Hopkins Hospital. We found the dura to be continuous with the deep dermis and the pia continuous with the superficial dermis and epidermis, the same arrangement observed in myelomeningocele. The development of eyes and cranial nerves, the absence of a bony calvarium, and the meningeal-cutaneous relationships found in this study support the idea that anencephaly can originate as an abnormality of mesenchymal structures and that the brain is secondarily lost to injury in utero because of its exposed position.

Birth-related expression of c-fos, c-jun and substance P mRNAs in the rat brainstem and pia mater: possible relationship to changes in central chemosensitivity.

In situ hybridization was used to characterize respiration-related areas of the brainstem activated around the time of birth as well as their postnatal sensitivity to CO2. Levels of mRNA corresponding to the immediate early genes (IEG), c-fos and c-jun, and of substance P precursor, ppt-A, were determined in rat fetuses (E21) and neonatal pups (1 h, 1 day and 6 days after normal birth) and after exposure to hypercapnia (12% CO2 for 1 h). Transient increases in c-fos mRNA were observed in the central chemoreceptor area of the ventral medullary surface (VMS), in the lateral reticular nucleus (LRN), in the nucleus of the solitary tract (NTS), and in the nucleus raphé pallidus (RPA) 1 h after birth. Increased expression of c-fos mRNA in the VMS could also be evoked by hypercapnia and this response was particularly pronounced 1 day after birth. On the other hand, c-jun mRNA could be detected already at E21 in the hypoglossal nucleus (XII) and LRN and these levels were not significantly altered at 1 h after birth. There was, however, an increase in the expression of c-jun mRNA in the pia mater surrounding the brainstem after birth. At 1 day after birth, c-jun mRNA levels had decreased in the LRN and pia mater, and later on (6 days after birth) in XII. Furthermore, the ppt-A mRNA level in NTS increased immediately after birth and remained high 1 and 6 days later. These results suggest that (a) the central chemoreceptor area of the VMS, as well as the NTS, LRN, RPA and pia mater are activated following birth; (b) the VMS, but not the other structures examined, can be activated immediately after birth by hypercapnia; and (c) increased expression of ppt-A mRNA may be related to the transition of respiratory control at birth.

On the origin and nature of the pituitary gland capsule.

Observations under the operating microscope confirming the presence of a pituitary capsule are reported. This capsule envelops the anterior lobe of the pituitary, the neurohypophysis, and the pituitary stalk. It merges along the stalk with the intracranial pia mater. The origin and nature of this capsule are discussed in light of the known facts of development of the pituitary gland and surrounding structures. It is concluded that the pituitary gland capsule is a derivative of the primitive pia mater.

Scanning electron microscopy of human cerebral meningeal stomata.

The human dura mater and pia mater were studied by using a scanning electron microscope and a computer image processing system (C.I.P.). The human cerebral meningeal stomata are located between the mesothelial cells of the cerebral meninges. They are round or oval in shape with diameters of 0.33-2.98 microns. The cerebral meningeal stomata are stable structures, scattered or clustered together. Their density in the dura mater is greater than in the pia mater (P < 0.01), and they are regularly distributed. The statistical analysis showed that the stomata diameter and distribution density in the dura mater are 1.34 microns and 381.55/0.1 mm2; while in the pia mater they are 0.88 micron and 195.06/0.1 mm2 respectively. The cerebral meningeal stomata are probably part of the cerebral prelymphatic capillary system, which undertakes the cerebral lymph drainage because there are no lymphatic vessels in the brain although yet there is lymph drainage. Thus, we believe that the cerebral meningeal stomata are involved in maintaining the physiological function of the brain as part of the cerebrospinal fluid (CSF) which absorbs the cerebral interstitial fluid.

[Rosette formation in explants of human embryonic neocortex (an electron-microscopic study)]. / Obrazovanie rozetok v éksplantatakh neokorteksa émbrionov cheloveka (élektronno-mikroskopicheskoe issledovanie).

Multiple invaginations and closed cavities (rosettes) were developed in 199 fragments of wall of human anterior cerebral vesicle. Contraction of neuroepithelial cells apexes after the principle of the gathered tobacco pouch was involved into the process. This confirms the previous suggestion of the authors on the similarity between the mechanisms of rosettes forming and neurulation. The participation of radial glia cells and neuroblasts in the reorganization of the neocortex germ was also studied.

Mutations in the beta-tubulin gene TUBB2B result in asymmetrical polymicrogyria.

Polymicrogyria is a relatively common but poorly understood defect of cortical development characterized by numerous small gyri and a thick disorganized cortical plate lacking normal lamination. Here we report de novo mutations in a beta-tubulin gene, TUBB2B, in four individuals and a 27-gestational-week fetus with bilateral asymmetrical polymicrogyria. Neuropathological examination of the fetus revealed an absence of cortical lamination associated with the presence of ectopic neuronal cells in the white matter and in the leptomeningeal spaces due to breaches in the pial basement membrane. In utero RNAi-based inactivation demonstrates that TUBB2B is required for neuronal migration. We also show that two disease-associated mutations lead to impaired formation of tubulin heterodimers. These observations, together with previous data, show that disruption of microtubule-based processes underlies a large spectrum of neuronal migration disorders that includes not only lissencephaly and pachygyria, but also polymicrogyria malformations.

A morphological study of the development of the leptomeninx of the fetal rabbit cerebrum.

The development of the leptomeninx of the rabbit cerebrum is similar to that of the rabbit spinal cord. At E12 and E14 the leptomeningeal cells resemble immature fibroblasts. At E16 glycogen granules appear in the leptomeningeal cell cytoplasm and are present in pial as well as arachnoid cells. The amount of glycogen decreases from E22 until by E28 it is rarely found in leptomeningeal cells. Leptomeningeal macrophages are present from E12 but granular pial cells were not observed prior to E16. Initially collagen in the pial layer consists of scattered fine fibrils but by E20 collagen is present in larger amounts and the fibrils are of larger diameter and banded.

[The middle cerebral artery system in fetuses: the distribution of the pial branches]. / Sistemul arterei cerebrale mijlocii la fat: distributia ramurilor piale.

The aim of our study is to point out the influence of the cerebral development on the evolution of the middle cerebral artery (MCA) system. Brains of 20 fresh fetuses (16-22 weeks) were injected with gelatin-ink China mixture, observed and dissected under surgical microscope in correlation with the evolution of the sylvian fossa. Ending pattern of MCA and territories of distribution of its branches were also discussed.

[Adrenergic innervation of the pial arteries of the human brain]. / Adrenergicheskaia innervatsiia arterii miagkoi obolochki golovnogo mozga cheloveka.

By means of glyoxylic acid adrenergic neural fibres have been studied in fetuses at the second part of pregnancy, in a 2-month-old boy, in a 18-year-old boy, in three men 30-, 34- and 40-year-old. According to the data of fluorescent microscopy, the fibres studied contain noradrenaline. In walls of the arteries 400 mkm and more in diameter there are double-layered adrenergic plexuses. In arteries of smaller diameter the adrenergic plexuses predominantly consist of longitudinal fibres. The adrenergic apparatus is situated within the external membrane; its development is more perfect in mature persons than in fetuses and in children before one year of age.

An ultrastructural study of development of the leptomeninx of the rabbit spinal cord.

Development of the rabbit spinal cord leptomeninges was examined in embryos and fetuses aged 12 to 30 d post-conception. In the early stages of development all mesenchymal cells surrounding the neural tube were structurally similar, resembling immature fibroblasts. At 16 d post-conception cells adjacent to the glia limitans showed little structural change, apart from an increase in the amount of rough endoplasmic reticulum, but cells in the presumptive arachnoid became packed with glycogen. By E22 glycogen was present in pial cells but never in the amounts found in the arachnoid. As development proceeded the amount of glycogen in the leptomeninges declined. Pial collagen increased both in amount and in fibre diameter with age.

Human telencephalic angiogenesis.

We provide new observations regarding the histogenesis and regional development of the human telencephalic microvasculature during the last half of gestation. Endothelium-lined trunks extending from pia to the subventricular plexus, evident early in gestation, persist during the last half of gestation. The courses of these trunks are modified and become complex as the bulk of the telencephalon, striatum, and thalamus increases and as gyri grow. New cerebral tissue is supplied by increasing numbers of shorter penetrating vessels. All extrastriatal vessels have many lateral right- and acute-angle branches that join nearby trunks and shorter vessels. Striatal vessel branches predominantly have acute angles. Most extrastriatal channels remain devoid of apparent muscularis until the final weeks of gestation. In contrast, striatal arteries begin to muscularize at about 24 weeks of gestation. Muscularization appears to occur in a centripetal direction and is apparent in the caudate at approximately 30 weeks' gestation. We did not identify transventricular, paraventricular, or recurrent arteries ending in deep white matter.

The vitreus, an intraocular compartment of the leptomeninx. Electron microscopic observations on rat eyes.

In the 16-day rat embryo and in 2 and 15 days old postnatal rats the macrophage- and fibroblast-like cells of the vitreus are not different from those of the developing leptomeninx. There is also no difference in the ultrastructure of the developing blood vessels. Both vitreus and leptomeninx are separated from the neuroectodermal tissues by a basal lamina. Regarding the embryonic development, the entire intraocular cavity except the lens, is a special cleared-out portion of the former mesenchymal tissues surrounding the central nervous system including the eye cup (meninx primitiva). It may be interpreted as analogous to a leptomeningeal cisterna. The largest part of this intraocular cisterna is the vitreus. The pathogenesis of diseases occurring simultaneously in the leptomeninx, the uvea, and the vitreus, possibly may have some relation to the common embryologic origin of the affected tissues.