6-Hydroxydopamine induced ectopia of external granule cells in the subarachnoid space covering the cerebellum. II. Differentiation of granule cells: a scanning and transmission electron microscopic study

Vitamin C Depletion in Prenatal Guinea Pigs as a Model of Lissencephaly Type II

Humans and guinea pigs are unable to produce vitamin C, with deficiency resulting in a well-known disorder of collagen synthesis. Pial basement membrane structure preservation is essential in the proper migration of neurons. In our study, intrauterine deprivation of vitamin C in guinea pig fetuses led to a collagen synthesis disorder, weakness, and finally a breach of pial basement membrane. We found excessive migration of the external germinal layer cells into the subarachnoid space of the cerebellum through defects in the pial basement membrane. The changes ranged from focal rupture of pial basement membranes to their complete disintegration. The loss of proper folia formation resulted in macroscopically visible flattening of the cerebellar surface. Different grades of dysplastic changes in the folia of the cerebellar cortex were observed in 2 experimental groups assigned different limits to mark the time of commencement and duration of vitamin C deprivation. The most severe form of dysplastic changes was characterized by marked irregularity of the cerebellar cortex similar to that in lissencephaly type II. Thus, prenatal vitamin C deficiency represents a novel animal model to study the effects of collagen synthesis on development of breaches in the pial basement membrane, disordered migration of neurons, dysplasia of cerebellar cortex, and the pathogenesis of lissencephaly.

Postnatal cerebellar granule cells of the white rat (Rattus norvegicus): a quantitative study, using design-based stereology

A stereological study was carried out on postnatal cerebellar granule cells of rats aged 6 and 10 days, for detecting whether and how much they would differ from those of young adult rats. The following parameters were estimated: number-weighted mean volume of the nucleus and of the soma; mean total surface area of the soma; mean absolute volumes per cell of total cytoplasm, mitochondria, Golgi apparatus, and cytosol; mean surface density of the rough endoplasmic reticulum (RER); mean total surface area of the RER. These values were compared between the two postnatal ages. In addition, those values were also analysed in comparison to the ones depicted in young adult rats (60 days), already published by our team, in order to detect similarities between them. It was noticed that, between 6 and 10 days, the mean surface density of the RER was the only parameter that did not change significantly. The comparison of each of the postnatal ages with 60 days revealed that, with the exception of the absolute volume of Golgi apparatus, significant differences were displayed concerning other organelles and cellular compartments. It was concluded that, although fine structural differences have been disclosed, from the stereological point of view postnatal granule cells at 10 days were practically similar to the young adult ones at 60 days. Some potential physiological implications have been considered.

The malformation of the cerebellar fissura prima: a tool for studying histogenetic processes

Due to its regular cytoarchitecture and the relatively low number of cell types, the development of the cerebellar cortex is a model of election for studies of morphogenetic processes. To unravel the cellular and molecular mechanisms that regulate cell development, migration and differentiation and the settling of local circuits, pertubation of the three-layered organization of the cerebellar cortex has been induced by X-ray irradiation or antimitotic drug. In this review we deal with some data about the incidence and development of the malformation of the cerebellar fissura prima of the rat, as an eligible model for histogenetic studies. The naturally occurring malformation of the fissura prima is characterized by the loss of the three-layered organization of folia and the presence of large masses of ectopic granule cells. The malformation appears to be under genetic control, since the incidence of affected animals is consistent over extended breeding cycles, although the target of the eventual mutation is unknown. The observation of development of the malformation in infant rats suggests that a defect in meninges, and in particular in the pia mater, is a primary contributing factor. The molecules responsible for this defect are not identified, but they must be involved in basal lamina stabilization or destabilization. In fact, there is evidence that meninges do develop during first stages of histogenesis and only later degenerate. A possible correlation with certain human pathologies that involve defects in foliation is discussed.

Neonatal 6-hydroxydopamine treatment affects GABA(A) receptor subunit expression during postnatal development of the rat cerebellum

Neurotoxic elimination of noradrenergic terminals by 6-hydroxydopamine (6-OHDA) leads to alteration of the granule cell layer formation. We have studied the developmental expression of GABA(A) receptor subunits in rat cerebellum after neonatal administration of 6-OHDA during the first postnatal month of life. 6-OHDA was injected subcutaneously. The expression of GABA(A) receptor subunits was studied by in situ hybridization and immunohistochemistry. The alterations were observed in the neocerebellum - the part of the cerebellum which starts development postnatally. The migration of granule cells was delayed, and the total area of the granule cell layer in the neocerebellum from 6-OHDA-treated rats was reduced to 22.6+/-5% of the corresponding area from control rats. In situ hybridization with subunit-specific antisense oligonucleotide probes was performed for alpha1, alpha2, alpha3, alpha5, alpha6, beta1, beta2, gamma1 and gamma2 subunits of the GABA(A) receptor. In neocerebellum, 6-OHDA treatment caused a significant reduction in the alpha1, alpha6 and gamma2 subunit mRNA levels. The expression of the other subunits was not changed. It has been shown that in the postnatal cerebellum alpha1 and alpha6 subunits can be detected in granule cells only when the cells had migrated to their final destination. Our findings indicate that a noradrenergic influence may be necessary for the normal maturation and migration of cerebellar granule cells.

Forebrain overgrowth (fog): a new mutation in the mouse affecting neural tube development

Forebrain overgrowth, fog, is a spontaneous autosomal recessive mutation in the mouse producing forebrain, lumbo-sacral, and facial defects. The defects appear to result from excessive growth or cellular proliferation leading to abnormalities in neural tube closure. Three unique features of the mutant are: (1) the growth of telencephalon cells into the surrounding mesenchyme, (2) presence of an encephalocele through the midline cleft in some mutants, and (3) dissociation of the tail defect from the caudal neural tube defect. We used an intersubspecific intercross between mice carrying the fog mutation and mice from an inbred Mus musculus castaneus strain (CAST/Ei) to map the fog mutation to mouse Chromosome 10 near D10Mit262 and D10Mit230 in a region with several potential candidate genes.

Novel forms of neuronal migration in the rat cerebellum

Infrared video microscopy of neonatal rat cerebellum (P0-P14) was used to directly visualize migrating granule neurons in relation to other cerebellar cells in a brain slice for up to 24 hr. Initially (P0-P5), granule neurons move along radial migration pathways of other neuronal fibers. These pathways are probably established by the bipolar granule neurons that attach to the external basement membrane via one process and extend another process toward the Purkinje cell layer. At P5-P8, a substantial number of granule neurons move horizontally and extend long parallel fibers. Both radially and horizontally migrating granule neurons move by nuclear translocation inside their preformed processes with a speed that varies between 6 and 120 microns/hr. In P10-P12 animals, the horizontally oriented granule neurons start to migrate radially. They move into the internal granule cell layer either along the radial pathways of other neuronal fibers or in contact with the matured glial processes. The radial neuronal migration pathways disappear by P14 whereas the glial cell processes are maintained and reach the basal lamina. These results describe novel radial and horizontal modes of neuronal migration that proceed independently of the physical glial guidance.

Colony-forming ectopic granule cells in the cerebellar primary fissure of normal adult rats: a morphologic and morphometric study

The morphology and degree of maturation of intrafissural colony-forming ectopic granule cells (EGCs) were investigated in the cerebella of normal adult rats. These EGCs exhibit a notable degree of morphological maturity, with 2-5 short dendrites radiating from a globular soma, typical dendritic claw-like terminals and descending axons. The EGCs receive the afferent input of mossy fiber terminals and their axons also establish homologous synapses with the postsynaptic dendritic spines of the Purkinje cells. Although these EGCs exhibit the typical morphology of mature granule cells, the morphometric analysis shows, however, a significant reduction in the number of dendrites (mean 3.30 +/- 0.60) and 'claws' (mean 3.66 +/- 0.12) in the EGCs with respect to the controls (mean 3.85 +/- 0.73 and 4.53 +/- 0.13, respectively). These differences may be indicative of a slight numerical mismatching between EGCs and afferent mossy fibers, which limit the input that the ectopic neurons receive. The karyometric study also showed that the average nuclear area in EGCs was significantly smaller (mean 19.38 +/- 0.24 micron2) than in controls (mean 21.88 +/- 0.24 micron2), suggesting a positive correlation between nuclear size and the number of postsynaptic dendritic terminals available in the granule cells. The influence of some morphogenetic factors on the degree of maturity attained by these ectopic neurons is discussed, particularly the interactions of mossy fibers and astrocytes with maturing EGCs.

Meningeal cells influence cerebellar development over a critical period

We have investigated the influence of meningeal cells on the development of the cerebellum by destroying these cells with 6-hydroxydopamine in hamsters of different ages. The ensuing foliation and lamination disruption in the cerebellar vermis is attributed to a disintegration of the cerebellar surface and a disorganization of the glial scaffold of the cerebellar cortex due to a loss of meningeal-glial interaction in stabilizing the extracellular matrix at the glia limitans superficialis (v. Knebel Doeberitz et al. 1986, Neuroscience 17:409-426). The severity of these cerebellar defects is correlated with the ontogenetic stage at which meningeal cells are destroyed, being greatest after treatment at postnatal day 1 and decreasing thereafter until day 5 and beyond, when no abnormalities occur, although all meningeal cells are destroyed throughout. The absence of cerebellar defects after destruction of meningeal cells at day 5 or later is associated firstly with the end of the period of branching morphogenesis of the cerebellum when all folial primordia are established, and, secondly, with the maturation of the glia limitans superficialis. These findings indicate that meningeal cells stabilize the cerebellar surface and glial scaffold over a critical period that ends, when the pattern of cerebellar foliation is established, and when the glia limitans superficialis has reached a mature state. Beyond this stage glial end-feet alone are sufficient to maintain the epithelial integrity of the cerebellum.

Influences of meningeal cells on brain development. Findings and hypothesis

Destruction of meningeal cells during brain development results in alterations of the extracellular matrix over the surface of the central nervous system, and a regional disruption of the external glial limiting membrane, associated with a reorganization of the glial scaffold. These changes of the glial organization have repercussions on neuronal migration and aggregation leading to a failure of neurons to segregate into spatially separated layers and lobules. We propose that meningeal cells stabilize the labile outer surface of the brain and the glial scaffold during development by specifically controlling the composition of the extracellular matrix at the interface with glial end-feet.

A Golgi and morphometric study of the ectopic granule cells in the molecular layer of the rat cerebellum

The morphological types of isolated ectopic granule cells (EGCs) were examined with Golgi-Rio Hortega staining in the cerebellar molecular layer of adult normal rats. The EGCs showed a significant reduction in the number of dendrites (mean 2.42 +/- 0.07) with respect to the controls (mean 3.97 +/- 0.09), and they usually exhibited poorly developed dendritic terminals. The karyometric analysis on semithin sections indicated that the average nuclear area in EGCs was significantly smaller (16.62 +/- 0.20 micron 2) than in normally positioned granule cells (21.08 +/- 0.24 micron 2).

Meningeal cells are involved in foliation, lamination, and neurogenesis of the cerebellum: evidence from 6-hydroxydopamine-induced destruction of meningeal cells

In the present paper we report on experiments conducted to find out if there is a positive correlation between the destruction of meningeal cells over the newborn rat cerebellum by 6-hydroxydopamine (6-OHDA) and the subsequent development of abnormalities in cerebellar fissuration, lamination, and granule cell number. Both destruction of meningeal cells and quality and magnitude of 6-OHDA-induced cerebellar defects show the same threshold sensitivity without further dose responsiveness. Blockade of neuronal uptake 1 for catecholamines with nomifensine prevents neither destruction of meningeal cells nor the development of abnormalities in cerebellar structure after 6-OHDA treatment. Blockade of extraneuronal uptake 2 for catecholamines with normetanephrine prevents both destruction of meningeal cells and the development of typical cerebellar abnormalities after 6-OHDA treatment. All three parallel experiments suggest that there is a positive correlation between the destruction of meningeal cells and the development of abnormal cerebellar structure, indicating that meningeal cells are involved in these defective morphogenetic processes, i.e., fissuration, lamination, and cell proliferation in the external granular layer. The preferential localization of defects in cerebellar fissures indicates that, in analogy to the mesenchyme surrounding other epithelia with a branching morphogenesis, the role of meningeal cells could be the production of interstitial collagen which is necessary to stabilize the epithelial basal lamina in the fissures.

Derivation of cerebellar Golgi neurons from the external granular layer: evidence from explantation of external granule cells in vivo

The present report provides evidence to challenge the traditional view that cerebellar Golgi cells are derived from the ventricular neuroepithelium, postulating instead that they originate from external granule cells. Supporting evidence for this assertion comes from three sources: 1) Typical Golgi cells are found in ectopic granule cell colonies, both outside the cerebellum (in the subarachnoid space) and also within the cerebellar cortex between fused folia. Because ectopic granule cell colonies are derived from external granule cells, which become displaced after treatment with 6-hydroxydopamine (6-OHDA), it was assumed that the ectopic Golgi cells also stem from such displaced external granule cells. 2) In order to demonstrate that Golgi cell precursors migrate from the external granular layer into the Purkinje cell plate, the development of the cerebellar cortex was studied over the period of Golgi cell genesis. On E19 the external granular layer in the rat is subdivided into an outer proliferative and an inner subproliferative zone. At the inner margin of the external granular layer, and in the marginal zone, radially oriented, darkly staining cells are present that exhibit all the characteristics of migrating neurons possessing a leading process oriented toward the Purkinje cell plate, a somatic cilium, and a close association with radial glia fibers. In later stages, these cells are also found deep to the Purkinje cell plate. Because Golgi cells arise during the period between E19 and postnatal day 2 in the rat (Altman and Bayer, '77, '78) and as the basket cells, the first neurons of proven origin from the external granular layer, are not produced before the second postnatal day (Altman, '72), the earlier migrating neurons are presumed to be Golgi cells. 3) Available data from cell kinetic 3H-thymidine studies show that there is no unequivocal evidence for Golgi cell genesis from the ventricular neuroepithelium, because, at the time of Golgi cell birth, ventricular and external granular stem cell populations are proliferating, and with the present methods it is not possible to decide which of these are the precursors of Golgi cells. Thus, taken together, the findings of this study show that Golgi cells are more likely to arise from the external granular layer than from the ventricular neuroepithelium. This concept would unify cerebellar histogenesis by proposing that projection neurons arise from the ventricular neuroepithelium, whereas all interneurons of the cerebellar cortex are descendants of the external granular layer.

6-OHDA-induced ectopia of external granule cells in the subarachnoid space covering the cerebellum. III. Morphology and synaptic organization of ectopic cerebellar neurons: a scanning and transmission electron microscopic study

The present report describes the ultrastructure, surface morphology, and synaptic connectivity of ectopically placed cerebellar neurons after treatment of newborn rats with 100 micrograms 6-hydroxydopamine (6-OHDA), administered intracisternally. In addition to granule cells which form the majority of ectopic neurons, neurons exhibiting the ultrastructure of basket/stellate cells are found in the subarachnoid space over the cerebellum. The ectopic neurons present an almost complete spectrum of homologous efferent and afferent connections. Parallel fiber synapses are found on thorns of spiny branchlets of Purkinje cell dendrites which also have grown out into the subarachnoid space, and on the somata and dendrites of basket/stellate cells. Many ectopic parallel fibers are seen to pass into the molecular layer of the underlying cerebellar cortex through defects in the pial surface, presumably connecting with intracortical postsynaptic partners. Synapses between ectopic Purkinje cell dendrites and basket/stellate cell axons are also observed; however, the source of these axons remains uncertain. Granule cell dendrites are engaged in glomeruli with mossy fibers. Moreover, ectopic granule cell colonies are densely innervated by noradrenergic fibers. Our results show that the technique of generating ectopia of external granule cells provides an additional model for investigating influences of epigenetic factors on the development of nerve cells.