Studies on ectopic granule cells in the cerebellar cortex--with a hypothesis as to their aetiology and pathogenesis

Cerebellar foliation in rats. 4. Mid-line distortion of the primary fissure in normal postnatal rats

Two types of mid-line distortion affecting the primary fissure of the cerebellum in foetal rats were recently described (Conradi 1987a). During the postnatal period, cells of the external granular layer (EGL) in distorted regions differentiated to granule cells, forming a heterotopic granule cell layer replacing the deeper portion of the fissure. Differentiated heterotopic granule cells were seen close to normal EGL. Formation and growth of lobules V and VI, apart from the partial fusion at the mid-line, was not affected by the distortions. The findings support the previous suggestion that foliation is relatively independent of formation of a fissure. A network of capillaries was formed in the heterotopic granule cell layer and was continuous with capillaries in the molecular layer of lobules V and VI. The frequency of capillaries in the heterotopias increased in parallel with an increased activity of oxidative enzymes, simulated that in the internal granule cell layer and was higher than that in the molecular layer. These findings support the suggestion of a correlation between nerve-cell differentiation and capillary growth.

Disorders of neuronal migration

Neuronal migration constitutes one of the major processes by which the central nervous system takes shape. Detailed knowledge about this important process now exists for different brain regions in rodent and monkey models as well as in the human. In the human, distinct genetic, chromosomal and environmental causes are known that affect neuronal migration, often in a morphologically distinct pattern, but the underlying pathological mechanisms are largely unknown. This review is intended to integrate our basic knowledge of the field with the accumulated intelligence on a large number of disorders and syndromes that represent the human part of the story.

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.

Orthostatic hypotension from a cerebellar gangliocytoma (Lhermitte-Duclos disease). Case report

A 49-year-old woman presented with orthostatic hypotension. Vertebral angiography and ventriculography revealed a tumor of the left cerebellar hemisphere. Ventriculoperitoneal shunting was followed by complete disappearance of the orthostatic hypotension. The tumor was subsequently removed and microscopic study showed Lhermitte-Duclos disease. Orthostatic hypotension has been rarely reported in association with tumors of the posterior fossa except for those tumors destroying the medullary centers and interrupting the baroreceptor reflex arc. This case is of interest because the tumor was restricted to the cerebellum. The authors have found no previous case in which orthostatic hypotension was a presenting symptom of Lhermitte-Duclos disease.

Neurogenesis and gliogenesis in the human brain

The primary germinal neural epithelium lines the ventricular system of the brain and shows intense mitotic activity. Neuroblasts migrate from a juxtaluminal position to establish neural centres and, when this task is completed, the definitive ependymal epithelial lining of the ventricles develops, shifting the remnants of the germinal epithelium to a subendymal location. The subendymal layer continues to proliferate giving rise to glia in the postnatal brain. Premature arrest of migration of neuroblasts leads to the accumulation of heterotopic nests anywhere along the migratory path. Within such ectopia, neurons may make normal connections or they may remain undifferentiated and later become tumorigenic. The subependymal layer has long been considered the site of original of periventricular gliomas. Some migrating neuroblasts establish secondary germinal centres far removed from the ventricular system. An example of this is the external granular layer of the cerebellum which, in man, persists for the first 6 months of life. Cerebellar neuroblastomas could originate directly from this layer in the neonate, or from remnants of this layer in older individuals.

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.

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

The present report describes the morphological differentiation of ectopic granule cells from external granule cells that have been induced to escape from the cerebellar cortex into the subarachnoid space by injecting neonatal rats with 100 microgram 6-hydroxydopamine (6-OHDA) into the cisterna magna. The following cell types were observed in the period between 5 and 25 days postinjection (dpi): (1) unipolar cells with one process bearing a growth cone at its tip; (2) bipolar cells with two thin beaded processes originating from opposite cell poles, bearing growth cones at their tips; (3) bipolar cells with a T-like process at one pole and a short process lacking a terminal growth cone at the opposite pole; (4) multipolar cells with one thin beaded process and two or more short processes bearing growth cones of a different morphology at their tips; (5) intermediate stages. In the late second week p.i., cell aggregates were observed that continually increased in size up to 30 dpi. On the basis of our light, transmission, and scanning electron microscopic findings, we interpret these cell types to be equivalent to the individual stages of granule cell differentiation that characterize axon formation, migration, and aggregation. In the period between 30 and 365 dpi, granule cells were almost exclusively organized into cell colonies of different sizes, but small cell clusters and single granule cells exhibited the scanning electron microscopic features of adult granule cells, i.e., a small spherical cell body, a single axon with parent axonal stem, T-junction, and parallel fiber, and dendrites engaged in synaptic glomeruli. The parallel fibers ran in fasciculi of different sizes, often parallel to each other, but without preferential orientation over the cerebellar surface. During migration and aggregation, the granule cells and their processes were associated with a substrate of glial sheets that in turn were connected to intracortical Bergmann glia fibers. Our findings indicate that (1) granule cells differentiate normally in an ectopic environment in the presence of glia, (2) ectopic Bergmann glia contain no directional information to guide aberrant migratory granule cells to their correct destination, (3) granule cells can survive outside the brain parenchyma for periods up to one year (the longest postinjection interval studied).

Formation of heterotopic granule cell in mouse cerebellum after neonatal administration of cytosine arabinoside

The mice, ICR-JCL strain, were injected s.c. with 30 mg/kg body weight of cytosine arabinoside at the age of 2, 3, and 4 days. The external granular layer of these mice was destructed selectively, and subsequently these mice developed abnormal cytoarchitecture in the cerebellum, such as disarrangement of Purkinje cells and heterotopic granule cells in the molecular layer. This study was undertaken to elucidate the mechanism in the formation of heterotopic granule cells. In the cerebellum of the treated mouse, some mossy fibers and glomerular collaterals of climbing fibers extended abnormally even into the molecular layer by the age of 10 days, since no granule cells migrated to the inner granular layer until about 10 days when granule cell production started again in the regenerated external granular layer. Subsequently, these fibers, i.e. axons which extended into the molecular layer, established synapses in the molecular layer with the dendrites of migrating granule cells. These granule cells had no need to migrate to the inner granular layer, and so they remained in the molecular layer as heterotopic granule cells.

A quantitative study of the composition of cerebellar cortical dysplasias

The areas of molecular and granular layers in four large cerebellar cortical dysplasias were measured with the planimeter and the Purkinje cells were counted. There was a relative deficit of molecular layer and a still greater deficit of Purkinje cells in three of the four dysplasias. The fourth one had normal numbers of Purkinje cells per molecular layer and a relative deficit of granular layer. These abnormal proportions in the composition of a dysplasia may relate to the timing of its origin.

Redirected perforant and commissural of connections of eutopic and ectopic neurons in the hippocampus of methylazoxymethanol-acetate treated rats

In an earlier study, it was reported that clusters of ectopic neurons developed postnatally in the hippocampus of rats which were exposed to Methylazoxymethanol acetate (MAMac) during fetal development (Singh, 1977b). This paper describes the perforant tract and commissural connections of hippocampal eutopic and ectopic neurons. These connections were traced with a reduced-silver method (Eager, 1970). Two observations of significance were made: (i) Ectopic neurons misplaced in stratum radiatum received terminals from axons in the perforant tract. The upper boundary for these redirected fibers was stratum pyramidale--approximately 350 mu outside the normal boundary which is situated near the hippocampal fissure. (ii) Ectopic neurons received a dramatically reduced commissural projection, compared with eutopic pyramidal neurons in Ammon's horn. Eutopic neurons in the hippocampus were found to receive afferent perforant tract and commissural fibers in the same way--i.e., density and distribution, as in control rats.

Presence of displaced neural elements within rat cerebellar fissures

Clusters of neural tissue have been observed within the cerebellar fissures of normal and experimentally treated albino rats and hooded rats, between the ages of 5 and 90 days. The observations of these intra-fissural clusters were made in conjunction with several series of experiments dealing with the effects of various treatments on cerebellar morphogenesis. More important is the fact that similar displaced clusters are present in normal, untreated animals. Clusters can show a considerable variation in size, ranging from a few closely packed cells, to clusters with many cells completely occupying the area between the two folial surfaces, The clusters consist of immature or mature granule cells, depending on the age. With increasing age a well-developed neuropil is seen with bundles of parallel fibers coursing among the granule cells, along with occasional Bergmann glial fibers, capillaries and pericytes. Synaptic contacts, possibly granule cells, are present within the cluster. The continued differentiation of the cells in the clusters is exemplified by the observation of a mossy fiber terminal at the base of a cluster. Connecting each cluster with the nearby parenchyma is a stalk consisting of granule cells, Bergmann glial fibers, and neurites. The basal lamina of the folial surface is interrrupted at this point. The presence of these clusters implies a failure of normal morphogenetic and migrational control mechanisms. Importantly, the cells of the clusters continue to differentiate and to make synaptic connections, despite their abnormal location.

Lack of evidence for glial cells originating from the external granular layer in mouse cerebellum

In 25 day mice cerebella, a quantitative electron microscopic analysis showed that glial cells were not seen among 749 cells counted in the molecular layer. Likewise, a light microscopic autoradiographic study showed that labelled oligodendroglia and/or astroglia in the cerebellum were not derived from the external granular layer (EGL). Previous claims, that these cells derived from the EGL, may have arisen because other cell types, i.e., endothelial cells, pericytes, microglia, and other ectopic granule cells may have been misidentified as oligodendroglia and/or astroglia. It seems likely that the EGL is a unique germinal cell layer in the mammalian nervous system because it gives rise only to neurons, whereas cerebellar astrocytes and oligodendrocytes are derived from the subventricular layer of the fourth ventricle, as first suggested by Cajal in 1911.

Effects of low-level x-irradiation on cat cerebella at different postnatal intervals. I. Quantitative evaluation of morphological changes

Whole head fractionated doses of 200 r and 150 r were initiated postnatally in five experimental age groups (birth, 1-week, 2-week, 3-week and 4-week) and continued over a period of 14 or 20 days to prevent reconstitution of the external granular layer. Animals irradiated at birth displayed minor deficits in behavior, which included ataxia, tremor, hypertonus and dysmetria, while animals irradiated at 1-week showed only mild symptoms of hypermetria. All other animals displayed no motor deficits. Animals irradiated at birth had smaller eyes and ears, a reduction in the size of the entire head and were susceptible to seizures. All animals were sacrified at 70 days of age. The cerebellum was found to be reduced in size and weight, the greatest deficit being seen in animals x-irradiated at the very early ages. Newborn condition animals were found to have large compliments of interneurons in the molecular layer, an established internal granular layer, and Purkinje cells were found to have a normal orientation, position, and to be unreduced in number or size. Total granule cell deficits were found to range from 83% at birth to 29% at four weeks. Quantitative changes for the molecular layer, internal granular layer, medullary layer, Purkinje cell to granule cell ratio, and granule cell density, all depicted the greatest changes in the newborn, 1-week and 2-week conditions. This experiment confirmed that the critical period in the development at which damage would result in behavioral abnormalities was from birth to five days, while for neuroanatomical abnormalities, this critical period was from birth to 18 days.

Studies on ectopic granule cells in the cerebellar cortex. III. An investigation into the restoration of the external granular layer after partial destruction

In order to strengthen a hypothesis concerning the occurrence of ectopic granule cells, one of the assumptions made was tested systematically. The reaction of the EGL to partial destruction by various single doses by hydroxyurea at various ages was followed. Under all conditions examined, re-population of the EGL takes place--rapidly after lower doses, slowly after high doses of HU. The phenomena observed are discussed with a view on the hypothesis mentioned. Re-population is beneficial, but may itself be a major pathogenetic factor in certain developmental malformations by upsetting the "time-schedule". The results are also of potential interest for an analysis of the regulation of normal cerebellar morphogenesis.