The influence of age upon the effect of early postnatal x-irradiation on the development of the cerebellar cortex in rats

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.

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.

Alterations in the postnatal development of the cerebellar cortex due to zinc deficiency. I. Impaired acquisition of granule cells

The effects of zinc deficiency on cerebellar development were investigated in suckling rats. In 21-day-old zinc-deficient rats the cerebellum as well as the whole brain was reduced in size. The cerebellar cortex was underdeveloped and showed a persistence of the external granule cell layer, a reduction in the thickness of molecular layer and a decrease in the area of the internal granule cell layer. Lobular variations were present. Along the posterior superior fissure there was approximately a 60% reduction in the number of granule cells and in the number of granule cells per Purkinje cell. It was suggested that the reduction in the number of granule cells was predominantly the result of impaired cell proliferation in the external granule cell layer. Comparisons with undernourished, pair-fed pups indicated that the effects of zinc deficiency could not be mediated totally through the reduced food consumption experienced by zinc deficient dams.

Experimental modification of cerebellar development in tissue culture: x-irradiation induces granular degeneration and unattached purkinje cell dendritic spines

Cerebellum explants from 15--16 day fetal mice were irradiated with 500--1000 rad X-ray and were grown in tissue culture for 12--18 days. At the dose of 500 rad, a reduction of granule cell population was observed in the explant. At the dose of 1000 rad, granule cells were mostly destroyed leaving Purkinje cells disarrayed and clustered. Electron microscopy of the explants revealed the presence of unattached dendritic spines of Purkinje cell. It was noted that although numerous free spines could be seen in X-irradiated cultures, the postsynaptic specialization of these spines occurred only in ones embedded in astrocytic cytoplasm. The results indicate that (a) the formation of dendritic spines is a property of the Purkinje cell which probably is preprogrammed and (b) the postsynaptic differentiation of the unattached spines is directly related to their contacts with astrocytes.

Membrane-fusions and cytoplasmic bridges in the cells of the developing cerebellum

In the developing cerebellum of the neonate rats membrane-fusions and cytoplasmic bridges between cells were observed. These membrane-fusions were characterized by the presence of loops of membrane and cytoplasmic bridges between the two limits of the membrane-fusions. They were found between Purkinje cells, Purkinje cells and the migratory cells, mitotically potent cells of the external granular layer, and differentiating granule cells of the internal granular layer. The membrane-fusions were found to be a transient developmental phenomenon. Issues pertaining to the universality of membrane-fusions, their significance in the induction for cell differentiation, and the problem of fixation artifacts are discussed.

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.

Differentiation of Bergmann glia cells in the cerebellum: a golgi study

The migration of Bergmann glia cells from the external granular layer to the Purkinje cell layer and the differentiation of their processes were analyzed in Golgi-Cox preparations of the cerebellum of the rat. During their migration they showed numerous cytoplasmic excrescences and long thick processes, which suggested that they may follow ameboid form of movement for their migration. The differentiation of their processes was observed to involve a series of complex events related to retraction of the thick apical processes from the tips of which sheaves of filiform processes emerged, growth in the number and the thickness of the filiform processes, resorption of some of these filiform processes, cyclic changes in the appearance, resorption and reappearance of excrescences on the filiform processes, and changes in size and shape of the growth cones. These findings are discussed in the context of the role played by the Bergmann glia cells in the postnatal developmental events in the cerebellum, and the dynamic nature of differentiation of their processes.

Resting and reactive macrophages in the developing cerebellum: an experimental ultrastructural study

The whole heads of 6 days old rats were exposed to 150R of X-ray irradiation to induce cell-death in the external granular layer of the cerebellum. The animals were sacrificed in a developmental sequence, and the tissue obtained from the cerebellum was prepared for electron microscopy. On the basis of the cytological criteria established for the macrophages in the cerebellum of the normal animals, characteristics of the resting and reactive macrophages and their transformation from one state to the other in the experimental animals are described. In their reactive state the macrophages showed hypertrophic and nuclear changes which were found identical to those seen in the reactive microglial elements. Issues pertaining to the ultrastructural characteristics of the macrophages, their endogenous presence in the developing cerebellum, and their transformation into the microglia cells are discussed.

Cerebellar microgyria

The histological features of cerebellar microgyria are described in a 15 month old infant. The bizarre appearance of this malformation seems to be the result of two basic factors: secondary fusion between preformed folia, and deficient and disorderly formation of the internal granule cell layer. The findings support the view according to which cerebellar microgyria is probably due to a pathological process involving the external granule cell layer during a relatively late period of cerebellar development.

Dark neurons: a significant artifact: the influence of the maturational state of neurons on the occurrence of the phenomenon

In cerebella of young rats, killed by decapitation, dark Purkinje cells were readily observed in animals from 16 days of age onward. In younger animals (from 0-12 days of age) these dark neurons were never observed. The author agrees with Cammermeyer and others that dark neurons are an avoidable artifact. The findings presented suggest, however, that the possibility of the occurrence of this type of artifact depends on the maturational state of the neurons concerned.