The development of murine retrovirus induced spongiform polioencephalomyelopathy was studied sequentially by electron microscopy. During the initial 30 days, viral infection of the central nervous system, as evidenced by viral budding from membranes, was limited to the endothelial cells and pericytes. Viral particles were observed in the lumen of blood vessels, extracellular spaces and astrocytic endfeet surrounding blood vessels, but no morphological evidence of productive infection was found in astrocytes or neurons during early development of vacuolation. The earliest lesions in the neuropil consisted of swelling of astroglia followed by vacuolation, initially in axons and dendrites and later in neuronal and astrocytic soma, where vacuoles appeared to arise from dilated cisternae of the Golgi apparatus. Vacuoles contained only amorphous debris and fragments of membranes. Virions budding aberrantly into vacuoles were seen only in mice surviving beyond 35 days. Numerous reactive astrocytes were observed, but inflammatory cells were absent. The ultrastructural changes were remarkably similar to those described in scrapie, Kuru, and Creutzfeldt-Jakob disease.
The external granular and molecular layers in the foetal cerebellar cortex of mice and rats were examined by electron microscopy for the presence of Bergmann glial fibres. Morphologically distinct Bergmann fibres were observed at embryonic day E 15 in the mouse and at E 17 in the rat. Even at prenatal stages of development these fibres have a considerable degree of cytological differentiation which permits their identification as glial elements. The glial fibres contain numerous microfilaments, some smooth endoplasmic reticulum, a few mitochondria and scant free ribosomes. They penetrate the molecular and external granular layers radially and terminate with endfeet at the cerebellar surface. The proliferative cells of the external granular layer possess cytoplasmic processes which are oriented randomly, do not have endfeet, and are morphologically distinct from the Bergmann fibres with which they intermingle. In conclusion, immature Bergmann glial cells are present well before birth in the rodent cerebellum.
A 10-fold reduction in the incubation period of murine neurotropic retrovirus spongiform polioencephalomyelopathy was effected by a 1,000-fold concentration of the cloned virus inoculum.
Mice that were deprived of rapid eye movement sleep for 2 days immediately after one-trial training in an inhibitory avoidance task and were given an electroconvulsive shock after deprivation displayed retrograde amnesia on a retention test given 24 hours later. Electroconvulsive shock produced no amnesia in comparable groups of animals that were not deprived of rapid eye movement sleep.
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.
The ventricular layer (VL) of foetal mouse cerebellum at days 13--15 of gestation was studied by light and electron microscopy. In Golgi-stained material, round or ovoid cells are located in the VL. These cells have ascending processes, which extend to the pial surface. Ultrastructurally, the ascending processes are electron-lucent, contain microfilaments, some smooth endoplasmic reticulum and scant free ribosomes. They appear to be immature glial processes, oriented radially away from the ventricle. The perikarya of these glial cells lie either in the ventricular or subventricular zones. Juxtaposed along the length of these radially oriented glial processes are unidentified cells, some of which are attached to the immature glial fibres by puncta adhaerentia. These cells are elongated or ovoid with a thin rim of cytoplasm containing few organelles. These unidentified cells may represent neuroblasts (Purkinje cells, Golgi cells, cells of the deep cerebellar nuclei) or glioblasts, (precursors of astrocytes and/or oligodendrocytes) at very early stages of development.
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