Neurons of layer Vb of rat sensorimotor cortex atrophy but do not die after thoracic cord transection.

Albino rats six weeks (wk) of age underwent transection of the spinal cord at the level of the seventh thoracic vertebra. They were killed ten wk later by several schedules of formaldehyde-glutaraldehyde, formaldehyde and formaldehyde-ethanol-acetic acid perfusion-fixation. Layer Vb of the sensorimotor cortex, the site of origin of corticospinal axons severed by the operation, was searched by light and electron microscopic methods for evidence of neuronal necrosis. Cord-transected rats were compared with control, unoperated animals of identical age. Nerve cell death was not evident to qualitative study, although shrunken, deeply-staining neurons of artefactitious origin occurred capriciously in paraffin sections when fixation was initiated with a dilute formaldehyde-glutaraldehyde solution. Quantitative light and electron microscopic studies were also negative for indications of neuronal death. However, mild somal atrophy could be substantiated for layer Vb neurons of cord-transected rats by light microscopic, morphometric methods. Neuronal atrophy was unaccompanied by qualitative or quantitative ultrastructural alterations. Subcellular organelles and the per cent of neuronal plasma membrane apposed by axosomatic boutons were unchanged. Neuroglia and neuronal processes always had a normal electron microscopic appearance.