An electron microscopic study of terminal degeneration in the neocortex of the cat

The nature and immediate postoperative course of experimental degeneration of axon terminals have been studied in the somatic sensory cortex. The first somatic sensory area was examined at intervals of 2 to 6 days following lesions in the thalamus, opposite cortex or ipsilateral second somatic sensory area. There is a characteristic sequence of degenerative changes which affects the terminals of each of the afferent fibre systems studied. This commences as a simple, though marked, increase in electron density of the axoplasm with no loss of synaptic vesicles and little alteration in the size or shape of the terminal. Following this, there is a progressive loss of vesicles and disruption of the mitochondria with shrinkage of the terminal and its compression, invasion and fragmentation by astroglial processes. There is evidence that many fragments are phagocytosed by the invading astroglia but a thin sliver always remains attached at the synaptic contact zone. Within the range of survival periods used, no changes affect the synaptic region nor the postsynaptic profile and if the latter is a dendritic spine, it is not detached from the parent dendrite. Changes in degenerating axons are similar, except that the largest thalamo-cortical fibres show a stage of neurofilamentous hyperplasia. In the cortex at a distance from the lesion only smaller astrocytic processes are involved in breaking down the degenerating products; close to a lesion, however, all astrocytic processes and perikarya become involved and many atypical glial cells which are difficult to classify as astrocytes or oligodendrocytes become visible; the vascular pericytes also display large heterogeneous dense bodies and other inclusions.