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Electron microscopy of non‐neuronal cellular changes accompanying neural degeneration in thalamic nuclei of the rabbit. I. Reactive hematogenous and perivascular elements within the basal lamina

Electron microscopic examination of several thalamic nuclei undergoing neural degeneration following ablation of their cortical projection fields revealed reactive changes in vessels unrelated to direct mechanical trauma. A series of 30 rabbits were studied at post‐operative survivals of one to 238 days. Many neurons display signs of retrograde atrophy within the first week accompanied by a distinct increase in the number of perivascular cells within the basal lamina surrounding venules and large capillaries. This reaction was optimally observed between 5 and 98 days post‐operatively and was limited to the circumscribed zones of degeneration. In early stages, some vessels displayed margination of hematogenous elements, predominantly agranular leukocytes, of which some examples were seen in phases suggestive of diapedesis. Cells within the perivascular space exhibited diverse morphology. In addition to the indigenous “pericyte” population, there were numerous agranular leukocytes, and a few plasma cells and eosinophils. Within the second to third week postoperatively, some perivascular elements become enlarged and display electron‐lucent lipid vacuoles and PTA‐positive dense bodies of complex morphology, many of which are clearly lysosomal in nature. This cellular reaction persists for three to four months, after which time the cytoplasm displays evidence of cholesterol crystals and lysosomal transformations suggestive of terminal or residual bodies. Morphological criteria for identifying the possible origin of vacuolated, lysosomeladen perivascular cells are presented for a consideration of their possible derivation from pericytes and/or hematogenous elements. Of the several hypotheses considered to account for these findings, it appears likely that lipid and protein end‐products of neural degeneration provide the stimulus for perivascular aggregation and subsequent transformation.