In vivo application of ethidium bromide to cockroach central nervous connectives caused extensive disruption of the neuroglia within 24 hr. Axonal conduction persisted following treatment with the glial toxin. A consistent feature of glial damage and repair was the prominent involvement of granule-containing cells. These cells (which were never seen in control cords) shared a number of cytological features with hemocytes that were seen adhering to and penetrating the neural lamella, in the early stages of glial damage. The granule-containing cells appear to serve dual functions: phagocytosis and structural repair. After 48 hr, granule-containing cells, or their processes, formed layers at the periphery of the connectives. By 4 to 6 days after treatment, the peripheral cells had assumed the morphological characteristics of normal perineurial cells and by 28 days were indistinguishable, ultrastructurally, from those of the perineurium of normal, untreated animals. These structural changes paralleled the re-establishment of the normal permeability properties of the blood-brain interface revealed by the exclusion of an extracellular tracer, ionic lanthanum, and electrophysiological observations
