Ultrastructural changes in cerebral cortical blood vessels of immature and adult rats after a single episode of neonatal hypoxia.

Newborn rats were exposed to a single episode of neonatal hypoxia and were studied at intervals from 4 hours to 3 months after the exposure. Endothelial cells showed unusual darkening of the cytoplasm, and protrusion of the soma into the lumen of the vessels. Fibroblasts showed unusually dilated cisternae of rough endoplasmic reticulum. The perivascular cells were also unusual, in that they had an 'activated' appearance as suggested by the presence of large vacuoles and secondary lysosomes, and were often connected to the vessel walls by only a thin foot process. The basal lamina often gave off long extensions into the neuropil or incompletely surrounded the cells of the vessel walls. The astrocytic foot processes were swollen and contained many glycogen granules and loosely packed glial filaments. All the above changes were present even at 3 months posthypoxia. The discontinuous basal lamina could allow substances to pass more easily from the bloodstream into the neuropil. It is also possible, in view of the frequent observations of perivascular cells that were connected to the vessel walls by only a thin foot process, that these extensions could be places where perivascular cells leave the vessels to enter the neuropil. It is postulated that increased movement of 'activated' perivascular cells into the neuropil could result in increased pruning of neuronal processes in the neuropil, after the episode of neonatal hypoxia.