Late glial swelling after acute cerebral hypoxia-ischemia in the neonatal rat: a combined magnetic resonance and histochemical study.

Secondary brain damage after transient cerebral hypoxia-ischemia (HI) is caused by a cascade of cellular events. In this study, complementary methods of magnetic resonance imaging and histochemistry were used to investigate the formation of cytotoxic and vasogenic edema during secondary brain damage induced by transient HI in 7-d-old rats. To elicit injury, 21 rats underwent right common carotid artery ligation followed by 1.5 h of 8% O2 exposure. Sequential apparent diffusion coefficient (ADC) and transversal relaxation time (T2) weighted magnetic resonance imaging were recorded for up to 3 d in 13 7-d-old rats. Eight animals were killed at various intervals between the end of HI and 21 h of recovery to perform histochemical assays using neuronal and astrocytic markers. Changes of the ADC revealed a biphasic function for the evolution of cytotoxic edema during the recovery period. At the end of HI, the ADC in the ipsilateral cortex was significantly decreased. Upon reoxygenation, it returned transiently to normal followed by a secondary, although less pronounced, decline after 8-48 h. After this, the ADC rose steadily. From 8 h of recovery, the proportion of vasogenic edema steadily increased as indicated by the T2 prolongation. At 21 h, the majority of glial cells showed immunoreactivity for glial fibrillary acidic protein and were of larger size, whereas the neurons were apoptotic. These results indicate that the delayed cerebral injury is accompanied by late glial swelling in conjunction with an enlarged interstitial space due to cell damage.