Temporal evolution of neuropathologic changes in an immature rat model of cerebral hypoxia: a light microscopic study

Abstract

The sequential evolution of neuropathologic changes was studied in an immature model of cerebral hypoxia-ischemia. According, 7-day postnatal rats were subjected to unilateral common carotid artery ligation combined with 2 h of hypoxia (breathing in 8% oxygen) and their brains were examined by light microscopy at recovery intervals ranging from 0 to 3 weeks. Immediately following hypoxia, a large area with a pale staining border was noted occupying most of the cerebral hemisphere ipsilateral (IL) to the occluded common carotid artery; in approximately half of the brains the dorsomedial cortex of the contralateral (CL) hemisphere was also involved. Most neurons in the pale area had nuclei containing a coarse granular condensation of chromatin. Within a few hours, the majority of neurons in the IL hemisphere had developed pyknotic nuclei and clear or eosinophilic perikarya. After 24 h these changes had evolved in the majority of brains into coagulation necrosis (infarction) in the IL hemisphere and foci of selective neuronal necrosis in the CL cortex. Within a few days infarcts became partially cavitated, and by 3 weeks a smooth-walled cystic infarct had developed. Activated microglia/macrophages and reactive astrocytes were first seen at 4 and 24 h, respectively. No parenchymal neutrophilic infiltrate was seen at any time point.

Secondary Energy Failure after Cerebral Hypoxia–Ischemia in the Immature Rat

“…The metabolic alterations described here are consistent with the evolving morphologic abnormalities that characterize the immature rat model of perinatal hypoxicischemic brain damage (Rice et al, 1981;Towfighi et al, 1995). Towfighi et al (1995) studied the temporal evolution of the neuropathologic changes in immature rats subjected to a similar interval of hypoxia-ischemia as rat pups in the present investigation.…”

“…After a 20-minute prewarming period, the animals were subjected to systemic hypoxia with 8% oxygen-92% nitrogen for 2 hours. This duration of hypoxia-ischemia typically is associated with infarction of the cerebral hemisphere ipsilateral to the carotid artery occlusion (Rice et al, 1981;Towfighi et al, 1995;Vannucci et al, 1997). Either at the terminus of hypoxia-ischemia or at specific intervals of recovery for up to 48 hours, randomized animals were decapitated directly into isopentane, −40°C.…”

“…Based on sequential neuropathologic analyses of immature rats during the early recovery interval after hypoxia-ischemia (Rice et al, 1981;Towfighi et al, 1995), we speculated that the secondary depletion of high-energy phosphate reserves follows rather than precedes brain tissue necrosis. We further suggested that the secondary decreases in both phosphocreatine and ATP do not denote a secondary energy failure of the brain but rather reflect a loss of total creatine and adenine nucleotides from the tissue and their conversion to creatinine and adenosine and other metabolites, respectively.…”

Failure to complete apoptosis following neonatal hypoxia–ischemia manifests as “continuum” phenotype of cell death and occurs with multiple manifestations of mitochondrial dysfunction in rodent forebrain

“…Even with strict attention to regional and chronologic detail, a variety of cell death morphologies have been observed within the forebrain after neonatal HI (Northington et al, 2001b, Sheldon et al, 2001. That many of the dying cells in the forebrain following neonatal HI exhibit a necrotic morphology is widely accepted (Towfighi et al, 1995). Excitotoxic neurodegeneration also occurs and defines some neurons undergoing an organized, non-apoptotic cell death following both glutamate injection and hypoxia-ischemia (Ishimaru et al, 1999).…”

“…It is clear that the developing brain is primed to respond to various forms of injury with activation of apoptosis signaling (Zhu et al, 2005), however, most studies of the neuropathology of neonatal HI fail to demonstrate prominent neuropathologic evidence for apoptosis in the developing forebrain during the acute phase of injury (Towfighi et al, 1995, Northington et al, 2001b. The coexistence of at least three forms of neurodegeneration (apoptosis, necrosis, and a hybridcontinuum form) following neonatal HI and glutamate receptor excitotoxicity have been reported (Northington et al, 2001b, Sheldon et al, 2001.…”