Objective The major form of MRI-defined white matter injury (WMI) comprises diffuse lesions where the burden of small necrotic foci (microscopic necrosis) is poorly defined. We hypothesized that myelination failure associated with diffuse WMI involves an aberrant injury response linked to arrested pre-oligodendrocyte (preOL) maturation in reactive astrocyte-rich lesions. Methods A retrospective autopsy series (1983–2000) was selected for cases with diffuse WMI and analyzed relative to prospectively-collected contemporary cases (2003–2010). Controls were age and region-matched to address regional variation in preOL maturation. Successive oligodendrocyte stages were analyzed with lineage-specific markers. Microscopic necrosis was quantified with microglial markers. Axon injury markers defined the burden of axonopathy. Extracellular matrix remodelling was defined by detection of hyaluronic acid (HA), an inhibitor of preOL maturation, and the HA receptor, CD44. Results In the contemporary case series, diffuse WMI was accompanied by a significant reduction in the burden of microscopic necrosis and axonopathy. Diffuse astrogliosis extended into the lesion surround with elevated HA and astrocyte-expressed CD44. The total population of OL lineage stages was significantly increased in lesions. This increase coincided with significant expansion of the preOL pool. Interpretation Although these data confirm that microscopic necrosis occurs in contemporary cases, the markedly decreased burden supports that it does not contribute substantially to myelination failure. The primary mechanism of myelination failure involves a disrupted cellular response whereby preOLs fail to differentiate in diffuse astrogliotic lesions. Pre-oligodendrocyte maturation arrest converts chronic WMI to a more immature state related to the burden of astrogliosis.

Objective-Abnormal myelination is a major pathological sequela of chronic periventricular white matter injury (PWMI) in survivors of premature birth. We tested the hypothesis that myelination failure in chronic hypoxia-ischemia-induced PWMI is related to persistent depletion of the oligodendroctye (OL) precursor pool required to generate mature myelinating OLs.Methods-A neonatal rat model of hypoxia-ischemia was employed where acute degeneration of late OL progenitors (preOLs) occurs via a mostly caspase-independent mechanism. The fate of OL lineage cells in chronic cerebral lesions was defined with OL-lineage-specific markers.Results-Acute caspase-3-independent preOL degeneration from hypoxia-ischemia was significantly augmented by delayed preOL death that was caspase-3-dependent. Degeneration of preOLs was offset by a robust regenerative response that resulted in a several-fold expansion in the pool of surviving preOLs in chronic lesions. However, these preOLs displayed persistent maturation arrest with failure to differentiate and generate myelin. When preOL-rich chronic lesions sustained recurrent hypoxia-ischemia at a time in development when white matter is normally resistant to injury, an approximately 10-fold increase in caspase-dependent preOL degeneration occurred relative to lesions caused by a single episode of hypoxia-ischemia.Interpretation-The mechanism of myelination failure in chronic white matter lesions is related to a combination of delayed preOL degeneration and preOL maturation arrest. The persistence of a susceptible population of preOLs renders chronic white matter lesions markedly more vulnerable to recurrent hypoxia-ischemia. These data suggest that preOL maturation arrest may predispose to more severe white matter injury in preterm survivors that sustain recurrent hypoxia-ischemia.Human periventricular white matter injury (PWMI) is the major form of brain injury and leading cause of cerebral palsy in survivors of premature birth. With advances in neonatal care, a changing spectrum of chronic PWMI has emerged. Whereas focal cystic necrotic lesions (periventricular leukomalacia; PVL) previously predominated, 1, 2 recent neuroimaging studies support that focal or diffuse noncystic myelination disturbances and cerebral gray matter atrophy are now the major lesions associated with chronic PWMI. [3][4][5][6] The critically-ill preterm neonate appears to be particularly susceptible to ischemic white matter injury related to developmentally-regulated susceptibility of preOLs to oxidative 4

Abstract-Survivors of premature birth have a predilection for perinatal brain injury, especially to periventricular cerebral white matter. Periventricular white matter injury (PWMI) is now the most common cause of brain injury in preterm infants and the leading cause of chronic neurological morbidity. The spectrum of chronic PWMI includes focal cystic necrotic lesions (periventricular leukomalacia) and diffuse myelination disturbances. Recent neuroimaging studies support that the incidence of periventricular leukomalacia is declining, whereas focal or diffuse noncystic injury is emerging as the predominant lesion. In a significant number of infants, PWMI appears to be initiated by perturbations in cerebral blood flow that reflect anatomic and physiological immaturity of the vasculature. Ischemic cerebral white matter is susceptible to pronounced free radical-mediated injury that particularly targets immature stages of the oligodendrocyte lineage. Emerging experimental data supports that pronounced ischemia in the periventricular white matter is necessary but not sufficient to generate the initial injury that leads to PWMI. The developmental predilection for PWMI to occur during prematurity appears to be related to both the timing of appearance and regional distribution of susceptible oligodendrocyte progenitors. Injury to oligodendrocyte progenitors may contribute to the pathogenesis of PWMI by disrupting the maturation of myelin-forming oligodendrocytes. There has been substantial recent progress in the understanding of the cellular and molecular pathogenesis of PWMI. The oligodendrocyte progenitor is a key target for preventive strategies to reduce ischemic cerebral white matter injury in premature infants. Key Words: hypoxia-ischemia Ⅲ oligodendrocyte Ⅲ prematurity P eriventricular white matter injury (PWMI) is the major form of brain injury and the leading cause of chronic neurological disability in survivors of premature birth. 1,2 Although major advances in the care of premature infants have resulted in striking improvements in the survival of very-low birth weight infants (Ͻ1.5 kg), when compared with the 1980s, improved survival has been accompanied by a significant increase in the number of preterm survivors with long-term neurological deficits. 3 In up to 25% of preterm survivors, the major consequence of PWMI is permanent motor impairment (ie, "cerebral palsy") ranging from mild to profound spastic motor deficits. 4,5 By school age, 25% to 50% of children with PWMI manifest a broad spectrum of cognitive and learning disabilities. 6 The period of highest risk for PWMI is between approximately 23 and 32 weeks postconceptional age. Premature infants with PWMI are at markedly increased risk for several others forms of brain injury, notably intraventricular hemorrhage and intraparenchymal hemorrhage. 1 Whereas medical interventions have resulted in a pronounced decrease in the incidence of intraventricular hemorrhage, 7,8 the incidence of PWMI is not decreasing. 9 Thus, PWMI is now the major neurological p...

Objective Previously we reported that exposure of 6-day old (P6) rhesus macaques to isoflurane for 5 hours triggers a robust neuroapoptosis response in developing brain. We have also observed (unpublished) that isoflurane causes apoptosis of cellular profiles in the white matter that resemble glia. We analyzed the cellular identity of the apoptotic white matter profiles and determined the magnitude of this cell death response to isoflurane. Method Neonatal (P6) rhesus macaques were exposed for 5 hours to isoflurane anesthesia according to current clinical standards in pediatric anesthesia. Brains were collected 3 hours later and examined immunohistochemically to analyze apoptotic neuronal and glial death. Results Brains exposed to isoflurane displayed significant apoptosis in both the white and gray matter throughout the CNS. Approximately 52% of the dying cells were glia, and 48% were neurons. Oligodendrocytes (OL) engaged in myelinogenesis were selectively vulnerable, in contrast to OL progenitors, astrocytes, microglia and interstitial neurons. When adjusted for control rates of OL apoptosis, the percentage of OLs that degenerated in the forebrain white matter of the isoflurane-treated group was 6.3% of the total population of myelinating OLs. Interpretation Exposure of the infant rhesus macaque brain to isoflurane for 5 hours is sufficient to cause widespread apoptosis of neurons and OLs throughout the developing brain. Deletion of OLs at a stage when they are just beginning to myelinate axons could potentially have adverse long term neurobehavioral consequences, that might be additive to the potential consequences of isoflurane-induced neuroapoptosis.

Although periventricular white matter injury (PWMI) is the leading cause of chronic neurological disability and cerebral palsy in survivors of premature birth, the cellular-molecular mechanisms by which ischemia-reperfusion contributes to the pathogenesis of PWMI are not well defined. To define pathophysiologic relationships among ischemia, acute cerebral white matter damage, and vulnerable target populations, we used a global cerebral ischemia-reperfusion model in the instrumented 0.65 gestation fetal sheep. We developed a novel method to make repeated measurements of cerebral blood flow using fluorescently labeled microspheres to resolve the spatial heterogeneity of flow in situ in three-dimensional space. Basal flow in the periventricular white matter (PVWM) was significantly lower than in the cerebral cortex. During global cerebral ischemia induced by carotid occlusion, flow to all regions was reduced by nearly 90%. Ischemia of 30 or 37 min duration generated selective graded injury to frontal and parietal PVWM, two regions of predilection for human PWMI. Injury was proportional to the duration of ischemia and increased markedly with 45 min of ischemia to extensively damage cortical and subcortical gray matter. Surprisingly, the distribution of PVWM damage was not uniform and not explained by heterogeneity in the degree of white matter ischemia. Rather, the extent of white matter damage coincided with the presence of a susceptible population of late oligodendrocyte progenitors. These data support that although ischemia is necessary to generate PWMI, the presence of susceptible populations of oligodendrocyte progenitors underlies regional predilection to injury.