Cerebral Blood Flow Heterogeneity in Preterm Sheep: Lack of Physiologic Support for Vascular Boundary Zones in Fetal Cerebral White Matter

Abstract

Periventricular white matter (PVWM) injury is the leading cause of neurologic disability in survivors of prematurity. To address the role of ischemia in PVWM and cerebral cortical injury, we hypothesized that immaturity of spatially distal vascular 'end zones' or 'border zones' predisposes PVWM to greater decreases in cerebral blood flow (CBF) than more proximal structures. We quantified regional CBF with fluorescently labeled microspheres in 0.65 gestation fetal sheep in histopathologically defined three-dimensional regions by post hoc digital dissection and coregistration algorithms. Basal flow in PVWM was significantly lower than in gyral white matter and cortex, but was equivalent in superficial, middle, and deep PVWM. Absolute and relative CBF (expressed as percentage of basal) did not differ significantly during ischemia or reperfusion between PVWM, gyral white matter, or cortex. Moreover, CBF during ischemia-reperfusion was equivalent in three adjacent PVWM levels and was not consistent with the magnitude of severity of PVWM injury, defined by TUNEL (terminal deoxynucleotidyltransferase-mediated dUPT nick end labeling) staining. However, the magnitude of ischemia was predicted by the severity of discrete cortical lesions. Hence, unlike cerebral cortex, unique CBF disturbances did not account for the distribution of PVWM injury. Previously defined cellular maturational factors, thus, appear to have a greater influence on PVWM vulnerability to ischemic injury than the presence of immature vascular boundary zones.

Ovine Proinflammatory Cytokines Cross the Murine Blood-Brain Barrier by a Common Saturable Transport Mechanism

“…Further, evidence also suggests that white matter lesions produced in the ovine fetus are similar to the lesions of periventricular leukomalacia, which represent an important antecedent of cerebral palsy in premature infants [26][27][28] . These findings [26,29] collectively suggest that the preterm ovine fetus is an excellent model for the brain damage observed in premature human infants [26,[29][30][31] . Therefore, models of neuroinflammation related to cytokine upregulation and BBB permeability in the ovine fetus may represent a more ethological model of human in utero pathogenesis because of the ovine's extended gestational length, ability for real-time physiological monitoring and the frequency of single fetuses as compared with the mouse [18,19,26,29,32] .…”

The Instrumented Fetal Sheep as a Model of Cerebral White Matter Injury in the Premature Infant

“…However, as cardiovascular compensation fails with time, substantial albeit variable reductions in BP occur. The residual brain blood flow has seldom been measured, but for brain injury to occur, pressures must fall below one third of normal for at least 10 to 15 minutes, depending on the animal's age and the model being used [41,109]. The importance of the central autonomic system and adrenal stress hormones remains unclear, as well as the systemic blood concentrations of metabolic substrates, such as glucose and products such as lactate.…”

“…To be able to analyze CBF disturbances in regions of periventricular white matter vulnerable to hypoxicischemic injury, we developed a novel method to achieve high resolution spatial blood flow measurements on the brains of immature fetal sheep. This was achieved by determining the location in the fetal brain of thousands of individual fluorescently labeled microspheres [41,109]. This high resolution blood flow technique is illustrated in Fig.…”

“…To seek evidence for vascular end and border zones in fetal cerebral white matter, we measured blood flow in histopathologically defined regions of injury in cerebral cortex and white matter in preterm fetal sheep [109]. Although white matter blood flow was lower than cerebral gray matter, there was no evidence for pathologically significant gradients of fetal blood flow within the periventricular white matter under conditions of global partial ischemia or reperfusion.…”

Bilateral Common Carotid Artery Ligation in Sheep. Could These Animals be Used as Human Models for Vascular and Cerebral Research?

“…These are carbon dioxide concentration, hydrogen ion concentration and oxygen concentration [32]. Several experiments with animal models to simulate cerebral ischaemia have been described [35][36][37][38], including those using sheep [39][40][41][42]. One of the most serious complications of abnormal cerebral hemodynamics is the development of brain oedema.…”