Moderate-Grade Germinal Matrix Haemorrhage Activates Cell Division in the Neonatal Mouse Subventricular Zone

Precise temporal and spatial control of the neural stem/progenitor cells within the subventricular zone (SVZ) germinal matrix of the brain is important for normal development in the third trimester and the early postnatal period. The high metabolic demands of proliferating germinal matrix precursors, coupled with the flimsy structure of the germinal matrix cerebral vasculature, are thought to account for the high rates of haemorrhage in extremely- and very-low-birth-weight preterm infants. Germinal matrix haemorrhage can commonly extend to intraventricular haemorrhage (IVH). Because neural stem/progenitor cells are sensitive to microenvironmental cues from the ventricular, intermediate, and basal domains within the germinal matrix, haemorrhage has been postulated to impact neurological outcomes through aberration of normal neural stem/progenitor cell behaviour. We developed an animal model of neonatal germinal matrix haemorrhage using stereotactic injection of autologous blood into the mouse neonatal germinal matrix. Pathological analysis at 4 days postinjury showed high rates of intraventricular extension and ventricular dilatation but low rates of parenchymal disruption outside the germinal zone, recapitulating key features of human "Papile grade III" IVH. At 4 days postinjury we observed proliferation in the wall of the lateral ventricle with significantly increased numbers of transient amplifying cells within the SVZ and the corpus callosum. Analysis at 21 days postinjury revealed that cortical development was also affected, with increased neuronal and concomitant reduced oligodendroglial differentiation. At the molecular level, we showed downregulation of the expression of the transmembrane receptor Notch2 in CD133+ve cells of the SVZ, raising the possibility that the burst of precocious proliferation seen in our experimental mouse model and the skewed differentiation could be mediated by downregulation of the Notch pathway within the proximal/ventricular domain. These findings raise the possibility that Notch regulation plays a critical role in mediating the response of the neonatal SVZ to ischaemic and haemorrhagic insults.