Cell proliferation in immature mouse brain following periventricular blood injection. A - Ki67 nuclear immunoreactivity (brown stained nuclei; hematoxylin counterstain) is prominent in the frontal periventricular subventricular zone (SVZ, arrow) of control (i.e. no blood injection) mice from all three genotypes (this example is wild type). B – Following blood injection, some of which extends into the frontal horn of the lateral ventricle (arrow), Ki67 immunoreactivity in the SVZ is reduced (this example is a PAR1 knockout). C – Quantitative analysis (ANOVA) shows the proportion of Ki67 immunoreactive cells in the SVZ was significantly reduced in the 24-hour heterozygous (Het; *p = 0.0050) and the 48-hour wild type (WT; *p = 0.0054) mice; the reduction also approached significance in the 48-hour knockout (KO; p = 0.0577) in comparison to controls. Bar = 100 μm.