CD34 and brain injury: CD34 is known to be ubiquitously expressed by vascular endothelial cells.19 Other cell types including endothelial progenitor cells and HSC are alsoknown to be CD34-positive.19,23,36,43 CD34 overexpression by reactive microglia has been reported inthe context of neurodegenerative conditions such as experimental amyotrophic lateralsclerosis in rats16 and traumatic brain injury in mice.17 However, evidence of CD34 expression in other CNS cell populations is scarcewithout a definitive identification of the cell origin.19,21