Hypoxia influences growth and HIF-1α activation in cancer stem cells from GBM. (A) GBM tumor spheres were grown either under normoxia or hypoxia for 2 or 5 days. Images of the tumor spheres were then taken using a digital camera mounted on an EclipseNet 2000 microscope. C: control normoxic cells. (B) GBM tumor spheres were grown either under normoxia or hypoxia for 17 h. Upper panel: Integrity of spheres was assessed in sections by hematoxylin/eosin staining. Lower panel: HIF-1α expression in sphere sections was studied by immunofluorescence as described in Methods. C: control normoxic cells.