3D analysis of intracortical microvasculature during chronic hypoxia in mouse brains

The purpose of this study is to determine when and where the brain microvasculature changes its network in response to chronic hypoxia. To identify the hypoxia-induced structural adaptation, we longitudinally imaged cortical microvasculature at the same location within a mouse somatosensory cortex with two-photon microscopy repeatedly for up to 1 month during continuous exposure to hypoxia (either 8 or 10% oxygen conditions). The two-photon microscopy approach made it possible to track a 3D pathway from a cortical surface arteriole to a venule up to a depth of 0.8 mm from the cortical surface. The network pathway was then divided into individual vessel segments at the branches, and their diameters and lengths were measured. We observed 3-11 vessel segments between the penetrating arteriole and the emerging vein over the depths of 20-460 μm within the 3D reconstructed image (0.46 × 0.46 × 0.80 mm(3)). The average length of the individual capillaries (<7 μm in diameter) was 67 ± 46 μm, which was not influenced by hypoxia. In contrast, 1.4 ± 0.3 and 1.2 ± 0.2 fold increases of the capillary diameter were observed 1 week after exposure to 8 % and 10% hypoxia, respectively. At 3 weeks from the exposure, the capillary diameter reached 8.5 ± 1.9 and 6.7 ± 1.8 μm in 8% and 10 % hypoxic conditions, respectively, which accounted for the 1.8 ± 0.5 and 1.4 ± 0.3 fold increases relative to those of the prehypoxic condition. The vasodilation of penetrating arterioles (1.4 ± 0.2 and 1.2 ± 0.2 fold increases) and emerging veins (1.3 ± 0.2 and 1.3 ± 0.2 fold increases) showed relatively small diameter changes compared with the parenchymal capillaries. These findings indicate that parenchymal capillaries are the major site responding to the oxygen environment during chronic hypoxia.