The effect of chronic hypobaric hypoxia (28 days, 455 Torr) on the organization of brain vessels was studied in Balb/c mice. In comparison to age-matched controls kept at sea level, emulsion-perfused capillaries in hypoxic mice showed marked dilation in all brain areas studied. Capillary length per unit volume of tissue (L_v) was increased in the cerebellar granular layer, the caudate nucleus, the globus pallidus, the substantia nigra, the superior colliculus, and the dentate gyrus. There was a selective increase of L_v in the hippocampus (CA1 strata pyramidale and lacunosum and CA3 strata pyramidale and oriens) and in somatosensory cortex layers V and VI, motor cortex layers II, III, V, and VI, and auditory cortex layers II and III. An increase in capillary surface area per unit volume of tissue was also determined in several brain areas, including layer IV of somatosensory cortex, where L_v was not significantly increased. The O₂ diffusion conductance and PO₂ in the tissues were estimated with a mathematical model. The remodeling of capillary diameter and length during chronic hypoxia accounts for the significant increase of O₂ conductance to neural tissues. Also the estimated tissue PO₂ in chronic brain hypoxia is markedly increased in the caudate nucleus and the substantia nigra compared with acute hypoxia. These results suggest that formation of new capillaries is an important mechanism to restore the O₂ deficit in chronic brain hypoxia and that local rates of energy utilization may influence angiogenesis in different areas of the brain.

whisker barrels; capillary remodeling

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