Ultrastructural concomitants of hypoxia-induced angiogenesis.

Prolong hypoxia results in structural and functional adaptive responses to improve tissue oxygen delivery. Structural changes within the brain include vascular proliferation and elongation. The aim of the current study was to investigate whether ultrastructural changes in capillary walls also occur as part of the adaptive response. Adult rats were exposed to 2 or 3 weeks of moderate hypobaric hypoxia at 0.5 atmospheres and their cerebral microvasculature examined using quantitative ultrastructural methods. We found that hypoxic rats had an 18% increase in their brain capillary diameter but no change in endothelial wall thickness, basement membrane thickness, or coverage of the endothelial wall by pericytes. The increased diameter of cerebral capillaries may plan an important role in decreasing the resistance to capillary perfusion which is brought about by the increased erythrocyte fraction in the blood of hypoxic rates. Ultrastructural features relevant to the blood-brain barrier were maintained in hypoxic rats. Pericytes, that are thought to form a second line of defense in the blood-brain barrier, maintained their numerical and size relationships to the endothelial cells. Endothelial junctions were unchanged and endothelial vesicles were somewhat lower in density than normal at 2 weeks of hypoxia, but had regained their normal density by 3 weeks. Mitochondria of the brain capillary endothelial cells maintained normal numerical and volume densities in hypoxia, but the mitochondria of the surrounding neuropil were decreased significantly by about 30%.