Ultrastructure of carotid body in rats living at a simulated altitude of 4300 metres.

We studied the ultrastructure of the carotid body of three normal rats and three rats living in a hypobaric chamber at a pressure of 460 mm Hg for 27, 28 and 35 days respectively. The type I cells of the carotid bodies of our hypoxic rats were enlarged due to an increase in the volume of their cytoplasm. Many of their dense core vesicles were vacuolated and the core was displaced eccentrically to become adherent to the limiting membrane of the vesicle. The concentration and distribution of dense core vesicles remained unaltered and there were no obvious changes in the mitochondria, ribosomes or Golgi apparatus. There was pronounced capillary dilatation in the carotid bodies of all three rats exposed to chronic hypoxia. This change was accompanied by attenuation of capillary endothelial cells and increased frequency of endothelial fenestrations. There were no structural changes in the type II cells.