A quantitative study of some ultrastructural features of the type I cells in the carotid bodies of rats living at a simulated altitude of 4300 metres

Summary A quantitative study was carried out on the ultrastructure of the type I cells of the carotid bodies of three normal rats and three rats living in a hypobaric chamber at an atmospheric pressure of 460 mm Hg for 27, 28 and 35 days. Point-counting methods were performed on electron micrographs of randomly selected cross-sections of type I cells. These sections always included the nucleus of the cell. The same electron micrographs were used to determine the number of mitochondrial cross-sections and electron-dense core vesicles appearing in each type 1 cell profile. The morphometric analysis disclosed that the mean cross-sectional area of the type I cells was 41.35 m³ in the untreated rats and 82.03 m³ in the rats exposed to chronic hypoxia. Assuming that this area of cross-section was in direct proportion to the volume of the cell, this result indicates an approximately three-fold increase in volume of the type I cells of the carotid bodies in hypoxaemic rats. There was no change in the volume proportion of the type I cells occupied by the mitochondria. However, as the cells had increased in volume in hypoxaemic rats, it was concluded that the number of mitocondria in each type I cell was increased. The concentration of electron-dense core vesicles was 21.9/m³ cytoplasm in the type I cells of untreated rats and 7.3/m³ in the hypoxaemic rats. The dense-core vesicles were increased in diameter in states of chronic hypoxia. Their mean diameter was 102.3 nm in normal rats and 117.0 nm in hypoxaemic rats. The enlargement of the type I cells and the increase in the number of mitochondria within each cell suggests that this depletion is more likely to be due to an increased rate of release of dense-core vesicles, than to a reduced rate of their synthesis.