Quantitative electron microscopic autoradiography of in vivo incorporation of 3H‐choline, 3H‐leucine, 3H‐acetate and 3H‐galactose in non‐ciliated bronchiolar (Clara) cells of mice

The participation of Clara cells in the formation of alveolar surfactant and their general metabolism were studied in mice by quantitative autoradiography under electron microscopy after in vivo incorporation of four precursors: 3H‐choline, 3H‐leucine, 3H‐acetate and 3H‐galactose. Tritiated choline, a specific precursor of the dipalmitoyl‐phosphatidylcholine‐active fraction of the surfactant, is not incorporated into Clara cells. In contrast, 3H‐acetate is first incorporated actively into the endoplasmic reticulum and Golgi apparatus of these cells and, subsequently, in its secretory granules. It is suggested that acetate follows the general metabolism of the cell and/or is transformed into cholesterol. Tritiated leucine is incorporated into the secretory granules, although part of it remains more permanently in the cells. Tritiated galactose is actively incorporated, passes through the Golgi apparatus and eventually appears in the secretory granules. These results suggest that the Clara cell does not synthetize the tensio‐active fraction of the lining layer, as does pneumocyte II, but participates in the synthesis and secretion of proteins, glucides and possibly the cholesterol belonging to the hypophase.