Smith, P., Heath, D., and Moosavi, H. (1974).Thorax, 29, 147-163. The Clara cell. The ultrastructure of the bronchiolar Clara cell was examined in normal adult and neonatal rats, in similar animals which had been acutely exposed to hypoxia, and in adult rats following repeated doses of the anorexigen chlorphentermine. The Clara cell has all the features of a secretory cell, the product of secretion accumulating within smooth cisternae at the apex of the cell. The apical region is then extruded into the bronchiolar lumen in a process of apocrine secretion. Acute hypoxia accelerates this secretion in adult rats but has little effect upon neonatal rats. Administration of chlorphentermine induces a hyperplasia of Clara cells which is associated with large quantities of phospholipid free within the alveolar spaces and in macrophages. The Clara cells also contain accumulations of what appear to be phospholipid. This suggests that the Clara cell secretes the phospholipid pulmonary surfactant.