The structural composition of the pulmonary acinus in the mouse. A scanning electron microscopical and developmental-biological analysis.

For analysis of the structural composition of the pulmonary acinus in the mouse, fixed lungs of adult mice were examined by scanning and transmission electron microscopy. The murine acinus proved to consist of one or two generations of rather short respiratory bronchioles and about three generations of alveolar ducts opening into alveolar sacs. The epithelial cells lining the respiratory bronchioles (except for a sharply demarcated zone in the initial portion of the first-order branch) have a cuboidal or squamous shape and are very probably of the same type as those lining the alveolar ducts and sacs, i.e., small and great alveolar cells and/or their precursor cells. Therefore, this respiratory bronchiole portion may be considered part of the respiratory unit or pulmonary acinus. The initial zone of the first-order branch is lined by columnar epithelium, i.e., bronchial epithelium (mainly Clara cells), and should be assigned to the bronchial system of the lung.