Alveolar cell hyperplasia in the lungs of smoking dogs

Hyperplasia of bronchiolar epithelium was observed in the lung parenchyma of 14 smoking dogs. Besides the increased number of cell layers in the bronchiolar epithelium, there was a virtual absence of glycogen in the cell cytoplasm. Sheetlike arrangements of squamous cells containing an abundance of tonofilament bundles were found lining alveoli. Type II cells were found adjacent to, and sometimes extending over the squamous cells. In all cases, a proliferation of type II cells was found in the alveolar epithelium, almost completely lining some alveoli. Acinarlike structures also lined by many type II cells were sometimes found near the pleura. Microinvasions of cytoplasmic processes into the underlying connective tissue often occurred through defects in the basal lamina. In some type II cells an unusual content of RER was observed. It possessed electron dense banding ≈ 180 A wide with a repeat at intervals of ≈ 540 A and was found in narrow cisternae and in bulbous dilatations of the cisternae of the RER. Fibrogranular material of the same appearance and periodicity was also found in the perinuclear space in some cells. Type II cells are further implicated as a reserve cell or stem cell in metaplastic alterations.