Distribution and function of cytoskeletal proteins in lung cells with particular reference to 'contractile interstitial cells'.

Cytoskeletal proteins are demonstrated in the interstitial cells of the lungs. These proteins appear in the cytoplasm as bundles of microfilaments, the individual filaments measuring 40--80 A in diameter. The presence of actin and myosin in these cells is demonstrated by immunofluorescence. Antiactin antibodies (AAA) obtained from patients with chronic aggressive hepatitis, as well as AAA and antimyosin antibodies prepared in the rabbit, are used. The major difference between the cytoskeletal proteins of interstitial cells and other cells of the alveolar tissue (type II epithelium, pericytes, and near the junctional complexes of endothelial cells) is that the microfilaments within the interstitial cells are organized into bundles forming tiny intracytoplasmic 'muscles'. Furthermore, they appear to be much more abundant and seem to anchor the cell on the alveolar basement membrane by hemidesmosome-like structures. These peculiar cytological features provide these cells with an important functional capacity. Being located in the 'pillars' which cross the capillary space, the contraction of interstitial cells may modify the alveolocapillary configuration in some circumstances. The physiological importance of such an 'active' alveolar motility is to provide the lung with a mechanism of autoregulation of the ventilation/perfusion (V/Q) ratio at alveolar level.