The influence of collapse of the lung parenchyma on the morphology of pulmonary blood vessels in the rat.

Abstract

During a search for resident subendothelial smooth muscle cells in pulmonary vessels of the rat we found that in expanded lungs the muscular pads in the veins, considered by some authors as sphincters, were hardly visible whereas in collapsed lungs they were very conspicuous. In a separate study intended to quantify the degree of collapse or expansion the left lung was examined in 5 rats with a collapsed and in 5 rats with an expanded lung: the expansion was produced by filling the airways by gravity with Methacarn fixative. The degree of expansion was determined by morphometry measuring the volume density of the tissue fraction of the pulmonary parenchyma in the microscopic sections: in the expanded lung the mean value was 8.5% (range 6.7-12.6%), in the collapsed lung 20.1% (range 18.7-22.3%), a highly significant difference (p < 0.000). Serial sections generally 60-100, 6-microns-thick, were stained by PAS, Sirius red hematoxylin and Verhoeff's elastic stains. Immunohistochemical staining was done with monoclonal antibody against alpha smooth muscle cell actin and desmin. Graphic reconstructions of representative vessels were performed. It was shown that the muscular media of the veins was interrupted and that the muscular pads corresponded to the contracted smooth muscle cell segments alternating with the noncontracted segments devoid of muscle. In the expanded lungs muscular pads were flattened and often hardly detectable. This indicates that the structures considered as sphincters are postmortem contraction rings in collapsed lungs.