THE INFLUENCE OF COLLAPSE OF THE LUNG PARENCHYMA ON THE MORPHOLOGY OFPULMONARY BLOOD-VESSELS IN THE RAT

During a search for resident subendothelial smooth muscle cells in pul monary vessels of the rat we found that in expanded lungs the muscular pads in the veins, considered by some authors as sphincters, were har dly visible whereas in collapsed lungs they were very conspicuous. In a separate study intended to quantify the degree of collapse or expans ion the left lung was examined in 5 rats with a collapsed and in 5 rat s with an expanded lung: the expansion was produced by filling the air ways by gravity with Methacarn fixative. The degree of expansion was d etermined by morphometry measuring the volume density of the tissue fr action of the pulmonary parenchyma in the microscopic sections: in til e expanded lung the mean value was 8.5% (range 6.7-12.6%), in the coll apsed lung 20.1% (range 18.7-22.3%), a highly significant difference ( p <0.000). Serial sections generally 60-100, 6-mu m-thick, were staine d by PAS, Sirius red hematoxylin and Verhoeff's elastic stains. Immuno histochemical staining was done with monoclonal antibody against alpha smooth muscle cell actin and desmin. Graphic reconstructions of repre sentative vessels were performed. II 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 non contracted segments devoid of muscle. In the expanded lungs muscular p ads were flattened and often hardly detectable. This indicates that th e structures considered as sphincters are postmortem contraction rings in collapsed lungs.