The pulmonary arteriole

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The terminal branches of the mammalian pulmonary arterial tree are called arterioles despite statements that these vessels in the adult have no smooth muscle. We examined these vessels in the cat (8 hr to 11 years) and man (24 hr to 76 years) by light microscopy and special staining to clarify the terminology problem. In both cat and man, the changes in arterioles from the newborn to the aged individual are similar and differ only in time course. At and shortly after birth, fetal pulmonary arterioles are found throughout the lung: Endothelial cells are polygonal and somewhat thickened, the smooth muscle of the media has plump nuclei with blunt ends and fine chromatin; the cytoplasm is pale and periodic acid-Schiff (PAS) negative; internal and external elastic laminae are present. Mixed in with these fetal vessels are a few arterioles which show occasional areas of PAS-positive material and nuclei which are elongating. Occasional arterioles are more strongly PAS positive and show further elongation of nuclei and increased condensation of chromatin. After weeks to months (species variation) almost all arterioles show wall thinning, flattened endothelial cells, smooth muscle cells which occupy most of the media with deeply stained elongated nuclei and scant but persistent cytoplasm which is deeply PAS positive, and persistence of an internal and external elastic layer. Such vessels persist in the aged cat and human. We have called these sequential changes in the pulmonary arteriole maturation changes. The functional correlates of these arteriolar alterations for the adult mammal are discussed.