STRUCTURAL ORGANIZATION OF PULMONARY-ARTERIES IN THE RAT LUNG

The structure of the normal pulmonary arteries in the rat was studied with light and electron microscopy after use of a newly devised techni que of perfusion fixation and tissue preparation. We distinguished two main types of artery in the rat lung on the basis of the structure of the media, an elastic artery and a muscular artery. The elastic arter y was characterized by an abundance of extracellular matrix in the med ia and by an oblique arrangement of smooth muscle cells to connect nei ghboring elastic laminae. It was subdivided into two segments, a class ical elastic and a transitional elastic segment. The muscular artery w as distinguished by a paucity of extracellular matrix in the media and by a circumferential arrangement of smooth muscle cells (or pericytes ) enclosing the lumina, and was subdivided into four segments, a thick muscular, an ordinary muscular, a partially muscular and a nonmuscula r segment. The smooth muscle cells in the muscular artery contained we ll-developed microfilament bundles compared with those in the elastic artery. Structural differences in smooth muscle cells and in extracell ular matrix in the media between the elastic and muscular arteries may re fleet the functional heterogeneity of pulmonary arteries in respon se to hypoxic pulmonary vasoconstriction and to vasoactive substances such as endothelium-derived relaxing and hyperpolarizing factors, and endothelin.