Normal postnatal development of the media of the rat hilar pulmonary artery and its remodeling by chronic hypoxia.

The present morphometric and electron microscopic study follows the development of the hilar intrapulmonary artery of rat form 12 hours to 36 days of age and its structural adaptation to hypobaric hypoxia (380 torr) when exposed for periods of 3, 7, 14, and 28 days starting at 8 days of age. In control animals a striking increase in medial thickness is seen at 11 and 15 days of age and is accompanied, at 11 days, by an increase in the organelles of smooth muscle contraction-the myofilaments-and at 15 days, by an increase in rough sarcoplasmic reticulum. From 15 days, a gradual increase in collagen fibrils and elastin occurs in the extracellular ground substance. With exposure to hypoxia, from day 14 medial thickness increases more than the age-matched controls due to an increase in extracellular ground substance and to thickening of the elastic laminae. At all times studied, smooth muscle diameter is similar to the age-matched controls, but the normal fluctuations in volume density of the contractile elements and the rough sarcoplasmic reticulum are blunted. In the extracellular ground substance, the matrix is increased and collagen fibrils reduced, both in concentration and diameter. Thus, in young hypoxic rats, the narrowed lumen and external diameter of the large muscular arteries is associated with abnormal collagen formation. The hypoxia-induced changes in the young rat are different from those reported in the adult in which medial thickening is the result of increased extracellular ground substance with a normal concentration of collagen fibrils and hypertrophy of the smooth muscle cells. In the young lung, the structural changes have more far-reaching effects than the adult since they also interfere with lung growth.