Human postnatal pulmonary arterial remodeling. Ultrastructural studies of smooth muscle cell and connective tissue maturation.

A quantitative light and electron microscopic study of the elastic conducting, small muscular resistance arteries (accompanying or just proximal to the terminal bronchioli) and respiratory unit pulmonary arteries was carried out on the autopsy specimens of 12 children (newborn to 2 years) and three adults, who died without cardiopulmonary disease. Between birth and 1 month, in muscular and respiratory unit arteries, wall thickness decreased due to a reduction in mean smooth muscle cell diameter (p less than 0.01) and a reduction in overlap between adjacent cells. In the elastic arteries, the distance between adjacent elastic lamellae decreased as smooth muscle cell diameter decreased (p less than 0.01 for both). At birth, smooth muscle cells appeared immature, synthetic rather than contractile organelles predominating. Smooth muscle cell filament (thick, thin, and intermediate) volume density increased by 12 to 13% by 6 months in the small muscular arteries and by 2 years in the elastic arteries (p less than 0.001 for both). Surface and cytoplasmic dense bodies increased in a similar manner (p less than 0.001). The findings indicate a postnatal increase in contractile myofilament and surface dense bodies. The amount of connective tissue in the subendothelium and media increased between 1 month and adulthood, collagen finally predominating in small muscular arteries and elastin in the conducting arteries. Collagen fibrils showed regional differences in size within the media, between media and adventitia, and between different types of artery, and increased in size with age (p less than 0.001). Thus, the intrapulmonary arteries from hilum to precapillary bed adapted structurally to extrauterine life. It is suggested that the high fetal pulmonary vascular resistance is due to the shape and arrangement of smooth muscle cells within the vessel wall, rather than to an excessive contractility of each cell. After 1 month, remodeling occurred more slowly with growth, involving an increase in wall thickness, connective tissue deposition, and smooth muscle cell maturation.