Conducting pulmonary arteries: structural adaptation to extrauterine life in the pig.

A quantitative light microscopical and ultrastructural study of the elastic and large muscular pulmonary arteries of 30 Large White pigs aged from birth to 6 months yielded light microscopical measurements on 120 arteries and 62,560 ultrastructural assessments, which composed a computerised database. After birth mean arterial medial thickness and mean smooth muscle cell diameter decreased during the first 4 days (p less than 0.01). Mean interlamellar distance decreased, reaching its nadir at 1-3 weeks (p less than 0.01). All these features increased between 3 weeks and adulthood (p less than 0.01). In the smooth muscle cells synthetic rather than contractile organelles were dominant during the first 3 weeks. Between 3 weeks and adulthood, however, smooth muscle cell myofilament volume density increased (p less than 0.0001). At all ages the smooth muscle cells of the outer lamellar units of elastic arteries had a greater myofilament volume density than those of adluminal units (p less than 0.0001). The amount of connective tissue increased between 3 weeks and adulthood, collagen and basement membrane increasing preferentially (p less than 0.0001, p less than 0.05, respectively). Thus the conducting pulmonary arteries, like the peripheral resistance arteries, adapt structurally to extrauterine life. Remodelling occurred rapidly after birth, and then gradually during growth, as connective tissue was deposited and smooth muscle cells matured.