Pulmonary arterial and alveolar development in normal postnatal rat lung.

The postnatal development of rat lung was studied by quantitative light microscopic techniques at 12 hours and at 3, 8, 11, 15, 22, and 36 days of age. Particular emphasis was placed on the pulmonary arterial circulation. At birth the lung is not the adult lung in miniature; adaptation occurs through a series of relatively fast steps. A spurt of alveolar multiplication occurs between 3 and 8 days of age, followed, between 8 and 11 days, by one of arterial multiplication. Muscular, partially muscular, and nonmuscular arteries are present within the acinus from birth and their proportion, in each intra-acinar region, is similar at all ages examined except in the alveolar wall where muscular arteries are not found before 11 days of age. Between 12 hours and 8 days, medial thickness of the intra-acinar arteries decreases. At about 15 days of age this decrease is followed, in both pre-acinar and intra-acinar regions, by an increase in medial thickness. Between 12 hours and 3 days, absolute weight of the right ventricle decreases. This pattern of growth indicates that, at certain times, lung development is especially susceptible to injury and, also, that the nature of the disturbance is determined by its timing.