Growth and development of the fetal Rhesus monkey. VI. Morphometric analysis of the developing lung.

Groups of fetal monkeys were studied at 50, 75, 100, 125 and 150 days gestational age, and postnatal monkeys were studied at 175 days gestational age and in adult life (full term gestation is 168 plus or minus 4 days). The ratios between the various tissues of which lung is composed were determined by light and electronmicroscopy. Characteristic changes in ratios were noted at each developmental age. Primitive bronchioles increased from 50 to 75 days, decreased as permanent bronchioles, alveolar ducts and alveoli developed, and were absent after 150 days. Mesenchymal tissue decreased progressively from 50 to 150 days. Alveoli were present at 100 days gestational age; alveolar lumens accounted for approximately 1/2 the volume of peripheral lung by 150 days, and 2/3 the volume of lung in adult animals. By 150 days alveolar epithelial cells could be identified as Type I (membranous) or Type II (granular) on the basis of the characteristic cytoplasmic lamellar inclusion bodies of the latter. The ratios of the Type II/Type I cells were .21-.23 at 150 and 175 days, and fell to approximately half that value in adult lungs.