(A) PuMA yields an intact lung microarchitecture. Day 0: bronchioles (B) were lined by epithelia that contact the basement membrane. Alveoli (A) were uniformly expanded throughout the lung, and the alveolar walls (AW, with arrows) were normal in diameter. The alveolar walls contained small numbers of migratory inflammatory cells, pneumocytes (type I and II), and endothelial cells. Blood vessels and alveolar capillaries were expanded by rbcs. Day 7: alveoli remained expanded. There were decreased numbers of migratory cells, pneumocytes, and endothelial cells in the alveolar walls, and many of those that remained contained pyknotic nuclei. Days 14 and 21: alveoli, airways, and large vessels (PA, pulmonary arteries; PV, pulmonary veins) remained expanded. Alveolar capillaries and rbcs were no longer discernible, and the alveolar walls contained fewer migratory cells and pneumocytes (loss of cellularity). Overall, lung microarchitecture was remarkably unchanged. (B) Movat stain was used to examine the connective tissue components of the lung culture. Black elastin fibers were present in large vessels, in the basement membrane supporting the airway epithelia, and within the alveolar interstitium. Black nuclei were scattered throughout the alveolar interstitium. Red muscle surrounded arteries and larger airways (B) and yellow collagen fibers were in the surrounding vascular submucosa and alveolar interstitium. Each of these components was identified at each time point. (C) TEM. Stromal elements composed of collagen microfibers (C) were evident from day 0 through day 21. Scale bars: 100 μm (A and B); 1 μm (C).