Development of terminal buds in the fetal mouse lung.

Development of the lungs in mammals begins with the formation of pharyngeal buds that undergo repeated branching to establish the bronchial tree. Late in fetal life the most superficial buds begin to form side branches that will develop into the respiratory surfaces of the lung; the alveolar ducts, alveolar sacs, and alveoli. The bronchial ciliated cells and Clara cells first appear on the seventeenth day of gestation in the mouse. In the respiratory region, a few lamellar bodies are found by 16 days but identifiable type II pneumocytes and extracellular surfactant are not found until 18 days. Flattened type I pneumocytes do not form until after birth, on day 19 or 20. The epithelial branches of the lung are surrounded by mesenchyme and covered by a capsule. At 16 days, the branches are separated by a compact mesenchyme that is highly cellular. By 18 days, the mesenchymal space begins to be reduced. By 19 days, the air passages expand and the mesenchyme forms a thin layer between branches. Little experimental work has been done on the role of the mesenchyme in differentiation of the respiratory surfaces. Since branching of the embryonic lung is influenced by the kind of mesenchyme surrounding the epithelium at early stages, it is likely that mesenchymal control is exerted at later stages in the fetus.