Terminal bronchiolar-alveolar (TB-A) units in circumfusion system cultures

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Abstract
Long-term (more than 75 days) recirculating circumfusion system cultures of 15-day fetal mouse lungs gave rise to terminal bronchiolar-alveolar (TB-A) units detached from the respiratory tree. TB-A units established in serum-free microenvironments under reconstructed cellophane membranes became especially flat and optically accessible in fast-flowing 6-chamber systems devoid of a pulse pressure. Time-lapse cinephase movies showed that the alveoli were maintained debris-free by the phagocytosis of alveolar macrophages which entered the alveoli by penetrating the pneumocytic walls. The terminal bronchioles with active ciliary cells communicated with the alveoli and together they formed a flat fluid-filled tubule-like closed system composed of ultrastructurally typical differentiated cellular constituents, viz. types I and II pneumocytes, endocrine-like cells, ciliary cells, Clara cells, and septal cells. TB-A units were observed throughout 150 days of cultivation at which time their phase morphological differentiation remained stable and their functional activity observable as smooth-muscle contractions, ciliation, and alveolar macrophage phagocytosis. A 1000-foot motion-picture film complementing the descriptions of the TB-A units and the alveolar macrophages in the text is deposited with the Motion Picture Film Rental Collection of the Tissue Culture Association, Inc., at the W. Alton Jones Cell Science Center, Lake Placid, New Hork.