The alveolar macrophage delivery system. Kinetic studies in cultured explants of murine lung.

The production of alveolar macrophages in a blood-free organ culture system has been studied to determine whether free macrophages undergo mitosis or whether their ongoing production is dependent upon continuing division of interstitial cells. Explants of murine long were attached to cellophane or glass; after 6 days of culture, a population of cells identified morphologically and functionally as macrophages appeared around the central tissue. These cells did not divide, and they disappeared 4 days after removal of the central lung explant. 3H-thymidine labeling of these peripheral macrophages was observed only when the central tissue was present and when the thymidine pulse was 24 hours in duration. Actual cell division was observed only in the interstitial cells of the explant. It is concluded that the interstitial cell population provides a continuing pool of precursor cells that divide and migrate outwards, creating a "steady state" system in which macrophage loss at the periphery is balanced by cell production at the center.