Flow-cytometric and ultrastructural analysis of alveolar macrophage maturation

Alveolar macrophages (AM) from adult and newborn rats were studied by flow cytometry and ultrastructural morphometry. We observed that the laser scatter and autofluorescent properties of newborn macrophages were different from those of adult cells. Relative to the adult AM, the forward-angle laser scatter obtained with the newborn AM was reduced; this optical measurement appeared to correlate with the smaller mean size, as determined by ultrastructural and electronic volume measurements. The diminished right-angle laser scatter (90 degrees angle) correlated with the presence of fewer small, irregularly shaped lysosomal structures in the newborn AM, compared with AM from adult animals. AM from 1-2-day-old rats displayed large vacuoles containing multilamellar structures, which proved to be less effective at scattering light. Cells from newborn rats were less autofluorescent, a finding that appeared to correlate best with the numbers of secondary lysosomes. Flow cytometry may be used to discern structural alterations that occur during the maturation of AM. These changes correlate well with quantitative ultrastructural analyses of these cells.