Pulmonary macrophages in birds (barn owl, Tyto tyto alba), domestic fowl (Gallus gallus f. domestica), quail (Coturnix coturnix), and pigeons (Columbia livia).

BACKGROUND: Birds have a limited number of resident macrophages in the normal steady-state respiratory tract. The discovery of phagocytes in lavages of lung from birds contrasts with findings that phagocytes are seldom seen in investigations in situ. An electron microscopic study was performed in the respiratory units, the parabronchi, and air capillaries in particular in several adult bird species to localize the seat of respiratory macrophages.
METHODS: Lung tissue of barn owl, domestic fowl, quail, and town and homing pigeons was subjected to standard processing for light and electron microscopy after immersion fixation, intratracheal instillation, and intravascular perfusion.
RESULTS: Clusters of macrophages were predominantly housed in the loose connective tissue at the floor of atria at the entrance to the infundibula and gas-exchange tissue proper. Scattered solitary phagocytes were also found in connective tissue of air sacs, interatrial septa, and adventitia of inter- and intraparabronchial arteries and veins and in peribronchial lymphoid tissue. Phagocytized foreign particulate material mostly consists of hard, dense, crystalline formations surrounded by a limiting membrane. The transport of small airborne particles occurs via the squamous atrial epithelium to the underlying macrophages. The macrophages are often accompanied by mast cells.
CONCLUSIONS: The present results demonstrate that avian respiratory macrophages are predominantly located in atrial connective tissue compartments and do not seem to migrate to the airway surfaces.