Orthogonal arrays of particles in the plasma membrane of pneumocytes

The presence of orthogonal arrays of particles (OAP) in pneumocytic membranes was studied using freeze-fracture replicas of lung tissue from turtles, frogs and various mammalian species, including man. OAP are typically present in the basal plasma membrane of pneumocytes in turtles and frogs, while they have only been detected in the plasma membrane of type I cells in human lungs among the mammalian species studied. The median of particles per OAP was 10 (range 4-40) in lower vertebrates and 8 (range 4-24) in man. The mean density of OAP per unit cell surface varied between 40 and 60/micron2. The frequent occurrence of OAP in pneumocytic membranes in the lungs of man, turtles, and frogs suggests that they are related to a specific, although undetermined pulmonary function which is restricted to the ciliated airway epithelium in those mammalian species lacking OAP in the alveolar epithelium.