Loss of square (orthogonal) arrays from cultured airway epithelial cells

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Abstract
Electron microscopic examination of freeze‐fracture replicas reveals distinctive arrangements and planar distributions of certain membrane constituents. In these replicas, the square (orthogonal) array of 6–7‐nm intramembranous particles is an easily recognizable membrane differentiation commonly observed in transepithelial ion‐transporting epithelia and in several mesenchymal cells, in which they were originally described. Recently, the occurrence, distribution, and number of square arrays have been documented in the lungs of mammals, turtles, and frogs. The current study confirms that there are more than 10 arrays per μm2 basal and lateral portion of the plasma membrane in the alveolar type I and ciliated cells of human, rat, guinea pig, turtle, and frog lung, and in type II pneumocytes of the turtle and frog lung. We have not observed them in mucous cells, brush cells, macrophages, or endothelial cells of any species examined, nor do they appear in the apical portion of the plasma membrane of any airway or alveolar lining cell. Upon elastase or other proteolytic dissociation of the alveolar cells of the mammalian lung, concomitant with breakdown and endocytosis of tight junction strands, the square arrays cluster and are internalized within the membranes of endocytic vesicles that are derived from the lateral and basal portions of the cell. Within 1 day of establishing the cells in tissue culture, tight and gap junctions reappear, but the square arrays do not. Nor do they return to cultures examined on subsequent days. We conclude that the square array is a widespread membrane differentiation throughout the lung and major airways; but extrapolating from results obtained from dog tracheal cells (Widdicombe et al., 1987), the square array configuration of particles is unnecessary for maintaining normal ion transport.