Number and distribution of plasmalemmal vesicles in the lung

Vesicles in the lung, particularly in capillary endothelial and squamous alveolar epithelial cells, are membrane differentiations that form a system of impressive aggregate size. Strong evidence from electron microscopic tracer studies indicates that they have the potential of acting as a transendothelial transport mechanism, but their precise level of activity and the significance of vesicular transport as a pathway different from hydraulic permeability have not been clarified. It has been proposed that vesicles may form either permanent or transient transcellular channels. The relative significance of vesicles as passage sites compared to the leaky intercellular endothelial junctions awaits clarification. Quantitation of individual cell size and number of vesicles reveals that in both endothelial and epithelial cells more than 70% of the total plasma membrane is located in vesicles. Coated pits and vesicles are readily identifiable in all cells of the lung parenchyma, although they are much less frequent than noncoated vesicles. These are important because they have been identified in cell biology as sites of receptor-mediated endocytosis and therefore offer the basis for the presence of a system of selective permeability and/or uptake of substances by the lung, which, so far, has not been explored.