FIGURE 2.

Scanning electron micrograph of the alveolar surface of a human lung showing protruding capillaries and two type II cells (Ep2) sitting in niches and characterized by a rim of microvilli. Small arrow points to cell body of a type I cell (Ep1) that covers several capillary meshes (yellow); the boundary of its cytoplasmic leaflet is marked by arrowheads outlining a small lip of the cell junction between adjoining cells (inset, compare Figure 1B). The surface area covered by this cytoplasmic leaflet is 1,300 mm². Scale bar = 10 μm (from Weibel, 2015); reprinted with permission of the American Thoracic Society.