Scanning electron microscopy of pulmonary alveolar capillary vessels.

The pattern of subepithelial vessels in pulmonary alveoli of rabbits has been studied using scanning electron microscopy. Alveolar capillaries form a network of interconnecting vascular rings, most of which surround the periphery of type II cells of the alveolar epithelium. Individual capillaries contributing to the formation of adjacent rings follow a corrugated course with angulations located on the sites of junction with other capillaries completing the rings; the capillaries are covered by type I epithelial cells which also extend into and form the alveolar lining at the peripheral area of the interstices of the capillary network. Single type II cells form the alveolar lining at the centre of vascular rings. The pattern of pulmonary alveolar capillaries revealed by scanning electron microscopy is thus similar to that postulated by Weibel (1963) on the basis of transmission microscopic studies.