Zonulae occludentes in alveolar epithelium and capillary endothelium of dog lungs studied with the freeze—fracture technique

Abstract

Intercellular junctions both in the alveolar epithelium and in the pulmonary capillary endothelium of the alveolar—capillary wall of dog lungs were observed in the electron microscope using the freeze—fracture technique. Only zonulae occludentes (tight junctions) were observed between cells in both alveolar epithelium and capillary endothelium and no other type of junctions were found. The zonula occludens between epithelial cells was a well-developed, bubble-like assembly of compartments made up of interconnecting junctional strands. The endothelial junction, on the other hand, was rather poorly developed with an approximately parallel arrangement of fewer strands with less frequent interconnections. The number of strands and the depth of the junction were measured on 20 alveolar epithelial and 45 capillary endothelial junctions. In the epithelium the mean depth of the junction was 0.261 ± 0.023 (SE) μ m and the number of junctional strands varied from 2–3 to 4–10. Mean depth of the endothelial junction was 0.166 ± 0.011 (SE) μ m, the number of strands varied from 1–2 to 3–4 with the most frequent occurrence of 2–3 strands. These observations suggest that the alveolar epithelium is “tight” while capillary endothelium is “leaky.” The fine structure of the zonulae occludentes in the pulmonary capillary endothelium is in favor of the hypothesis that they are the site of the small pore system which has been proposed by physiologists.