Microfilament-associated adhering junctions (6 nm F-maculae adherentes) connect bovine pulmonary fibroblasts in vivo

Fibroblasts in the pulmonary alveolar septa of neonatal and mature cattle form spot-shaped intercellular junctions with each other where 6 nm microfilaments adhere to the plasma membranes. The junctions have variable cleft widths (10-20 nm) and a diffuse periodic intracleft substance. Impinging 6 nm microfilaments and plaque specializations were present in thin sections of junctions ranging from 0.4 to 0.6 micron in length. The microfilaments involved in junctions are parts of either cortical cytoplasmic webs or highly organized bundles. Two or more fibroblasts may form one complex of junctions and multiple junctions may occur between two fibroblasts. It is proposed that the in vivo fibroblast junction be named a 6 nm F-macula adherens, based on size of the associated microfilaments and type of junctional specialization.