Cytoarchitecture of the bronchiolar wall in the rat lung: scanning electron microscopic studies

The cytoarchitecture of rat bronchiolar walls was directly visualized from the adventitial side by scanning electron microscopy after removal of collagen and elastic fibers with a KOH-collagenase digestion method. The bronchioles were surrounded by cord-shaped smooth muscle fibers, which ran circularly to the long axis of the bronchiolar tree. Branches of the bronchial artery were located outside the bronchiolar smooth muscle layer, ran toward the periphery, gave off smaller branches one after another, and finally formed capillary networks both inside and outside the smooth muscle layer. Collecting venules arising from the capillary network ran independently of arterioles, but were gradually accompanied with arterioles as they became thickened. Peripheral nerves covered with a perineurial sheath were also present outside the bronchiolar smooth muscle layer; they gave off branches toward the periphery, and finally entered the bronchiolar smooth muscle layer. Lymphatic vessels were sometimes found at the bifurcation of bronchioles. These findings provide important basic data on the structure and functions of the bronchiolar wall.