Respiratory bronchiolus (human)

Stain: Hematoxylin and eosin. A longitudinal section shows the lumen of a respiratory bronchiolus (1) with an irregular lining. It is covered by low columnar-cuboidal cells, a few pouches within the same lumen are lined by thin alveolar epithelium as found in the surrounding alveoli (2). Thin arrows (→) point to light-stained Clara cells. At (3) more or less continuous bundles of smooth muscle (at this level individual differences appear depending on among others age, health). At (4) area with discontinuous muscle bundles. (5) indicates pulmonary arteries. Note that most capillaries (C) within the extensive vascularized alveolar walls are dilated.