This field shows a respiratory bronchiole with a few connections (arrows) to the underlying alveoli. The respiratory bronchiole is lined with a simple epithelium composed of lightly stained cells, the Clara cells (Cl), showing a spherical nucleus and a slightly granulated cytoplasm. These cells secrete a serous fluid.

These Clara cells are next to bundles of smooth muscle fibres (*) seen here in cross section.

A few ciliated cells (arrowhead) are at the borderline of a terminal and a respiratory bronchiole.

Stain: H–­­E
Magnification: ×500