Airway smooth muscle (ASM) is an integral component of the primordial lung. It differentiates
from the mesenchyme as a ring of cells around the base of the epithelial bud that express
smooth muscle-specific proteins. These rapidly form into interlocking bundles that
progressively become wider and more compact along the bronchial tree to the trachea. Their
orientation is perpendicular to the long axis of the airway. The ASM exhibits rhythmic
contractility (ie it is a phasic-type smooth muscle) soon after formation, and the spontaneous …