Three-dimensional mapping of smooth muscle in the distal conducting airways of mouse, rabbit, and monkey

Airway smooth muscle remodeling is implicated in a number of constrictive pulmonary diseases such as asthma and may include changes in smooth muscle orientation and abundance. Both factors were compared in the normal distal bronchioles of the mouse, rabbit, and rhesus monkey (respiratory bronchioles included). Airway smooth muscle was measured by using a three-dimensional approach employing confocal microscopy and whole-mount cytochemistry with fluorochrome-conjugated phalloidin, a probe for polymerized actin. Smooth muscle orientation had a wide range of angles along the airway, but the distribution was conserved among species and among distal airway generations. At the bifurcation of proximal bronchioles, smooth muscle was nearly parallel to the longitudinal axis of the airway. Smooth muscle abundance was significantly different between species (abundance was less in the monkey compared with the mouse and rabbit), and there was a trend for abundance to decrease with each more distal airway generation. This study defines the normal distribution of smooth muscle in three test species and provides a basis for future comparisons with the diseased state.