Airway smooth muscle: an architect of the lung?

This review examines the rationale for increased study of the prenatal development of airway smooth muscle (ASM). With a critical role in asthma and airway remodelling, ASM has been extensively investigated. Nevertheless, there is a paucity of studies looking at prenatal ASM development and function. A working party report from the National Heart Lung and Blood Institute identified this issue and has recommended further investigation. The impetus for this call stems not only from an appreciation that childhood and adulthood disease may have their origins in fetal life, but also because recent studies indicate that prenatal ASM regulates lung development (via its phasic contractility and growth factor production). Moreover, recognition that phasic ASM contractility is the prenatal norm has raised interest in the idea that postnatal reactive airways disease may likewise be a periodic oscillatory phenomenon. Finally, bronchial thermoplasty represents an exciting new therapy for refractory asthma. However, the mechanism for its effects is unclear. Recent studies show that prenatal ASM contractility is governed by a hierarchy of pacemakers within proximal airways. If such a pacemaker arrangement persists postnatally, it is possible that bronchial thermoplasty achieves its distal airway effects via ablation of controlling proximal pacemaker centres. Hence, study of prenatal ASM may allow us not only to develop ways to stimulate prenatal lung growth, but also to understand and develop new therapies for reactive airways disease.