Abstract: Objective To investigate the developmental pattern of the airway smooth muscle in mice and the expression characteristics of different muscle-specific proteins. Methods Serial sections of mouse embryos from embryonic day 10 (ED10) to embryonic day 18 (ED18) were stained with monoclonal antibodies against α-SMA, α-SCA and Desmin. Results With the foregut gradually separating into trachea and esophagus, α-SMA positive cells were detected in the posterior wall of the initial segment of the trachea at ED12 and the expression intensity was tapered off towards the caudal segment. A few α-SMA and α-SCA positive cells were observed in the mesenchyme surrounding the developing pulmonary vein. At ED13, the expression intensity of α-SMA in the posterior wall of the trachea became stronger, while the very weaker expression of α-SCA and Desmin was only initiated. At ED14, strong α-SMA expression showed C-shaped patterns in the wall of the left and right bronchi. However, staining intensity of α-SCA and Desmin at the same segment was weaker than that of α-SMA. In addition, the muscle cells of the pulmonary vein showed strong expression of α-SMA. At ED15, α-SMA positive cells were found in the wall of small bronchioles. Between ED17 and ED18, with progression towards the terminal bronchioles, expression intensity of α-SCA and Desmin became weaker in the airway smooth muscle while the expression of α-SMA became stronger. Conclusion The development of the airway smooth muscle begins from the upper segment of the trachea at ED12 and gradually extends distally. At ED18, airway smooth muscle cells have extended to the terminal bronchioles. The expression of Desmin marks the formation of smooth muscle cytoskeleton structures and implies the further maturation of the airway smooth muscle. Besides, it may help airway smooth muscle

Key words: Embryo, Airway smooth muscle, Development, Immunohistochemistry, Mouse