Background: Collectively coordinated ciliary activity constantly propels the airway surface liquid, which lines the luminal surface of the vertebrate respiratory system, in cranial direction – constituting mucociliary clearance, the primary defence mechanism of our airways. Our contemporary understanding on how the quantitative characteristics of the metachronal wave field determines the resulting mucociliary transport is still limited, which is partly due to the sparse availability of quantitative observational data. Methods: We employed high-speed video reflection contrast microscopy to simultaneously image and quantitatively characterize the metachronal wave field as well as the mucociliary transport in excised bovine, porcine, ovine, lapine, turkey and ostrich samples of the luminal tracheal wall. Advanced image processing techniques were used to determine the ciliary beating frequency (CBF), the velocity and the wavelength of the metachronal wave as well as the mucociliary transport velocity. Results: The mucociliary transport direction was found to strongly correlate with the mean wave propagation direction in all six species. The CBF yielded similar values (10−15 Hz) for all six species. Birds were found to exhibit considerably higher transport speeds (130−260 μm/s) than mammals (20−80 μm/s). While the average transport direction significantly deviates from the tracheal long axis (TLA) in mammals, no significant deviation from the TLA was found in birds. In comparison to mammals, longer metachronal wavelengths were found in birds. Finally, the metachronal waves were found to propagate at about 4−8 times the speed of mucociliary transport in mammals, whereas the metachronal waves propagate at about the speed of mucociliary transport in birds. Conclusions: The tracheal mucociliary clearance mechanism is based on a symplectic metachronsim in all examined species. The mucociliary transport in birds is fast and roughly follows the TLA, whereas the transport is slower and proceeds along a left-handed spiral in mammals. The longer wavelengths and the lower ratio between the metachronal wave speed and the mucociliary transport speed provide further evidence that the mucociliary clearance mechanism operates differently in birds than in mammals.
