The tracheobronchial tree is lined by a mucociliary epithelium containing millions of multiciliated cells. Their integrated oscillatory activity continuously propels an overlying pollution-protecting mucus layer in cranial direction, leading to mucociliary clearance – the primary defence mechanism of the airways. Mucociliary transport is commonly thought to co-emerge with the collective ciliary motion pattern under appropriate geometrical and rheological conditions. Proper ciliary alignment is therefore considered essential to establish mucociliary clearance in the respiratory system. Here, we used volume electron microscopy in combination with high-speed reflection contrast microscopy in order to examine ciliary orientation and its spatial organization, as well as to measure the propagation direction of metachronal waves and the direction of mucociliary transport on bovine tracheal epithelia with reference to the tracheal long axis (TLA). Ciliary orientation is measured in terms of the basal body orientation (BBO) and the axonemal orientation (AO), which are commonly considered to coincide, both equivalently indicating the effective stroke as well as the mucociliary transport direction. Our results, however, reveal that only the AO is in line with the mucociliary transport, which was found to run along a left-handed helical trajectory, whereas the BBO was found to be aligned with the TLA. Furthermore, we show that even if ciliary orientation remains consistent between adjacent cells, ciliary orientation exhibits a gradual shift within individual cells. Together with the symplectic beating geometry, this intracellular orientational pattern could provide for the propulsion of highly viscous mucus and likely constitutes a compromise between efficiency and robustness.

气管支气管树由含有数百万多纤毛细胞的黏液纤毛上皮排列。它们的综合振荡活动在头颅方向不断推动上覆的污染保护粘液层，导致粘液纤毛清除——气道的主要防御机制。黏液纤毛运输通常被认为在适当的几何和流变条件下与集体纤毛运动模式共同出现。因此，正确的纤毛排列对于在呼吸系统中建立粘液纤毛清除至关重要。在这里，我们将体积电子显微镜与高速反射对比显微镜结合使用，以检查纤毛方向及其空间组织，以及参考气管长轴（TLA）测量异时波的传播方向和牛气管上皮上的粘液纤毛运输方向。纤毛方向是根据基体方向 (BBO) 和轴索方向 (AO) 来衡量的，这两个方向通常被认为是一致的，两者都等效地指示有效中风和黏液纤毛运输方向。然而，我们的结果表明，只有 AO 与黏液纤毛运输一致，发现其沿着左手螺旋轨迹运行，而发现 BBO 与 TLA 对齐。此外，我们表明即使相邻细胞之间的纤毛方向保持一致，纤毛方向也会在单个细胞内逐渐发生变化。