Spatial Intra- and Intercellular Alignment of Respiratory Cilia and its Relation to Function

Ciliary alignment is considered necessary to establish respiratory tract mucociliary clearance, and disorientation is often associated with primary ciliary dyskinesia. Therefore, there is an urgent need for a detailed analysis of ciliary orientation (CO). We used volume electron microscopy to examine CO relative to the tracheal long axis (TLA) by measuring the inter- and intracellular basal body orientation (BBO) and axonemal orientation (AO), which are considered to coincide, both equivalently indicating the effective stroke direction. Our results, however, reveal that only the mean BBO is aligned with the TLA, whereas the AO determines the effective stroke direction as well as the mucociliary transport direction. Furthermore, we show that even if the mean CO is conserved across cell boundaries, a considerable gradient in CO exists within individual cells, which we suspect to be crucial for the emergence of coordinated ciliary activity. Our findings provide new quantitative insight into CO and correlate this new structural information with mucociliary function.