On the mechanics of mucociliary flows. II. A fluorescent tracer method for obtaining flow velocity profiles in mucus

Abstract

The transport of particles by cilia lining vapor-filled tubes such as the trachea, bronchi, and upper bronchioles requires that the particles be carried by mucus [Sade et al, 1970], and the force that causes mucus flow is generated by the beat of the underlying cilia. The means by which the ciliary beat force is transmitted to the mucus is not clear. Lucas and Douglas [1934] proposed that cilia penetrate the mucus enough to push it forward in a conveyor-belt fashion. This view is consistent with recent EM studies [Yoneda, 1976; Reissig, Bang, and Bang, 1978] that suggest the serous layer is thinner than the length of a cilium (5-6 µm). In contrast, Ross and Corrsin [1974] developed a theoretical model for mucociliary transport based on the assumption that mucus persists as a "blanket" carried by the serous fluid, which is in turn propelled by the cilia. If one accepts current simplified models which take into account the viscoelastic properties of mucus, the Lucas and Douglas model is the more reasonable concept. Accordingly, the two most recent fluid mechanical models for mucociliary transport incorporate ciliary tip penetration as a central requirement. One [Blake and Winet, 1980] favors the average depth of penetration as the critical force-generating factor, whereas the other [Yates et al, 1980] favors the average number of cilia penetrating per wavelength. There appear to be no articles describing tests of these theoretical models with measurements of mucus and serous fluid below the air-mucus interface. The primary reasons for this deficit are the following: a) Epithelium viewed from the side must be folded over and placed in narrow chambers where mucus blankets tend to adhere to the glass walls, and b) epithelium viewed from above must be observed through mucus which refracts and scatters light unevenly such that one cannot resolved tracer particles in the mucus reflected or transmitted light. We chose to avoid these optical barriers by utilizing fluorescent tracer particles to investigate mucociliaryt flow profiles.