The Propulsion of Mucus by Cilia

Abstract

The presence of cilia on epithelia of the respiratory tract was reported more than 150 yr ago, and the two-layer model of mucus transport was put forward more than 50 yr ago. However, it is only in the last 10 yr or so that the motion of mucus-propelling cilia of the mammalian respiratory system has been adequately described, and fluid dynamic studies have developed far enough to allow descriptions of the mechanisms by which ciliary movement is coupled to mucus transport. In this review, scientific developments on the study of cilia and mucus, and interactions between them, are drawn together to further understanding of mucociliary clearance mechanisms of the respiratory tract. The study of the cilia incorporates a discussion of the internal mechanics and biochemistry of the ciliary axoneme, the physical principles of the beat pattern, and the (weak) metachronal coordination of cilia in the lung. Mucus rheology plays a central role in mucociliary transport with the rheologic properties of the mucus determining the effective functioning of this clearance mechanism. Theoretical models provide information on the mechanical principles of the beat pattern as well as providing reliable estimates of the transport rates. Although airflow is not thought to contribute to mucus transport in the normal state, high frequency ventilation and coughing may make significant contributions.

Controlled surface-induced flows from the motion of self-assembled colloidal walkers

“…Asymmetry arises from the velocitydependence of cilia deformation that results in different filament geometries for the fast, forward stroke compared to that for the slow, backward stroke (4)(5)(6). Cilia carpets perform tasks such as the locomotion of unicellular eukaryotes (e.g., Paramecium) (7), the transport of mucus in the trachea (8), the development of microscopic flows during the early stages of embryonic development (9), and even the directed transport of the egg along the mammalian oviduct into the uterus (10). One particularly interesting feature of an individual filament, as well as the cooperative motion of cilia carpets (cilium), is the creation of a local and dynamic flow that can transport nutrients or other substances in a controlled fashion over length scales hundreds of times the size of a single filament.…”

Dinámica y patología del barrido mucociliar como mecanismo defensivo del pulmón y alternativas farmacológicas de tratamiento

“…Su espesor de entre 5 y 10 μm es crítico ya que permite la función de los cilios que miden 6 a 7 μm. Si esta capa aumenta su espesor, los extremos de los cilios no alcanzan la capa de moco y no logran su propulsión, y si disminuye, el moco se adhiere a los cilios pudiendo bloquear su movimiento 9,19,20 (Figura 2).…”

Host defence capacities of pulmonary surfactant: evidence for ‘non‐surfactant’ functions of the surfactant system

“…Some unexplained coordination exists among cilia in a region of the lower airways, such that unidirectional waves of movement are able to propel the overlying mucus toward the more proximal airways. This results in a blanket of mucus that moves up the respiratory tree to be expelled by coughing from the trachea or to be cleared by propulsion into the hypopharynx and swallowed [84]. In the air-exchange area, which begins beyond or distal to the respiratory bronchioles, mucociliary activity and effective clearance by coughing are lost.…”