Airway Surface Liquid and Respiratory Mucus - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 772

12.6 Mucus Motility

Once delivered in the airway lumen, mucus is driven by cilia of the respiratory
epithelium that beat in a coordinated manner within the intraciliary fluid layer
(epithelial lining fluid) beneath a viscoelastic (more or less continuous) mucus film.
Ciliated cells of the respiratory mucosa line the respiratory tract from the nose
and nasal sinuses down to terminal bronchioles. Ciliated columnar cells outnumber
the other cell types. The percentage of ciliated cells in human airway epithelium
decreases from approximately 80% to 50% in the tracheal epithelium to 20% in the
bronchiolar compartment. Ciliated cells are very vulnerable to injury by infection
and pollutant exposure. Interleukins IL4 and IL13 increase the population of goblet
cells and decrease that of ciliated cells.
Each cell has 200 to 300 cilia at its apical surface that beat in unison, propelling
mucus and entrapped foreign material toward the oropharynx. Some substances such
as cigarette smoke are ciliotoxic.

12.6.1 Respiratory Cilium

Respiratory cylindrical cilia have a caliber of 0.10 to 0.25 m, a height of 0.5

to 7 m[ 1568 ]. The cilium

originates in a corpuscule beneath the plasma membrane. Between cilia, cyto-
plasmic projections (microvilli) can be observed. The distal, narrowed end of
cilia is associated with the glycocalyx. Microtubule capping structures at tips of
respiratory cilia that are firmly attached to specific peripheral A-tubule and central
pair microtubules restrict microtubule sliding.
The cilium is covered by a ciliary membrane that is continuous with the plasma
membrane, as it is a specialized extension of the cell membrane. An array of
microtubules (Vol. 1 - Chap. 6. Cell Cytoskeleton), the axoneme , runs longitudinally
inside the cilium, with a characteristic arrangement of 9 peripheral doublets of
microtubules and 2 central single microtubules (Fig. 12.2 ). Axonemal microtubules
end in the basal body within the cell cortex.
Each outer doublet microtubule consists of tubule-A and -B. Each tubule-A
supports inner and outer dynein arms and a radial spoke that has a head close
to one of the 2 single microtubules of the central sheath complex. In addition,
2 sets of links — the interdoublet linkers — join adjacent microtubular doublets.
Nexin and radial spoke linkages serve as circumferential and radial connections.
Two inner and outer dynein arms that yield axonemal ATPases exist on each outer
microtubular pair implicated in sliding of microtubular pairs relative to one another.
Dynein undergoes cyclical shape changes in the presence of its preferred substrate
AT P Mg to propel adjacent microtubules tipward. Accessory structural elements
(radial spokes, interdoublet linkers) are supposed to provide shear resistance to
microtubular sliding. Tektins are proteins associated with microtubules.

m, with a mean separation between cilia of 2 to 5