Biomedical Engineering Reference
In-Depth Information
reader, the following section gives a condensed summary of airway mucus,
mucins, and mucin (MUC) genes.
II. AIRWAY MUCUS, MUCINS, AND MUC GENES
In healthy individuals, a thin film of slimy liquid protects the airway surface
(6). The liquid is often referred to as 'mucus' and is a complex, nonhomoge-
neous dilute (1-2)% aqueous solution of salts, enzymes and antienzymes,
oxidants and antioxidants, bacterial products, antibacterial agents, cell-
derived mediators and proteins, plasma-derived mediators and proteins,
and cell debris such as DNA. The mucus forms a bilayer comprising an
upper gel layer and a lower sol layer. A thin layer of surfactant appears
to separate the gel and sol (7,8). Cilia beat in the sol layer, often termed peri-
ciliary fluid. Inhaled particles are trapped in the gel layer and, by transporta-
tion on the tips of beating cilia, are removed from the airways, a process
termed mucociliary clearance (Fig. 3). Airway mucus requires an optimal
combination of viscosity and elasticity for efficient ciliary interaction. Vis-
coelasticity is conferred primarily by high molecular weight mucins that
comprise up to 2% by weight of the mucus (9). Airway mucins are primarily
produced in, and secreted by, goblet cells in the surface epithelium (10) and
by mucous cells in the submucosal glands (11). Mature mucins are long
thread-like molecules composed of monomers joined end-to-end by disul-
phide bridges (Fig. 4). These threads form a 'tangled network' (12) that con-
tributes to the formation of the mucus gel. The mucin monomers comprise a
highly glycosylated linear peptide sequence, termed apomucin, that is
encoded by specific mucin (MUC) genes. Nineteen human MUC genes
are reported to date, namely MUC1, 2, 3A, 3B, 4, 5AC, 5B, 6-9, 11-13,
Figure 3
Mucus movement on cilia in bovine trachea in vitro. Note differences in
bending of the cilia at different stages of the beat cycle. CC, ciliated cell; GC, goblet
cell.
