Airway Surface Liquid and Respiratory Mucus - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 752

12.3 Mucus Delivery

Mucus is secreted by secretory cells of the respiratory epithelium. Mucus is released
by exocytosis in respiratory conduits down to terminal bronchioles. The mucus
extends over the tops of the cilia in a layer of thickness of about 5

m. Mucus
has also been observed to exist as droplets, flakes, and plaques in airways. Droplets
(size 4 ), and then plaques, but does

not necessarily form a complete mucus blanket (mucus rafts). Nasal mucus has a
thickness greater than 200

m) can aggregate into flakes (size 10-70

m.
Mucus is secreted in healthy upper airways primarily by submucosal glands. It
contains defense molecules mixed with mucins, electrolytes, and water. Mucus traps
pathogens, impedes their proliferation using molecular (e.g., lysozyme) and cellular
(e.g., neutrophils and macrophages) defenses, and sweeps them out of airways using
ciliary beats.

12.3.1 Secretory Cells

Mucous and serous secretions form the protective airway surface fluid. Airway
mucus is secreted by mucous cells, goblet cells of the respiratory epithelium
and mucous cells of the submucosal glands. In large airways, the major part of
respiratory mucus is produced by submucosal glands, whereas epithelial goblet cells
yield a smaller contribution.
The secretory cell population is usually defined by 3 cell types according to
their microscopical features — non-ciliated bronchiolar Clara, goblet, and serous
cells. Serous and goblet cells reside principally in trachea and bronchi. Secretory
cells of airways are classically identified according to morphological characteristics,
such as the density and type of secretory granules and the proportion of smooth to
rough endoplasmic reticulum. However, secretory cells are characterized by a great
phenotype (structural, molecular, and functional) changeability.
Secretoglobins (ScGb1a1-ScGb1a2, ScGb1c1, ScGb1d1-ScGb1d2, and Sc-
Gb1d4, ScGb2a1, and ScGb3a1-ScGb3a2) that are expressed in secretory epithelia
can serve as cell markers. Secretoglobin-1A1 (ScGb1a1), or Clara cell secretory
protein (CCSP), 3 is synthesized in cells that also produce mucins. Secretaglobin-
3A1 is, like mucins, expressed selectively in secretory cells of large bronchi in
humans [ 1532 ]. On the other hand, secretaglobin-3A2 is an early molecular marker
for bronchiolar Clara cells.

3 A.k.a. uteroglobin (UGb), Clara cell phospholipid-binding protein (CCPBP), Clara cells 10-kDa
secretory protein CC10, and urinary protein-1 (UP1). Secretoglobin-1A1 is the founding member
of the secretaglobin family. Secretory cells of the conducting airway epithelium produce dif-
ferent members of the secretoglobin family, such as ScGb3a1 and ScGb3a2 in neonatal human
lungs [ 1532 ].