two layers: (1) Pericellular fluid (PCF) - the layer next to the cells, 
sometimes referred to as the "sol" phase; and (2) mucus - the upper layer, 
referred to as the "gel" phase (Figures 1.9.2-A, 1.9.2-B). 
(1) PCF is approximately 6 /zm in depth and approximately the height of 
cilia (Massaro, 1989). It is believed that PCF allows cilia to 
move freely, with the tips of cilia imbedded into the upper mucus 
layer, permitting the cilia to propel the mucus toward the pharynx 
(see below). The composition of PCF is not fully defined, but 
includes proteins as well as small molecules. These components are 
derived from plasma, produced locally, and include alveolar epi 
thelial fluid molecules that have moved to the airways. 
(2) Mucus - in normal airways, the upper mucus layer is a discontinu- 
ous blanket forming mucus islands floating on the PCF (see Basbaum 
and Finbeiner, 1989; Kaliner et al., 1988; Lopez -Vidriero et al. , 
1977 for reviews). Mucus is a complex viscoelastic material that 
contains mucins, proteins, lipids, and proteoglycans. It traps 
inhaled particulates and, together with the cilia that propel the 
mucus upward, serves a major defense mechanism to remove inhaled 
material and endogenous debris from the lung. In the phase of 
their forward beat, the cilia grasp the mucus blanket with fine 
claws at their tips. 
It is likely that all columnar airway epithelial cells contribute to airway 
ELF. The goblet and serous cells are the major contributors to the mucus 
layer (at least the mucin part) (Basbaum and Finbeiner, 1989; Kaliner et 
al., 1988; Lopez-Vidriero et al., 1977). The physiologic stimuli that regu- 
late mucus production of the bronchial epithelium are largely unknown. In 
the submucosal glands, macromolecular secretion can be modified by variety 
of neural influences as well as by a variety of mediators (Kaliner et al., 
1988). Glandular serous cells and goblet cells appear to be under somewhat 
different control. In addition to mucus and PCF, airway ELF likely also 
contains components of alveolar ELF that has been transported upward. 
1.9.5 Airway Epithelium in CF 
As the disease progresses, the mucus obstruction, infection and neutrophil- 
dominated inflammation takes its toll on the normal epithelial architecture 
(Figure 1.9.5-A). There is no quantitative data available from biopsy 
samples of individuals with CF. To quantitate these changes in the surface 
epithelium of the large airways, a fiberoptic bronchoscope was used to 
collect cytologic brush samples of the bronchial epithelium of individuals 
with CF (18 years or older, all with moderate lung disease) compared to 
normal, non-smokers. For comparison, the nasal epithelium was evaluated 
(Table 1.9.5-B). The data demonstrates that while the nasal epithelium is 
similar in normals and those with CF, there are significant changes in the 
proportions of epithelial cell types in the large bronchi, with CF individ- 
uals having fewer ciliated cells, but increased proportions of basal, 
secretory and undifferentiated columnar cells. 
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