Biomedical Engineering Reference
In-Depth Information
The number of mucous secreting goblet cells in the airways is variable and
increases in chronic bronchitis and after inflammatory stimuli (2,8). The
small mucous granule cell that has been described in the midepithelial layer
may be a precursor of the goblet cell (9).
Basal cells are closely attached to the basement membrane. They do
not have a columnar shape and do not normally reach to the airway lumen.
It has been postulated that the primary function of basal cells is to enhance
the attachment of columnar cells to the basement membrane (10). Basal cells
have numerous attachment sites to the basement membrane through hemi-
desmosomes and an abundance of desmosomes, which mediate attachment
to columnar cells. While columnar cells attach to the basement membrane
directly, the attachments to basal cells may be more important in maintain-
ing mechanical integrity. There is recent evidence that basal cells are hetero-
geneous in the expression of some genes (11). The population of basal cells
likely contains progenitor cells that can be precursors of the other cell types
during injury and repair (11,12).
Clara cells and serous cells are other types of secretory cells. In
humans, Clara cells are found primarily in the distal, terminal, and respira-
tory bronchioles (13). The defining characteristics of Clara cells are electron-
dense granules. The main function of Clara cells is likely to provide intralu-
menal secretions for the distal, nonciliated bronchioles. Clara cells are
thought to secrete surfactant proteins, an antileukoprotease, and Clara cell
specific peptides such as Clara cell secretory protein (CCSP) (14-18). The
CCSP is thought to modulate inflammatory and secretory processes. Clara
cells are thought to be progenitor cells during development and it has been
suggested that a stem cell for the peripheral airway has Clara cell character-
istics (19,20). It has also been shown that CCSP-expressing stem cells that
localize to the neuroepithelial body (NEB) can contribute to renewal of
the proximal bronchiolar epithelium (20). It has been shown that Clara cells
are replaced by mucous-producing cells in the bronchioles of smokers
(21,22). Excess mucous in nonciliated airways and a reduction in CCSP
may impair clearance of secretions from these small airways. Serous cells
are found primarily in submucosal glands but can be found in the surface
epithelium of fetal human airways (23).
Pulmonary neuroendocrine cells are rare cells in the small airways of
humans. They are hard to identify without immunohistochemical techni-
ques. The subject of pulmonary neuroendocrine cells was comprehensively
reviewed in 1991 (24). The role of these cells in normal pulmonary function
is difficult to assess, in part, because of their rarity. Pulmonary neuroendo-
crine cells do proliferate in the setting of airway damage. The mediators pro-
duced by these cells include serotonin, bombesin, and somatostatin that can
have profound effects on cell growth and development. Increased release of
mediators derived from these cells has been suggested to play a role in the
response to cigarette smoke and in the development of lung cancer (25-27).
