Schematic diagram of pathology in cystic fibrosis. In cystic fibrosis, secretion of CI" anions into the lumen of the bronchial tree is markedly decreased because of a defective or nonexistent chloride channel protein. Na* resorption from the lumen of the bronchial tree is then increased, causing movement of water into
the cell. As a result, the mucous layer within the bronchial tree becomes dehydrated and viscous. This thick mucus is difficult to move by the mucociliary escalator mechanism, and it clogs the lumen of the bronchial tree, obstructing airflow.
dehydrated viscous mucus defective CI" channel cytoplasm. Clara cells secrete a surface-active agent, a lipoprotein, that prevents luminal adhesion should the wall of the airway collapse on itself, particularly during expiration. In addition, Clara cells produce a 16-kDa protein known as Clara cell protein (CC16), which is an abundant component of the airway secretion. CC16 is used as a measurable pulmonary marker in bronchoalveolar lavage fluid and serum. Secretion of CC16 into the bronchial tree decreases in lung injury (because of damage of Clara cells), while serum levels of CC16 may increase because of leakage across the air—blood barrier.
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