Figure 1812

Diagram of a Clara cell between bronchiolar ciliated epithelial cells.

The nucleus is in a basal location. Rough-surfaced endoplasmic reticulum (rER), a Golgi apparatus, and mitochondria are chiefly in basal and paranuclear locations of the cell. Smooth-surfaced endoplasmic reticulum (sER) and secretory vesicles are chiefly in the apical cytoplasm. One of the secretory vesicles is shown discharging its contents onto the surface of the cell.

Cystic fibrosis (mucoviscidosis) is a chronic obstructive pulmonary disease of children and young adults. It is an autosomal recessive disorder that affects the viscosity of the secretion of the exocrine glands. Almost all exocrine glands secrete abnormally viscid mucus that obstructs the glands and their excretory ducts. The primary cause of cystic fibrosis is a genetic defect in the Cl~ channel protein, which results in abnormal epithelial transport of CI .

The course of the disease is largely determined by the degree of pulmonary involvement. At birth, the lungs are normal. However, the defective CI channel protein in the bronchial epithelium causes decreased CI" secretion and increased Na+ and water reabsorption from the lumen (Fig. 18.13). As a result, the "mucociliary escalator" malfunctions, with consequent accumulation of an unusually thick, viscous mucous secretion. The pulmonary lesion is probably initiated by obstruction of the bronchioles. Bronchiolar obstruction blocks the airways and leads to thickening of the bronchiole walls and to other degenerative changes in the alveoli. Because fluids remain trapped in the lungs, individuals with cystic fibrosis have frequent respiratory tract infections. The recent cloning of the cystic fibrosis gene could lead to the use of gene therapy in the future.

ATPase chloride channel normal mucus

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