Page 681 - Atlas of Histology with Functional Correlations

functions. They secrete one of the surfactant-like lipoproteins that coat the

bronchial epithelium and break down (via proteolytic enzymes) the luminal
stickiness of mucus for more efficient respiration. These lipoproteins also
serve as tension-reducing agents in the alveoli helping to reduce the collapse
of the airway walls. Clara cells also function as stem cells that replace lost or
injured bronchial ciliated and nonciliated epithelial cells. These cells also

secrete proteins and lysozymes into the bronchioles to protect the lung from
inhaled toxic substances, oxidative pollutants, or inflammation and transfer
immunoglobulins into bronchiolar lumina.

FIGURE 17.19 | Low-Power Ultrastructure of

Lung Showing Portion of Bronchiole Wall and

Adjacent Alveoli

This lung was perfused with fixatives, and, as a result, the capillaries are empty
and do not exhibit any blood cells. This low-power ultrastructure of the lung
shows a section of the bronchiole wall and the alveoli. The lumen of the

bronchiole (14) is lined with the secretory, dome-shaped Clara cells (1, 8) and
ciliated cells (2, 9). The cytoplasm of the Clara cells (1, 8) contains dense-
staining secretory granules. The very thin capillaries (5, 11) with empty lumina
are adjacent to the very thin cytoplasm of alveolar cells (6, 13) (type I

pneumocytes) that line the lumina of alveoli (7, 12). Surrounding the wall of the
bronchiole is a layer of connective tissue (10), smooth muscle cells (3), and a
blood vessel with a white blood cell (4) in its lumen.

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