Page 269 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

Respiratory system (apparatus respiratorius) 251

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11.19 Wall of a bronchus (pig). The respiratory epithelium rests upon a lamina propria of loose connective tis-
sue with coiled, tubular mucous glands that occasionally protrude through to the hyaline cartilage fragments.
Smooth muscle encircles the bronchus forming a contractile ring. Haematoxylin and eosin stain (x80).

With continued branching of the bronchioles, the oriented smooth muscle cells. This muscle layer is impor-
pseudostratified columnar epithelium becomes flatter, tant for regulating ventilation of the conducting airways.
eventually transforming into simple columnar or cuboidal Contraction of the circular muscle rapidly constricts the
epithelium (Figures 11.20 and 11.21). In addition to ciliated bronchioles, increasing resistance to air flow (Figures
epithelial cells, the bronchiolar epithelium contains non- 11.20 and 11.21) (excessive constriction results in bronchial
ciliated columnar cells (Clara cells) (Figure 11.25). These asthma in humans).
are most numerous in the smaller bronchioles. Released In the final segments of the conducting airways, the
by the apocrine mode, the secretory product of Clara cells terminal bronchi, the epithelium is reduced to a single
contains proteolytic and mucolytic enzymes that reduce layer of cuboidal cells and the number of elastic fibres
the viscosity of the bronchial mucus. In contrast to the in the lamina propria increases. In conjunction with the
bronchi, glands and cartilage are absent from the walls well-developed bronchiolar muscle, the constrictive
of the bronchioles. The loss of goblet cells is followed effect of the elastic fibres contributes to folding of the
by the disappearance of cilia. Bronchioles have a distinct mucosa.
muscle layer comprising several sheets of mainly circularly

11.20 Wall of a terminal bronchiole (schematic). Occasional goblet cells may be observed in the proximal por-
tions. In contrast to the bronchi, subepithelial glands and cartilage are absent.

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