Background information respiration | Anatomy | Microscopic structures | Bronchial system

Histologic constituents of bronchi and bronchioli

The mucosa of bronchi, as in the trachea, consists of an epithelium (ciliated pseudostratified columnar with goblet cells), a basement membrane, and a lamina propria.

Goblet cells in bronchi and bronchioli are less numerous than in the trachea. They are usually filled with mucous secretory droplets, which are discharged into the lumen where they form a mucous blanket on the epithelial surface. It has been shown in experimental models that goblet cell mucus is necessary for ciliary action. In the absence of enough mucus, the cilia fail to have a beat-like motion. Their action can be restored by the addition of mucus.

In bronchi, the C-shaped cartilage of the trachea is replaced by separate plates of cartilage. At the same time, the lamina propria becomes surrounded by a band of smooth muscle fibres which are arranged spirally and criss-cross one another. The smooth muscle can be considered as a separate layer, the muscularis, lying between the mucosa on the one side and the submucosa, fibrocartilage plates and adventitia on the other.

Serous and mucous glands are present in the submucosa of bronchi, but their numbers decrease with each division into smaller orders of bronchi. The adventitia or peribronchial layer contains many elastic fibres and separates the wall of the bronchus from pulmonary parenchyma. It permits bronchi to move independent of other lung parenchyma.

In contrast to bronchi bronchioli lack cartilage and glands and generally have a diameter less than 1 mm. Three layers can be distinguished: mucosa, muscularis, and an outer layer. Due to the absence of cartilage and contraction of smooth muscle the mucosa is highly folded. It is lined with a simple cuboidal epithelium which – besides ciliated and mucous-secreting cells – includes Clara cells, whose role is uncertain.

The muscularis is the thickest layer. It has thick bands of smooth muscle, which completely encircle the bronchiole. The connective tissue of the thin outer layer is continuous with the parenchyma of the lung so that these passages move with the lungs.

Literature:

Klinke R, Silbernagel S. Lehrbuch der Physiologie. 2001. Thieme: Stuttgart

Ross MH, Gordon KI, Pawlina W. Histology: A text and atlas. 2002. Lippincott Williams and Wilkins: Philadelphia

Microscopic structures of bronchial walls







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