The trachea is formed of four layers: a mucosa including epithelium and lamina propria, a submucosa, a fibrocartilage layer and an adventitia.

The mucosa consists of a pseudostratified, ciliated, columnar epithelium overlying an elastic lamina propria. Cells of the tracheal epithelium may change to another type in response to an irritation such as chronic cough. The basement membrane, on which the epithelium rests, is the thickest in the body.

Ciliated columnar cells containing 200-300 cilia per cell at their apical surface outnumber the other cell types. Cilia are found throughout the tracheobronchial tree and protect the respiratory surface from dirt and airborne infection. The beating of cilia (1,000 strokes per minute) directs the secretions toward the oropharynx for expectoration or swallowing. Some substances, such as cigarette smoke, are ciliotoxic.

The submucosa is made of connective tissue with numerous mixed (serous and mucous) glands of the tubuloalveolar type. Ducts from these glands pass through the lamina propria to empty onto the epithelial surface. Particles get entrapped in the mucus which floats on the serous secretions.

The fibrocartilage layer consists of a series (16-20) of C-shaped cartilage rings which prevent the trachea from collapsing. As is typical of cartilage, the rings are surrounded by a band of dense connective tissue called perichondrium, which merges with the submucosa and the adventitia. The open end of each cartilage ring is directed posteriorly and is closed by a transverse band of smooth muscle, the trachealis muscle. Contraction of this muscle reduces the tracheal diameter and increases intrathoracic pressure during coughing. The area between the rings is occupied by fibroelastic connective tissue.

The adventitia is a layer of connective tissue that binds the trachea to adjacent structures in the neck and mediastinum. It contains the largest blood vessels, nerves and lymphatics.

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 the trachea

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