The laryngotracheal groove or sulcus appears in the proximal foregut at the third week (3 mm embryo, stage 10).1 The laryngotracheal groove progresses caudad and the lateral ridges progress cephalad to form the pri-mordium of the trachea (Figure 6-1). The pulmonary primordium appears and bulges ventrally from the foregut. Complete separation of trachea and esophagus occurs by 11 to 14 mm (sixth week). The tip of the tracheal primordium buds asymmetrically, left and right, at the 4 mm stage, to provide bronchial primordia. Mesenchymal proliferation by cells lining the coelomic cavity provides the tissue from which cartilage, muscle, and connective tissue will develop. Epithelial-mesenchymal interrelationships are essential for bronchial and pulmonary development to occur. The tracheal bifurcation moves gradually downward from the neck to the level of the fourth vertebra. Cartilage appears in the trachea at 10 weeks.
When the laryngotracheal groove appears, the forerunner of the glottis also appears as a median slit in the pharyngeal floor between the fourth and sixth branchial arches. The epiglottic primordium lies anteriorly and the arytenoid swellings lie laterally prior to their more medial migration. Ventricular buds are solid at first. A T-shaped slit appears, which opens into a lumen by the eighth week. Vocal cords are seen at 3 months. Thyroid and cricoid cartilages appear between 5 and 7 weeks. The laryngeal cartilages derive from the fourth and fifth arches.
figure 6-1 Development of larynx and trachea at about the sixth to eighth weeks. The laryngeal slit lies in the floor of the pharynx. Numerals indicate arches. The foregut is separating into the trachea and esophagus. Stem bronchi and branches are present. Specific stages are altered to show general relationships. Adapted from Gray SW and Skandalakis JE.1
Failure of complete separation of the foregut into respiratory and alimentary components is the most common defect and produces tracheoesophageal fistula (TEF). At the upper end, the larynx may fail to reopen, producing atresia (a fatal anomaly), or it may fail to form a complete posterior septum, producing a laryngotracheoesophageal cleft. Tracheal atresia, stenosis, esophageal atresia, and tracheoesophageal fistula occur more distally. The relatively separate processes of laryngeal development and budding of bronchi and pulmonary development allow for malformations of the trachea, such as agenesis and stenosis in the presence of a normal larynx and bronchial tree.
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