[15

In scanning electron microscopy studies of chick embryo morphology, Kluth et al.1 • found that the esophagus and trachea normally develop and separate as a result of cranial, ventral, and dorsal folds that arise in the foregut. The descending paired cranial folds represent the primitive larynx. The caudal ascending ventral fold, which appeared to correspond to the tracheoesophageal septum in earlier reports, separates the trachea from the esophagus. The dorsal pharyngoesophageal fold demarcates the primitive pharynx from the esophagus. EA with distal fistula is proposed to result from excessive ventral invagination of the pharyngoesophageal fold, creating an upper esophageal pouch and preventing the cranial folds from descending to meet the ventral fold, thus maintaining a connection between esophagus and trachea. Kluth et al. proposed that isolated atresia may be explained by developmental disorders of the esophageal circulation as seen with intestinal atresia.

[16] [16] O'Rahilly and Mullen ' proposed an alternate hypothesis for the normal and abnormal development of the respiratory and esophageal tracts based on the examination of more than 100 human embryos in the Carnegie Collection. The yolk sac, from which most of the digestive tract develops, is noted between 9 and 13 postovulatory days, and by day 20, most embryos possess a foregut.1 ' By day 22, a longitudinal median pharyngeal groove develops in the ventral aspect of the foregut, and by day 26, the lung bud forms from this sulcus. On day 28, the lung bud is clearly separated from the digestive tract, quickly becomes paired, and descends caudally into the mesenchyme ventral to the foregut. The part of this mesenchyme that comes to lie between the respiratory and digestive tubes constitutes the tracheoesophageal septum,

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