DEVELOPMENT OF THE ESOPHAGUS
As early as the fourth week in development, the esophagus of the human embryo is merely a sphincter or constricted part of the primitive foregut, between the pharynx and stomach, as observed by Keith (1948) (Fig. 122-1). During the sixth and seventh weeks of gestation, the esophagus undergoes rapid elongation as cephalic development separates the head and neck from the thorax. The elongation is facilitated by development of the lungs and pleural cavities, which push the stomach dorsally and inferiorly (Fig. 122-2). Keith suggested that the esophagus is of dual origin, with an upper retrotracheal part originating from the pharyngeal portion and an infratracheal part originating from the pregastric segment of the foregut. During the sixth week of development, the esophagus is only 2 mm long,...