Primitive intestinal loop
Figure 13.5 Embryos during the fourth (A) and fifth (B) weeks of development showing formation of the gastrointestinal tract and the various derivatives originating from the endodermal germ layer.
pharyngeal gut (Fig. 13.5). The tracheoesophageal septum gradually partitions this diverticulum from the dorsal part of the foregut (Fig. 13.6). In this manner the foregut divides into a ventral portion, the respiratory primordium, and a dorsal portion, the esophagus (see Chapter 12).
At first the esophagus is short (Fig. 13.5A), but with descent of the heart and lungs it lengthens rapidly (Fig. 13.5B). The muscular coat, which is formed by surrounding splanchnic mesenchyme, is striated in its upper two-thirds and innervated by the vagus; the muscle coat is smooth in the lower third and is innervated by the splanchnic plexus.
Esophageal atresia and/or tracheoesophageal fistula results either from spontaneous posterior deviation of the tracheoesophageal septum or from some mechanical factor pushing the dorsal wall of the foregut anteriorly In its most common form the proximal part of the esophagus ends as a blind sac, and the distal part is connected to the trachea by a narrow canal just above the bifurcation (Fig. 13.7A). Other types of defects in this region occur much less frequently (Fig. 13.7, B-E) (see Chapter-12).
Atresia of the esophagus prevents normal passage of amniotic fluid into the intestinal tract, resulting in accumulation of excess fluid in the amniotic sac (polyhydramnios). In addition to atresias, the lumen of the esophagus may narrow, producing esophageal stenosis, usually in the lower third. Stenosis may be caused by incomplete recanalization, vascular abnormalities, or
accidents that compromise blood flow. Occasionally the esophagus fails to lengthen sufficiently and the stomach is pulled up into the esophageal hiatus through the diaphragm. The result is a congenital hiatal hernia.
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