80. The Foregut: Esophagus and Stomach

  1. Introduction: the foregut derivatives are the pharynx and its derivatives (see Unit Two), the lower respiratory tract (see Unit Three), the esophagus, the stomach, the duodenum as far as the entrance of the common bile duct, the liver, the pancreas, and the biliary apparatus. All except for the pharynx, respiratory tract, and upper esophagus are supplied by the celiac artery
  2. The foregut extends from the buccopharyngeal membrane to the duodenum, is initially located in the median sagittal plane, and is attached by mesentery to the anterior and posterior abdominal walls. It consists of a cranial segment, the pharyngeal gut or pharynx,* which extends from the buccopharyngeal membrane to the tracheobronchial diverticulum; and a caudal segment, extending from the diverticulum as far caudally as the liver bud outgrowth from the duodenum
      1. The esophagus is partitioned from the trachea by the tracheoesophageal septum
        1. The esophagus is initially very short, but elongates rapidly, reaching its final relative length by about week 7
          1. Elongation is a result of cranial body growth (ascent of the pharynx), development of the heart, and retroflexion of the head
        2. The entoderm of the esophagus initially proliferates and almost obliterates the lumen, but recanalizes near the end of the embryonic period
        3. The striated muscle in the upper two-thirds of the esophagus is derived from the mesenchyme of the caudal branchial arches (innervated by cranial nerve X); the smooth muscle of the lower third of the esophagus develops from the surrounding splanchnic mesenchyme (innervated by the visceral nerve splanchnic plexus derived from neural crest cells)
      2. The stomach first appears as a fusiform dilatation of the caudal portion of the foregut in week The primordium soon enlarges and broadens ventrodorsally. Its position and appearance change as a result of the different rates of growth in various regions of its walls, as well as changes in position of the surrounding organs
        1. The positional changes are explained most easily by assuming that the stomach rotates around a longitudinal and an anteroposterior axis
          1. Around the longitudinal axis, the stomach carries out a 90? clockwise rotation, causing its left side to face anteriorly and its right side posteriorly. (This explains why, in the adult, the left vagus nerve supplies its anterior or ventral wall and the right vagus nerve its posterior or dorsal wall)
          2. Anteroposterior axis rotation displaces the pyloric part of the stomach to the right and upward and the cephalic or cardiac portion to the left and downward slightly, resulting in the future duodenum coming to be retroperitoneal
        2. The dorsal border of the stomach grows faster than the ventral one and produces the greater and lesser curvatures of the stomach
        3. Since at this stage of development, the stomach is attached to the posterior body wall by the dorsal mesogastrium, longitudinal rotation pulls the dorsal mesogastrium to the left and helps form the omental bursa or lesser sac (a peritoneal pouch found behind the stomach)
          1. As the embryo lengthens, the caudal part of the septum transversum thins and becomes the ventral mesentery or mesogastrium. It attaches the stomach and duodenum to the ventral wall of the abdominal cavity
        4. The stomach thus assumes its final position, and its long axis now runs from above left to below right. The greater curvature faces downward, and the lesser curvature faces upward and to the right
    2. *The cranial segment or pharynx is discussed under the development of the branchial arches, clefts, and pouches, and lower respiratory system
the foregut: esophagus and stomach: image #1