Introduction
ower foregut begins at the level of the lung buds with the esophagus, followed by the stomach and the superior part of the duodenum. It extends as far as the liver bud and the pancreas, the bile passages also arise from it.
Between stage 14 (ca. 33 days) and 20 (ca. 49 days) a massive lengthening of the whole intestines in a cranio-caudal growth gradient takes place: in the beginning it is mainly the esophagus and stomach that lengthen. Thus a shift of these structures relative to the vertebral column occurs. One also speaks of a descent of the stomach.
This lengthening not only leads to a relative relocation with respect to the spine but it also coupled to the so-called "stomach rotation".
Esophagus
At the level of the last pharyngeal pouch of the embryo the foregut is suddenly very narrow and gives out ventrally the lung bud. This is the point where the esophagus originates which, in the beginning, is very short. During the lung development and the descent of the heart it lengthens considerably. The esophagus continues in the spindle-shaped stomach and the duodenum with the ventrally budding liver and the dorsally budding dorsal pancreas anlage.
(see development of the liver and of the pancreas).
In the early stages of development the epithelium of the esophagus is composed of multiple rows. At the end of the embryonic period large vacuoles arise in the almost closed lumen. In the early fetal period these merge and the esophagus becomes recanalized. The epithelium is now a multi-layered, prismatic ciliated epithelium. In the middle of the pregnancy is finally replaced by the definitive, layered squamous epithelium.
The muscle layers of the esophagus arise in the late embryonic period. The lamina muscularis mucosae and the inner layer of the ring musculature arise subsequently out of cells of the local splanchnopleurae.
The striated muscles of the outer layer of the circular musculature and the outer longitudinal muscles stem from the mesoderm of the last pharyngeal arch.
The entire musculature is innervated by the vagus nerve (cranial nerve X of the 4th pharyngeal arch).