Fate and Specification of the Ventral Endoderm in and Outside the Foregut of the Avian Embryo at 9-12 Somite Stage.

The fate and specification of the endoderm lining the ventral foregut and outside the foregut cranial to the anterior intestinal portal of the 9-12 somite stage avian embryo were analyzed to determine whether this endoderm was specified according to its fate. The ventral foregut endoderm contributed to the ventral endoderm of the gut from the pharynx to the duodenum, and to the thyroid, respiratory organ, and liver. Pre-pharyngeal cells were distributed throughout the ventral foregut, with pre-thyroid cells lying medially in the anterior to middle part and pre-pharyngeal pouch cells lying laterally. Progenitors of posterior organs from the esophagus to the liver were found in the posteriormost quarter, with those of more posterior organs located in more posterior and medial locations. In the endoderm outside the foregut, progenitors of the liver, ventral jejunum, and extraembryonic endoderm were found, with those of more posterior organs taking more anterior locations. When cultured with somatic mesoderm, most parts of the ventral foregut endoderm showed pharynx/esophagus-like differentiation, whereas the middle part developed thyroid-like follicles. The posteriormost part exhibited stomach-like differentiation, as well as intestinal and pancreatic differentiation, which also appeared from the endoderm outside the foregut. It showed no or a rare appearance of a hepatic cord-like structure or albumin, respectively, but often developed bile duct-like Hex-expressing epithelia. However, the expression of Nkx2.1 but not of Hex, which is characteristic of the pulmonary epithelium, was not observed. These endoderms are likely to be specified in almost complete accordance with their fate, except for the respiratory organ.