Endodermi

Endoderm is the innermost of the three primary germ layers formed during vertebrate embryogenesis. It arises during gastrulation when ventralizing endodermal cells migrate through the primitive streak and replace the hypoblast, establishing a lining that will develop into the gut tube. The endoderm gives rise to the epithelium of the gastrointestinal and respiratory tracts and to several internal organs.

During development, the endoderm differentiates into the foregut, midgut, and hindgut, which form progressively the lining of the pharynx, esophagus, stomach, intestines, liver, pancreas, and parts of the respiratory tract. The foregut also contributes to the epithelial lining of the trachea and lungs, as well as the liver, pancreas, and biliary system. Endodermal cells from the pharyngeal region contribute to glands such as the thyroid, parathyroids, and thymus. The urinary bladder and parts of the urinary tract receive their epithelial lining from endodermal tissue as well.

Key regulatory factors directing endoderm formation include signaling pathways such as Nodal and subsequent transcription factors like Sox17 and FoxA2, which promote endodermal identity and organ-specific differentiation. Interaction with surrounding mesoderm and ectoderm is essential for proper patterning and organogenesis, including spatial cues that specify foregut, midgut, and hindgut derivatives.

Clinically, defects in endodermal development can underlie congenital abnormalities of the gut, liver, pancreas, thyroid, or thymus. Understanding endoderm formation is fundamental to developmental biology, organogenesis, and regenerative medicine.