Development of the endoderm and its differentiation

Formation of Trilaminar Disc (Gastrulation) - Development of the endoderm

During week 3 of gestation, a process called gastrulation occurs. It is basically when all 3 germ layers of an embryo (endoderm, mesoderm, and ectoderm) form.

So basically, the bilaminar disc (epiblast + hypoblast) turn into the trilaminar embryo containing the 3 known basic germ layers.
By the end of the 15th-16th day, something called a primitive streak forms in the embryo along with an elevation found on the cephalic end of the streak called the primitive node which is surrounding the primitive pit.
Epiblast cells migrate through the streak (invagination) and displace hypoblast cells. These replacing cells from the epiblast form the definitive endoderm.
Some epiblast cells lie between the epiblast and newly formed endoderm; this forms the mesoderm. Cells remaining in the epiblast layer form the ectoderm.

The endoderm forms the mucosal linings (epithelial and glandular tissue) of the respiratory, digestive, and urogenital systems.

The endoderm starts forming the primitive gut tube after gastrulation via cephalocaudal and lateral folding of the embryo where the endoderm then becomes a tube lined with simple cuboidal cells.
Cephalocaudal folding: forms the foregut (cranially) and hindgut (caudally) pockets.
Lateral folding: forms the midgut which is connected to the yolk sac via the vitelline duct.
Derivatives of foregut:
- Pharynx —> proximal duodenum.
- Lower respiratory tract (trachea —> lungs).
- Liver, pancreas, gall bladder, and bile ducts.
- Thymus, parathyroid gland, and thyroid gland (except for C cells).
Derivatives of midgut:
- Distal duodenum —> proximal 2/3 of transverse colon.
Derivatives of the hindgut:
- Distal 1/3 of transverse colon —> upper anal canal (above pectinate line).