Formation of the Primitive Gut Tube
of the Embryo
Toward the end of the third week (19-20 days) the embryo is flat and consists of three germ layers - ectoderm, mesoderm and endoderm . When observed from a dorsal viewpoint, several features are observable in a linear sequence from cranial to caudal. Most cranially is a mass of mesoderm the septum transversum and the adjacent bulge of the developing heart. Just caudal to this is the oral membrane where ectoderm and endoderm are fused together. A similar area of ectoderm-endoderm fusion is present at the caudal end of the embryo, the cloacal plate. Just caudal to this is the body stalk. The most prominent feature from a dorsal viewpoint is the neural plate. Its rapid expansion in a cranial caudal direction drives the cephalocaudal flexion of the embryo. As a result the heart as well as the oral and cloacal membranes and body stalk become ventrally positioned. Cephalocaudal folding of the embryo creates a cranial and a caudal pocket of endoderm which represent the primitive foregut and hindgut. The oral membrane is located at the cranial end of the foregut, while the cloacal membrane is at the caudal end of the hindgut. At this time the endoderm of the midsection of the embryo retains a broad connection with the yolk sac.
of the Embryo
The embryo folds laterally in tandem with cephalocaudal folding. Lateral folding is thought to be driven by the expansion of the paraxial mesoderm and its subdivsion into somites. Expansion of the amnion may also have a role. As the lateral folds approach the ventral midline they enclose the intraembryonic ceolom and obliterate its connection with the extraembryonic ceolom. The broad connection of the endoderm with the yolk sac becomes narrowed forming the vitelline duct. This area of the primitive gut tube is termed the midgut. The vitelline duct along with the body stalk extends into the newly formed umbilical cord.
Formation of the
GI Tract and Associated Organs
As a result of folding, the primitive gut is completely internalized. The primitive gut tube is subdivided into three regions, the foregut, midgut and hindgut. Initially the foregut undergoes the most extensive expansion and will generate a variety of derivatives. The midgut undergoes extensive growth and differentiation during the 6th to 11th week of development. The hindgut is also forming its derivatives at this time.
- Overall summary
of foregut development (particularly the intra-abdominal portion)