Overview:

Formation of the Primitive Gut Tube

Cephalocaudal Folding of the Embryo
Animation: Start with a flat embryo of 3 weeks pointing out oral and cloacal membranes, heart, neural plate, septum transversum, body stalk and highlighting endoderm. Begin a sequence of diagrams showing the 3-4 week embryo folding in a cranial-caudal direction. The embryo body should be transparent to show the endoderm folding into the foregut and hindgut. Alternatively this could be viewed from a midsagittal perspective. Check Moore (p84) & Langman (p99 & 271) for examples.

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.

Lateral Folding of the Embryo
Animation: Start with an external view of a day 19-20 embryo and segway to a cross section at about mid embryo. In a sequence of drawings show lateral folding and closure of the ventral body wall (see Moore p 84; Langman p 100). The combination of the cephalocaudal folding and the lateral folding closes the ventral wall around the body stalk sort of like drawing a purse string around a purse. Show the dorsal mesentery forming which suspends the gut.

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
Annimation: Begin with a drawing of the 4 week embryo having the body transparent so that the primitive gut tube is visible Langman p100 fig 5.18A, . Highlight the primitive gut tube with each of its three main subdivisions, foregut, midgut and hindgut. Show the dorsal mesentery associated with the entire gut tube and ventral mesentery (septum transversum) associated with the foregut. Show the aorta and its three major arteries to the gut which are found within the dorsal mesentery: celiac trunk to foregut (only supplies intra-abdominal part of the foregut) , superior mesenteric to the midgut and inferior mesenteric to the hindgut (Langman p 273; Moore p 272 12-1B, 12-2A.

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.

Foregut Development
Possible sequences to animate:

- Overall summary of foregut development (particularly the intra-abdominal portion)
- Formation of the stomach Shaping due to asymmetric dilation Rotation showing formation of greater omentum, spleen, lesser sac & lesser omentum, vagus nerves
- Formation of the duodenum showing how it rotates, becomes retroperitoneal (parts 2 & 3) and temporary occluded and recannalized
- Formation of the liver showing budding into septum transversum, breaking up of vitelline vessels, formation of gall bladder, common bile duct, falciform ligament
- Formation of the pancreas showing bud formation, rotation, fusion of dorsal and ventral buds, formation of main and accessory ducts, union with common bile duct, association with duodenum