Endoderm - GI Development

Tube-within-a-tube

Ingressing epiblast cells form a definitive endoderm. The barrier between hypoblasts and the cells of the true gut endoderm are a single cell layer which can be shown via fluorescent tagging of epiblast cells.

The tube-within-a-tube forms because the embryo and amniotic sac grow faster than the secondary yolk sac. The folding occurs cranially to caudally AND cross-sectionally at the same time.

Organs from the GI tract

GI Organs

Accessory organs

Blood supply to each section

Foregut: Celiac artery
Midgut: Superior mesenteric artery
Hindgut: Inferior mesenteric artery

Formation of the stomach

Region of the foregut expands posteriorly around week 4. At week 6, the stomach then undergoes a 90deg clockwise rotation. This rotation brings its ventral and dorsal mesenteries with it. The portion of mesentery attached to the greater curve of the stomach is called the dorsal mesogastrium and forms the greater omentum. The portion from the lesser curve of the stomach is called the ventral mesogastrium and forms the lesser omentum.

Formation of the liver

This buds into the ventral mesentery week 4-5, forming hepatic chords. The growth causes the ventral mesentery to be divided into two structures: the lesser omentum and the falciform ligament.

Out of the hepatic duct come the pancreatic bud and the gallbladder.

Vocab:

• Falciform Ligament: the part of the ventral mesentery connecting the liver to the anterior body wall
• Lesser Omentum: the part of the ventral mesentery connecting the lesser curvature of the stomach to the liver
• Septum Transversum: the membrane separating the thoracic and abdominal cavities which eventually becomes the diaphragm

Formation of the pancreas

Week 5-6. The pancreas comes from the pancreatic bud formed with the creating of the hepatic duct (liver). It arises from the fusion of the dorsal bud and the ventral bud. Fusion occurs at the same time as the clockwise rotation of the other organs.

The dorsal bud then disintegrates and the ventral bud becomes the main pancreatic duct.

With the pancreas, solid epithelial clusters form.

Pancreatic cell types

Acinar cells: Secrete digestive enzymes (trypsinogen, chymotrypsinogen, lipase, amylase, (deoxy)ribonuclease, gelatinase, elastase)
Ductal cells: Line the pancreatic duct and enable enzymes to be secreted into the duodenum.
Islet cells: Endocrine cells

• Alpha: Glucagon
• Beta: Insulin
• Gamma
• Delta
• PP cells (lol)

Clinical pearl: Gestational diabetes

Fetus is exposed to high glucose concentrations from the mother. This results in the overproduction of insulin by fetal pancreatic cells and results in congenital hyperinsulinism.

Result: Hyperplasia and damage to islet cells. Can cause infant diabetes and other pancreatic issues

Formation of intestines

Week 7: The primary intestinal loop performs a 90deg rotation and herniates into the umbilicus.
Week 11: The intestines have completed 270deg of rotation. The cecum now resides inferior to the liver.
Week 12: The cecum descends to its final position, forming the ascending colon.

If the vitelline duct does not retract you get Meckel's Diverticulum.

Meckel's Diverticulum rule of 2s:

• 2% of population
• 2% have symptoms
• 2x more common in males
• 2 feet proximal to terminal ileum
• 2 inches long
• 2 types of abnormal lining

Mesenteries

Derived from the mesoderm which hold the organs of the endoderm in place within the body. The gut tube is incased in a mesentery (has a ventral and dorsal end) when the splanchnic mesoderm meet. The mesoderm will also form the peritoneal cavity which contains the various GI organs.

Dorsal mesentery → Greater omentum
Ventral mesentery → Lesser omentum

Types:

• Somatic
• Splanchnic
• Intermediate