Development of the gastrointestinal system

During week 3, the embryo is a flat disc made up of three germ layers: the endoderm, the mesoderm, and the ectoderm.

From the endoderm, which you can think of as being the belly side of this three-layered embryo pancake, a fluid filled bubble called the yolk sac forms and grows alongside the developing embryo.

The digestive system starts forming when the embryo folds along its vertical and horizontal axes—so it rolls up on itself, and turns into a tubular structure that looks a bit like a little shrimp.

Folding also pinches the yolk sac, sort of like squeezing a balloon through a ring, so a part of it goes inside the embryo and forms the primitive gut tube.

The primitive gut tube is initially sealed off at both ends—the buccopharyngeal membrane at the top separates the tube from the primitive mouth, and the cloacal membrane at the bottom separates it from the primitive anus.

This tube is divided into three parts, based on the arterial blood supply.

The first portion is the foregut, and it’s nourished by the celiac artery.

The middle portion, the midgut, is nourished by the superior mesenteric artery.

For a short while, the midgut communicates with the yolk sac through the vitelline duct, which is eventually incorporated into the umbilical cord.

Finally, there’s the last portion, the hindgut, which is nourished by the inferior mesenteric artery.

The hindgut ends with the cloaca, which is the primitive common drainage site for the urinary, genital, and digestive systems.

The foregut gives rise to the superior part of the digestive tube—up to and including the first half of the duodenum, as well as the liver, the gallbladder, and the pancreas.

At the very top of the foregut, starting at the buccopharyngeal membrane, there’s the primitive pharynx, which is initially just five sets of symmetrical pharyngeal arches.

These pharyngeal arches turn into various bones, muscles, and cartilages of the head and neck, and the last two arches give rise to the final pharynx.

Below the pharynx, the foregut gives rise to the esophagus.

In this region, there’s an outpouching of endoderm called the lung bud, and it sprouts from the anterior wall of the foregut.

During week 4, the tracheoesophageal septum forms a barrier that separates the lung bud from the foregut; the anterior compartment develops into the trachea and lungs, and the posterior compartment develops into the esophagus.

The esophageal epithelium and glands derive from the foregut endoderm, whereas the esophageal muscles derive from the surrounding mesoderm.

The epithelium proliferates and initially fills up the lumen of the esophagus, turning it into a solid rod of tissue, but by week 8, a process called recanalization occurs, and the esophagus turns it into the hollow tube we know and love.

Under the esophagus, there’s the primitive stomach which starts out as a small dilation of the foregut.

The stomach has a dorsal, or posterior border, and a ventral, or anterior border.

The dorsal border is anchored to the posterior body wall by a two-layered sheet of mesoderm tissue called the dorsal mesogastrium, and the ventral border is anchored to the anterior body wall by another two-layered sheet, the ventral mesogastrium.

Starting in week 5, the liver grows between the layers of the ventral mesogastrium, and the spleen between the layers of the dorsal mesogastrium.

The dorsal border of the stomach grows a lot faster than the ventral boder and forms the greater curvature of the stomach, while the ventral border becomes the lesser curvature.

Alright, so if we switch to a top-down view of this developing stomach, we see that as it grows, it undergoes a 90-degree, clockwise rotation along its length, pulling the dorsal mesogastrium and ventral mesogastrium with it.

This moves the greater curvature to the left side of the body, and the lesser curvature to the right side, and the stomach now has an anterior and a posterior face.

As the stomach rotates, the ventral mesogastrium becomes the lesser omentum.

The dorsal mesogastrium grows longer and bends as the stomach rotates, forming a peritoneal cavity derivative called the omental bursa between the stomach and the posterior body wall.

The omental bursa communicates with the great peritoneal cavity through a small opening known as the omental foramen.

The omental bursa grows and fills with peritoneal fluid, developing two projections: the upper recess, which extends behind the developing liver, and the lower recess, which extends downwards over the developing intestines.

Eventually, the sheets of dorsal mesogastrium that form the lower recess fuse and form the greater omentum.

Finally, the stomach rotates once more, but this time in a frontal plane—so imagine looking at it from the front now.