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.