Endoderm

The endoderm is the innermost germ layer, which makes it easy to remember the body parts it gives rise to: stuff inside our bodies, specifically, spaces and cavitations like the gut tube and body cavities that house our internal organs.

Endoderm also forms things like the lining of the ear canals, the trachea and respiratory tract, and parts of the bladder and urethra.

During week 4 of development, the embryo folds in two directions.

In the longitudinal plane, the embryo folds slightly at the cranial and caudal ends, so it looks less like a pancake and more like a little shrimp.

This is the very beginning of its curling into the fetal position.

This folding shapes part of the yolk sac into a gut tube, with the remainder of the yolk sac remaining connected not at the cranial or caudal end, but just in the middle.

Now let’s switch to the transverse plane, looking at a spot near the middle of our embryo where you can see the yolk sac.

In the transverse plane, the lateral plate mesoderm splits into a dorsal layer called the parietal (or somatic) mesoderm layer, and a ventral layer called the visceral (or splanchnic) mesoderm layer.

The parietal layer of mesoderm follows the ectoderm and forms the chest wall and abdominal body wall of the embryo.

The visceral layer of mesoderm follows the endoderm and forms the gut tube.

More specifically, the endoderm becomes the epithelial cell lining of the gastrointestinal tract while the visceral layer of mesoderm becomes the muscular wall.

The mesoderm also gives rise to serous membranes that become the visceral and parietal pleura.

Two layers of the visceral pleura come together to form the mesentery, a flap of tissue that suspends the gut tube in the abdominal cavity.

Interestingly, the mesentery was only recently officially classified as a body organ!

Meanwhile, the ectoderm and parietal layer of mesoderm fold around the dorsal side of the embryo, with the two sides meeting up and seamlessly coming together in the midline to form the anterior body wall.

Well, we say “seamlessly”, but actually, the yolk sac still pouches out a little.

Once that happens, we end up with the hollow gut tube inside the embryo’s body, so in a sense embryonic folding creates a tube inside a tube.

The gut tube is divided into three regions: foregut, midgut, and hindgut.

When the oropharyngeal membrane breaks down in week 4, the foregut (including the pharynx) connects with the mouth, which forms from the ectoderm layer.