Development of the umbilical cord

During development, the fetus is connected to the placenta via the umbilical cord, a long, flexible stalk that has two small arteries and a large vein.

Deoxygenated fetal blood flows through the umbilical arteries to the placenta where it picks up oxygen and glucose while dropping off carbon dioxide.

Oxygenated blood then heads back towards the fetus’s heart through the umbilical vein.

When the baby is born the umbilical cord is no longer needed, so it’s cut off, leaving the navel or belly button.

In week 2 of development, the blastocyst has two parts—an inner part called the embryoblast, and an outer part called the trophoblast, which develops into the cytotrophoblast and syncytiotrophoblast.

The embryoblast has two layers called the epiblast, which contains the amniotic fluid, and the hypoblast, which contains the yolk sac filled with vitelline fluid that can nourish the embryo.

Cells from the epiblast layer start to differentiate into extraembryonic mesoderm cells, so-named because they are outside of the developing embryo.

These are some of the earliest mesoderm cells, and they start to form even while the embryoblast itself is a bilaminar disc.

These mesoderm cells line the inside of the cytotrophoblast and syncytiotrophoblast and form the chorion.

As development progresses, a space called the chorionic cavity develops between the embryoblast and the chorion, and these two structures are connected by a short band of extraembryonic mesoderm called the body stalk.

The body stalk contains the umbilical vessels and is the first of three structures that make up the umbilical cord.

In week 3 of development, the embryo folds in two directions.

In the longitudinal plane, there is a cranial and caudal fold, so that the embryo now looks less like a pancake and more like a little shrimp.

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

The endoderm layer, which derives from the epiblast, forms the gut tube and soon divides into three parts—the foregut, the midgut, and the hindgut.

The midgut is open to the yolk sac via a connection called the vitelline duct, which is the second structure in the umbilical cord.

Around the same time, the hindgut grows a little outpocketing called an allantois, which grows towards the umbilical cord and drains the bladder.

The allantois is the third structure in the umbilical cord.

In week 4 of development, the amniotic cavity folds down and around the embryo.