Development of the umbilical cord

Definitions & Key takeaways

The umbilical cord is the conduit between the developing fetus and the placenta. It contains two arteries and one vein, allowing for nutrient and oxygen exchange and waste removal. The development of the umbilical cord starts at around week three and is fully formed by week 7 of gestation. At this time, it consists of the stalk, vitelline duct, and umbilical vessels.

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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 that’s 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 which is filled with vitelline fluid that can nourish the embryo.
Cells from the epiblast layer start to differentiate into extraembryonic mesoderm cells, 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, and a space begins to form what will eventually become the chorionic cavity.
The embryoblast is connected to the chorion by a short band of extraembryonic mesoderm called the body stalk - this 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 it looks less like a pancake and more like a little shrimp.
The 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.
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.
The body stalk, the vitelline duct, and the allantois get pushed together and form the umbilical cord, and they emerge out of the umbilical ring, which is a fibrous ring of tissue that develops on the abdominal wall at the spot where they emerge from the fetus.
Between weeks 4-8, the cells lining the amniotic cavity which originally derive from the epiblast cells, start to generate a lot of amniotic fluid.
This causes the amnion to swell and take over most of the space in the chorionic cavity. As the amnion expands like a big bubble, it folds in and covers the body stalk and vitelline duct, forming an outer membrane for the umbilical cord.
Now briefly, around week 6 of development, the fetal intestines grow so quickly that a part of the intestine squeezes through the umbilical ring into the umbilical cord.
This is called physiological umbilical herniation, meaning it’s a normal occurrence, and by the end of the third month of development the intestines withdraw completely back into abdominal cavity.
So looking at the umbilical cord in cross-section - there’s the outer layer of amnion, and then there’s the vitelline duct which has its own arteries and veins as well as the yolk sac.
There’s also the allantois, and finally the two umbilical arteries and a single umbilical vein. After the formation of the umbilical cord, the vitelline duct and yolk sac starts to shrink and and eventually they disappear.
In rare cases, the vitelline duct won’t regress all the way, and will leave behind a tiny remnant midgut - an outpouching of the intestines that’s called a Meckel’s diverticulum.
The allantois continues developing into the bladder and leaves behind just a remnant called the urachus in the fetus, and then renamed as the median umbilical ligament once the baby is born.
So all that remains within the umbilical cord is the umbilical vein, the two umbilical arteries, and a gelatinous substance called Wharton's jelly which protects the umbilical vessels.
After birth the umbilical vein becomes the liver’s round ligament, and the umbilical arteries become the medial umbilical ligaments.
To keep it straight - think of the fact that there are two umbilical arteries, so “all” the umbilical arteries become medi“all” ligaments, whereas there’s only “an” allantois which turns into the medi“an” ligament.
All right, as a quick recap… The umbilical cord is made up of three key structures - the body stalk, the vitelline duct, and the allantois.
There is also a normal process called physiological umbilical herniation where the intestines briefly go into the umbilical cord as well.
The vitelline duct regresses completely but in some cases will leave behind a Meckel’s diverticulum. The end of the allantois nearest the umbilical cord turns into the median umbilical ligament.
And umbilical vein becomes the round ligament of the liver, while the two umbilical arteries become the medial ligaments.