15. Formation of the fetal membranes and the placenta. Fetal circulation

Learning objectives

What are the foetal membranes?
What is the amnion, and what is it derived from?
What is the chorion, and what is it derived from?
What is the fate of the chorionic cavity?
What are the parts of the placenta?
Which layers make up the basal plate?
What forms the cotyledons, and how many are there?
Which layers make up the chorionic plate?
What are the projections out of the chorionic plate, and what are their functions?
Describe the placental circulation
What does the umbilical cord contain?
What is the function of the ductus arteriosus, and what is its adult remnant?
What is the function of the ductus venosus, and what is its adult remnant?
What is the function of the foramen ovale, and what is its adult remnant?

Formation of the foetal membranes

The foetal membranes, the amnion and the chorion, are extra-embryonic tissues which aren’t a part of the mature newborn itself but play important roles during pregnancy. They rupture during birth.

Amnion

The amnion is the innermost foetal membrane, meaning that it is in contact with the amniotic fluid, the foetus, and the umbilical cord. It’s an avascular structure. The amnion surrounds the amniotic cavity. Together they form the amniotic sac, which contains amniotic fluid, which the foetus is swimming in.

The amnion is derived from the epiblast.

Chorion

The chorion is the outermost foetal membrane, meaning it is in contact with the uterine wall. The amnion and chorion are initially separated by the chorionic cavity, but as more and more amniotic fluid is produced, the amniotic sac expands. This causes the chorionic cavity to become smaller and smaller and eventually obliterate, at which point the amnion and chorion will adhere to each other and fuse. At this point they’ve formed the amniochorionic membrane.

The chorion is formed by extraembryonic mesoderm and the two layers of the trophoblast, cytotrophoblast and syncytiotrophoblast.

Formation of the placenta

The placenta is an organ which facilitates nutrient and gas exchange between the mother and the foetus. It has two parts: the chorionic plate, which originates only from the embryo, and the basal plate, which originates mostly from the uterus.

The space between these two plates is called the intervillous space and is filled with lakes of maternal blood. This space contains villi, which originate from the chorionic plate.

Basal plate

The basal plate is made up of decidua and trophoblast. The decidua is the name of the uterine endometrium after it has undergone the decidua reaction, which occurs after implantation.

There are three parts of decidua, the decidua basalis, capsularis, and parietalis. The latter two are on the opposite side of the foetus (that is, the back of the foetus faces them), and as such aren’t part of the basal plate and therefore aren’t very important. The capsularis and parietalis later fuse together, causing the uterine cavity to be obliterated. The decidua basalis is the part of the decidua which lies on the embryonic pole (that is, the front of the foetus faces it), making it the only part of the decidua which is part of the placenta.

Septa form in the basal plate, which grow toward the chorionic plate but never fuse with it. These septa divide the intervillous space into 15 – 20 so-called cotyledons.

Chorionic plate

The chorionic plate is made up of the chorion, with which the amnion will later fuse. The chorionic plate contains embryonic vessels.

The part of the chorionic plate which is in contact with the basal plate develops villi, making it “bushy” in appearance. This part is of the chorionic plate is called chorion frondosum, from the Latin word for “bushy”. The part of the chorionic plate which is not in contact with the basal plate initially develops villi, but they regress. This part of the plate is therefore smooth, giving it the name chorion laeve.

All villi at this point are definitive or tertiary, meaning that they consist of an outer syncytiotrophoblast layer and capillaries inside. The villi lose the cytotrophoblast layer and stroma after month 4, meaning that after month 4, the placental barrier consists of only a layer of syncytiotrophoblast and capillary endothelium.

Some villi are anchoring villi, meaning that they grow into and attach to the basal plate, anchoring the chorionic plate to it.

The foetal circulation

Placental circulation

Maternal and foetal blood never come in contact with each other, although they do come close. Maternal blood enters the intervillous space through the spiral arteries. Deoxygenated and nutrient-poor blood from the foetus is transported to the villi.

There, oxygen and nutrients diffuse from the maternal blood through the walls of the villi and into the foetal blood, while carbon dioxide diffuses the other way. Foetal blood, now oxygenated and nutrient-rich, travels back to the foetus through the umbilical vein. Deoxygenated and nutrient-poor maternal blood is drained by endometrial veins.

The umbilical vessels

The umbilical cord consists of gelatinous connective tissue known as Wharton jelly, the urachus, two umbilical arteries, and one umbilical vein.

In contrast to the normal adult physiology the umbilical arteries transport deoxygenated and nutrient-poor blood. These arteries transport deoxygenated blood from the foetus to the placenta. The umbilical arteries are connected to the dorsal aorta.

The umbilical vein transports oxygenated and nutrient-rich from the placenta to the foetus. This vein connects both to the portal vein, which will enter the liver, and to the ductus venosus. After birth, the umbilical vein will form the round ligament of the liver.

The foetal shunts

There are several shunts in the foetal circulation – these shunts allow blood to bypass parts of the foetus which the blood doesn’t need to pass through in the embryonic life. After birth, these shunts are closed, restoring the normal adult pattern of circulation. There are three important foetal shunts, the ductus arteriosus, the ductus venosus, and the foramen ovale.

Ductus arteriosus

The ductus arteriosus is an embryonic artery which connects the pulmonary artery to the distal part of the aortic arch. The ductus causes blood to bypass the pulmonary circulation, as the lungs aren’t ventilated during embryonic life anyway.

Shortly after birth muscles in the wall of the ductus arteriosus contract, causing it to functionally close. After 1 – 3 months the ductus arteriosus has been replaced by fibrous tissue, forming the ligamentum arteriosum. This ligament does not transport any blood and is simply a remnant of the ductus arteriosus in the adult.

Ductus venosus

The ductus venosus is a shunt which connects part of the umbilical vein directly to the inferior vena cava. During foetal life, not all of the oxygenated and nutrient-rich blood needs to pass through the foetal liver. As such, the ductus venosus exists to shunt approx. 1/3 of the blood coming from the umbilical vein directly to the inferior vena cava, bypassing the liver.

After birth, the ductus venosus closes and becomes the ligamentum venosum.

Foramen ovale

Foramen ovale is an opening in the septum between the atria of the foetal heart. This foramen shunts blood from the right heart to the left, bypassing the pulmonary circulation.

In most people the foramen ovale closes after birth, forming the oval fossa, but a surprisingly high 25% of the adult population may have a so-called patent foramen ovale.

Summary

What are the foetal membranes?
- Extra-embryonic membranes which protect the foetus and partake in the placenta
What is the amnion, and what is it derived from?
- The innermost foetal membrane, which forms the amniotic sac around the foetus
- It’s derived from epiblast
What is the chorion, and what is it derived from?
- The outermost foetal membrane, which is in contact with the uterine wall
- It’s derived from extraembryonic mesoderm and trophoblast
What is the fate of the chorionic cavity?
- It obliterates after the chorion and amnion fuse
What are the parts of the placenta?
- The basal plate, the intervillous space, and the chorionic plate
Which layers make up the basal plate?
- Decidua basalis and trophoblast
What forms the cotyledons, and how many are there?
- Placental septa which project out of the basal plate
- 15 – 20 cotyledons
Which layers make up the chorionic plate?
- Chorion, which is made up of extraembryonic mesoderm and trophoblast
What are the projections out of the chorionic plate, and what are their functions?
- These are the villi, which facilitate gas and nutrient exchange between foetal and maternal blood
- There are also anchoring villi, which anchor the chorionic plate to the basal plate
Describe the placental circulation
- Deoxygenated and nutrient-poor foetal blood enters the villous capillaries by the umbilical arteries
- Maternal blood from spiral arteries enters the intervillous space
- Gas and nutrients are exchanged across the villous wall
- Oxygenated and nutrient-rich foetal blood returns to the foetus by the umbilical vein
- Maternal blood in the intervillous space is drained by endometrial veins
What does the umbilical cord contain?
- Wharton jelly, urachus, two umbilical arteries, one umbilical vein
What is the function of the ductus arteriosus, and what is its adult remnant?
- The ductus arteriosus shunts blood from the pulmonary artery to the aortic arch, bypassing the pulmonary circulation
- The adult remnant is the ligamentum arteriosum
What is the function of the ductus venosus, and what is its adult remnant?
- The ductus venosus shunts blood from the umbilical vein to the IVC, bypassing the liver
- The adult remnant is the ligamentum venosum
What is the function of the foramen ovale, and what is its adult remnant?
- The foramen ovale shunts blood from the right heart to the left, bypassing the pulmonary circulation
- The adult remnant is the oval fossa