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Topic 15: Formation of the fetal membranes and the placenta. Fetal circulation
Terms in this set (20)
Extra-embryonic membranes which protect the fetus and is a part of the placenta
The fetal 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.
What are the fetal membranes?
- The innermost fetal membrane, which forms the amniotic sac around the fetus
-it is in contact with the amniotic fluid, the fetus, and the umbilical cord. It's an avascular structure.
- It's derived from epiblast
What is the amnion, and what is it derived from?
- The outermost fetal membrane, which is in contact with the uterine wall
- It's derived from extraembryonic mesoderm and trophoblast
What is the chorion, and what is it derived from?
It obliterates after the chorion and amnion fuse
What is the fate of the chorionic cavity?
The basal plate, the intervillous space, and the chorionic plate
What are the parts of the placenta?
Trophoblast (cytotrophoblast, syncytiotrophoblast), on the one side, and of maternal tissue (decidua basalis) on the other side
Which layers make up the basal plate?
- Placental septa which project out of the basal plate
- 15 - 20 cotyledons
What forms the cotyledons, and how many are there?
Chorion, which is made up of an outer layer of trophoblast and an inner layer of vascularized extraembryonic mesoderm.
The chorionic plate forms the fetal side of the placenta
Which layers make up the chorionic plate? What is the chorionic plate?
- These are the villi, which facilitate gas and nutrient exchange between fetal and maternal blood
- There are also anchoring villi, which anchor the chorionic plate to the basal plate
What are the projections out of the chorionic plate, and what are their functions?
- 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
Describe the placental circulation
Wharton jelly, urachus, two umbilical arteries, one umbilical vein
What does the umbilical cord contain?
- The ductus arteriosus is a normal blood vessel that connects two major arteries — the aorta and the pulmonary artery
The lungs are not used while a fetus is in the womb because the baby gets oxygen directly from the mother's placenta. The ductus arteriosus carries blood away from the lungs and sends it directly to the body.
- The adult remnant is the ligamentum arteriosum
What is the function of the ductus arteriosus, 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 ductus venosus, 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
What is the function of the foramen ovale, and what is its adult remnant?
The exchange of nutrients and gases between the fetus and the mother occurs in the placenta
- Blood will flow through the umbilical vein and enter the fetus
- The blood will bypass the liver, by flowing through the ductus arteriosus and into inferior vena cava
- From the inferior vena cava the blood will enter the right atrium, but there directed towards the foramen ovale and flow directly into the left atrium
- From the left atrium it will go into the left ventricle, and then the ascending aorta, where it will mix with venous blood
- From the superior vena cava, the blood will enter the right atrium, and then through the right ventricle enter the pulmonary trunk
- Since the resistance is so high in the pulmonary vessels, the blood will bypass the pulmonary circulation, and flow through the ductus arteriosus and into the ascending aorta
- Only around 35% of the mixed blood will supply the embryo, the rest will return to the placenta through the umbilical arteries
Explain the fetal circulation
Caused by the stop of placental blood flow and the beginning of respiration
- Closure of the umbilical arteries
- Closure of the umbilical vein and the ductus venosus
- Closure of the ductus arteriosus
- Closure of the foramen ovale
Circulatory changes at birth?
The distal part will form the medial umbilical ligament and the proximal part will fromt he superior vesical arteries
Closure of the umbilical arteries?
The umbilical vein will become the ligamentum teres hepatis, and the ductus venosus will become the ligamentum venosus
Closure of the umbilical vein and ductus venosus?
Will become the ligamentum arteriosum
Closure of the ductus arteriosus?
An increased pressure in the left atrium, and decreased pressure in right atrium causes the septum secundum to cover the foramen ovale, eventually fusing with the septum primum
Closure of the foramen ovale
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