Embryology of the Placenta (Easy To Understand)

The development of the placenta and the foetal blood circulation explained in 15 minutes.

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Intro 0:00
Sponsor 0:17-1:31
Development of Placenta 1:31-10:44
Circulation of the Fetus 10:44-14:36
Function of the Placenta 14:36-14:46
Thank you 14:46-15:28

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SUMMARY OF THE VIDEO FOR YOUR NOTES
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Placenta development begins during implantation of the blastula. This blastula contains an inner and outer cell mass. The inner cell mass further is differentiated into the epiblast, which the 3 germ layers are derived from, and the hypoblast. The outer cell mass contains the trophoblast cells, and it is the trophoblast that forms the placenta

Once implantation begins by about day 6, the trophoblast cells interact invade the epithelium of the uterus. This invasion is deep, and by day 8 the trophoblast will differentiate into the syncytiotrophoblast and the cytotrophoblast.

Syncytiotrophoblast produces human chorionic gonadotropin hCG by week 2 and this is what is tested on pregnancy tests

By day 9 lacunae form within the syncytiotrophoblast. As the syncytiotrophoblast continues to invade and erode materal tissues, blood is now allowed to enter the lacunae from the spiral arteries, and thus the earliest fetal circulation is established by the end of week 2.

As this occurs the cytotrophoblast forms the primary chorionic villi which penetrate the syncytiotrophoblast again. Soon, extraembryonic mesoderm will grow into the primary chorionic vili to form secondary coronic vili, and they become tertiary villi when embryonic blood vessels form into the embryonic mesoderm.

The tertiary villi grow towards the decidua basalis which is the maternal component of the placenta and anchor into it. These anchoring villi further grow into the intervillous space to increase the surface area between maternal and foetal placenta to increase the amount of nutrient exchange. These villi are called branching vili, and this is exactly how you should imagine it. The branching villi spread around to increase the surface area for nutrient and metabolite absorption from the maternal circulation.

Cytotrophoblast cells invade maternal spiral arteries replacing the maternal endothelium, this is important as these cells are now larger in diameter thus reducing the resistance of blood flow to the foetus. When the cytotrophpoblast do not completely differentiate in this way, pre-eclampsia can occur which can have disastrous consequences for mother and baby.

The ductus arteriosus is important when treating some life threatening congenital heart defects. One example is when a baby is born with transposition of the great arteries. In this condition, the ascending aorta and the pulmonary trunk are switched, leading to an inability for oxygenated blood to leave the pulmonary circulation and enter the systemic circulation. Prostaglandin E2 is often administered to maintain the patency of the ductus arteriosus until a surgical correction can be made. This allows for blood to mix between the two distinct circulatory circuits.

There are three shunts in the blood circulation of a foetus. These are the ductus venosus, ductus arteriosus, and the foramen ovale.