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The Placenta: The Often Forgotten, Underrated Organ

The placenta is a specialised organ which is often unappreciated despite its important role as a lifeline during foetal development. In this article, I hope to give this disposable organ some more appreciation and attention concerning its huge impact on maternal and foetal health.

By Asmita Anand

Published 4:20 PM EST, Sat June 26, 2021

Introduction

Many take the role of the placenta for granted. Often discarded as ‘an afterthought of afterbirth’, it’s the only temporary organ with its role of nourishing and protecting the foetus restricted to that of human gestation. Despite its short lifespan, it plays a vital and significant part in the healthy development and growth of the foetus and to the maintenance of mankind.

In recent years, a lot of the attention surrounding placentas have tended to consist of the consumption of freeze dried placenta pills after celebrities have given birth. While some may argue it has nutritional value, a review on human placentophagy [1] discovered no evidence to support the claimed benefits and instead, a higher risk of ingestion of harmful pathogens.

In some cultures, placentas also play an important role in ancient traditions such as burial or placenta prints (art). Although we may have been unaware of the placenta’s vital functions back then, it was always considered more than just medical waste.

The Role Of The Placenta

The placenta plays a critical role during pregnancy, yet many are unaware of the importance of this organ which magically appears. Its role ranges from sustaining the foetus through substance exchange, foetal protection from pathogens and infections to metabolic transfer and hormone secretion.

The main functional units of the placenta are the finger-like chorionic villi, which increase the surface area for substance exchange. Nutrients (such as glucose, fatty acids and amino acids) from the mother are transferred via active transport across the placenta to the foetus through the umbilical cord. Oxygen passively diffuses across the placenta to the foetus, who’s foetal lungs aren’t taking part in gas exchange while in utero. The placenta will also remove waste products including carbon dioxide, water and urea between the maternal and foetal plasma.

The placenta also provides immune protection against infectious diseases to the foetus through the transfer of immunoglobulins from the mother. Lastly, the placenta is hugely responsible for the secretion of various hormones important for foetal growth and development, which prepare the foetus for life outside of the uterus. For example, it acquires the production of both oestrogen and progesterone from the corpus luteum and the first hormone it produces is hCG. [2]

Overall the placenta performs the functions of the lung, liver, gut, kidney and the endocrine system. To sum up, its multifunctionality is awesome and not appreciated enough.

The Formation Of The Placenta

The development of the placenta in pregnancy generally begins once the blastocyst has implanted in the uterine wall. Some cells from the blastocyst, known as the outer trophoblast cells, form the placenta while the others (inner cell mass) form the foetus. Hence the blastocysts consist of both these two distinct differentiated embryonic cell types (the outer trophoblast cells and the inner cell mass) . [3] The placenta is unique as it partly develops from both maternal tissue but also foetal tissue. This also means that the placenta is foreign to the mother’s system as it comprises a mix of both maternal and paternal DNA.

At the end of pregnancy the placenta will undergo several changes to decrease exchange between maternal and foetal circulatory systems. These include the increase of fibrous tissue in the core of the villus and the increase in thickness of foetal capillary basement membranes. [3]

Extraordinarily, the placenta is not rejected by the mother’s body as the trophoblasts do not trigger a maternal immune response. If maternal and foetal blood were to mix, then the mother’s immune system would inevitably kill the foetus. This is because the foetus may not have the same blood type as the mother and an immune system attack in the foetal blood supply can be caused by direct mixing of blood. The syncytiotrophoblast, the outermost layer of the placenta, facilitates the separation of the two bloodstreams.

How And Why Has It Evolved?

All living mammals except marsupials and monotremes are considered to be placental. [4] However mammals didn’t always have one and the placenta was only estimated to have evolved around 150 million to 200 million years ago.

Thus placentas have given mammals a slight advantage over other animal kingdoms by allowing them to supply their offspring with essential nutrients and oxygen vital for the development of the mammalian brain.

If it weren’t for some special viruses, humans may still be laying eggs. Evolutionary scientists have mapped the genomes of cells in the placenta and found that ‘syncytin’, a protein which allowed these cells to fuse into a wall, appeared to come from an ancient retrovirus. Unlike normal viruses, retroviruses are able to enter cells and insert copies of its RNA genome into the DNA of the host. This leads to a change in the genome of that cell. [5] The RNA is converted into DNA during infection allowing it to integrate into the chromosome of the cell. In the case of the placenta, the virus eventually created a viral protein which gave the ability for cells to fuse into a wall that we know now as the placenta. Over many generations this viral DNA must have been passed on until the entire population had evolved, turning it into a fully endogenous retrovirus. This has made retroviral infection a driver of placental mammal evolution. Ed Chuong describes this process ‘as a molecular domestication of an ancient retrovirus element’. Apart from the placenta, remnants of ancient viral infections found in our DNA are also adding more evidence that viral genes are assisting the evolution of new species. [6]

Challenges To Our Understanding Of The Placenta

There are multiple reasons as to why scientists do not know much about the human placenta. A problem that arises when trying to study the placenta is that it is not only logistically but also ethically difficult to study due to its strong link with foetal development. It is difficult to apply research since placentas differ between animals and humans as evidenced from animal studies as well. Since many of us lack this important organ (not being pregnant or a woman), it  accounts for a reason as to why it has been so understudied.

What Is The Future Of Placental Research?

Despite being understudied in the past, scientists are now striving to achieve a better understanding of the behaviour of the placenta and more information on the prevention, detection and treatment of pregnancy problems.

The gatekeeping role of the placenta facilitates its exchange of nutrients and waste between maternal and foetal circulatory systems. However we are yet to uncover the intricacies as to how the placenta is able to do this without triggering an immune response. As Y.W Loke wrote in “Life’s Vital Link”, “This ‘immunological paradox of pregnancy’ has preoccupied immunologists for well over half a century and still the solution remains tantalizingly beyond reach”. “This normally peaceful co-existence between placenta and mother throughout pregnancy is one of the most extraordinary phenomena in reproductive biology.” [7] There is hope that if we are able to understand how the placenta is able to resist rejection, it could lead to a better understanding of preventing organ rejection in transplant patients.

The placenta could also unlock better explanations of how cancer is able to evade the immune system, leading to the possibilities for new cancer treatments. This is due to the similarities between pregnancy and cancer, such as the rapid proliferation, invasion of the host and evasion of the immune system. [8] A study conducted at the Wellcome Sanger Institute [9] found that placental tissue had about five times as many mutations to a single DNA ‘letter’ to body cells which generally have a high rate of mutation. This opens up potential questions as to why the placental tissue is so mutated. Tim Coorens, who was part of the team who conducted the study, suggests its “disposability might provide a clue: as it only “lives” for nine months, it doesn’t need to invest precious resources into repairing itself”.

Furthermore, the placenta is important to our understanding of the health of babies. For example, the size of the placenta in relation to the baby’s birth weight can indicate foetal death risk. [10] There is also hope that by studying the placenta in real time physicians may be able to diagnose complications that arise from pregnancy, such as pre-eclampsia, which is when blood flow to the placenta is decreased.

Conclusion

It is no question that the placenta is critical to human life. Arguably it is one of the most important organs in the human body, impacting both the lifelong health of the mother and child. Fortunately, the Human Placenta Project is currently researching more into the placenta’s development and function through monitoring in real time, to uncover more information about both the least understood and studied organ. [11] As we continue to research more into this wonderfully unique organ, hopefully the “afterbirth” will no longer be an “afterthought.”

Asmita Anand, Youth Medical Journal 2021

References

[1] Farr, Alex et al. “Human placentophagy: a review.” American journal of obstetrics and gynecology vol. 218,4 (2018): 401.e1-401.e11. doi:10.1016/j.ajog.2017.08.016

[2] “Placental Structure, Function and Drug Transfer.” ScienceDirect, 1 Apr. 2015, linkinghub.elsevier.com/retrieve/pii/S1743181617300070.

[3] “Placenta and Placental Development – Clinical Relevance.” TeachMePhysiology, 6 Mar. 2021, teachmephysiology.com/reproductive-system/fetal-physiology/placental-development.

[4] Britannica, The Editors of Encyclopaedia. “Placental mammal”. Encyclopedia Britannica, 19 Feb. 2021, https://www.britannica.com/animal/placental-mammal. Accessed 2 June 2021.

[5] Wikipedia contributors. “Retrovirus.” Wikipedia, 2021, en.wikipedia.org/wiki/Retrovirus.

[6] Mitra, Avir. “How the Placenta Evolved from an Ancient Virus.” WHYY, 31 Jan. 2020, whyy.org/segments/the-placenta-went-viral-and-protomammals-were-born.

[7] Loke, Y. Life’s Vital Link: The Astonishing Role of the Placenta. Illustrated, Oxford University Press, 2018.

[8] George, Alison. “The Cancer-Fighting Multi-Organ: 9 Ways the Placenta Is Amazing.” New Scientist, 1 Feb. 2021, www.newscientist.com/article/2170038-the-cancer-fighting-multi-organ-9-ways-the-placenta-is-amazing.

[9] Coorens, Tim. “Somatic Mutations Reveal Widespread Mosaicism and Mutagenesis in Human Placentas.” BioRxiv, 1 Jan. 2021, www.biorxiv.org/content/10.1101/2021.01.26.428217v1.

[10] Stranden, Anne Lise. “Your placenta may reveal risk of getting preeclampsia.” sciencenorway, 25 Mar. 2020, sciencenorway.no/illness-pregnancy/your-placenta-may-reveal-risk-of-getting-preeclampsia/1659196.

[11] “Human Placenta Project.” NICHD, www.nichd.nih.gov/research/supported/HPP/default. Accessed 3 June 2021.

Fard, Maggie Fazeli. “Book Discusses the Most Underrated Organ, the Placenta.” Washington Post, 5 Aug. 2013, www.washingtonpost.com/national/health-science/book-discusses-the-most-underrated-organ-the-placenta/2013/08/05/73f5fc4c-f83e-11e2-8e84-c56731a202fb_story.html.

Nelson, D Michael. “How the placenta affects your life, from womb to tomb.” American journal of obstetrics and gynecology vol. 213,4 Suppl (2015): S12-3. doi:10.1016/j.ajog.2015.08.015

Racaniello, Vincent. “Retroviruses Turned Egg-Layers into Live-Bearers.” Virology Blog, 14 Dec. 2017, www.virology.ws/2017/12/14/a-retrovirus-gene-drove-emergence-of-the-placenta.

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