Human placenta shown in uterus connected to fetus with umbilical cord
Human placenta baby side.jpg
Human placenta from just after birth with the umbilical cord in place
Precursordecidua basalis, chorion frondosum
Anatomical terminology

The placenta is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas and waste exchange between the physically separate maternal and fetal circulations, and is an important endocrine organ, producing hormones that regulate both maternal and fetal physiology during pregnancy.[1] The placenta connects to the fetus via the umbilical cord, and on the opposite aspect to the maternal uterus in a species-dependent manner. In humans, a thin layer of maternal decidual (endometrial) tissue comes away with the placenta when it is expelled from the uterus following birth (sometimes incorrectly referred to as the 'maternal part' of the placenta). Placentas are a defining characteristic of placental mammals, but are also found in marsupials and some non-mammals with varying levels of development.[2]

Mammalian placentas probably first evolved about 150 million to 200 million years ago. The protein syncytin, found in the outer barrier of the placenta (the syncytiotrophoblast) between mother and fetus, has a certain RNA signature in its genome that has led to the hypothesis that it originated from an ancient retrovirus: essentially a virus that helped pave the transition from egg-laying to live-birth.[3][4][5]

The word placenta comes from the Latin word for a type of cake, from Greek πλακόεντα/πλακοῦντα plakóenta/plakoúnta, accusative of πλακόεις/πλακούς plakóeis/plakoús, "flat, slab-like",[6][7] with reference to its round, flat appearance in humans. The classical plural is placentae, but the form placentas is more common in modern English.

  1. ^ Jin M, Xu Q, Li J, Xu S, Tang C (September 2022). "Micro-RNAs in Human Placenta: Tiny Molecules, Immense Power". Molecules. 27 (18): 5943. doi:10.3390/molecules27185943. PMC 9501247. PMID 36144676.
  2. ^ Pough, F. H.; Andrews, Robin M.; Cadle, John E.; Crump, Martha L.; Savitsky, Alan H.; Wells, Kentwood D. (2004). Herpetology (3rd ed.). Pearson. ISBN 978-0-13-100849-6.[page needed]
  3. ^ Mitra, Avir (31 January 2020). "How the placenta evolved from an ancient virus". WHYY. Retrieved 9 March 2020.
  4. ^ Chuong, Edward B. (9 October 2018). "The placenta goes viral: Retroviruses control gene expression in pregnancy". PLOS Biology. 16 (10): e3000028. doi:10.1371/journal.pbio.3000028. PMC 6177113. PMID 30300353.
  5. ^ Villarreal, Luis P. (January 2016). "Viruses and the placenta: the essential virus first view". APMIS. 124 (1–2): 20–30. doi:10.1111/apm.12485. PMID 26818259. S2CID 12042851.
  6. ^ Henry George Liddell, Robert Scott, "A Greek-English Lexicon", at Perseus Archived 2012-04-05 at the Wayback Machine.
  7. ^ "placenta" Archived 2016-01-30 at the Wayback Machine. Online Etymology Dictionary.

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