Placenta

The placenta (pl.: placentas or placentae) is a temporary <a href="/facts/Embryo/9QHnLVLN">embryonic</a> and later <a href="/facts/Fetus/evocHd0Y">fetal</a> <a href="/facts/Organ_(biology)/FUYsLaxz">organ</a> that begins <a href="/facts/Embryonic_development/1lFY4EoW">developing</a> from the <a href="/facts/Blastocyst/WkRkxdzc">blastocyst</a> shortly after <a href="/facts/Implantation_(embryology)/k6XJfNMv">implantation</a>. It plays critical roles in facilitating nutrient, gas, and waste exchange between the physically separate maternal and fetal circulations, and is an important <a href="/facts/Endocrine_system/PDvN5cpj">endocrine organ</a>, producing <a href="/facts/Hormones/WFnBnINJ">hormones</a> that regulate both <a href="/facts/Maternal_physiological_changes_in_pregnancy/GaUkbglx">maternal</a> and fetal <a href="/facts/Physiology/fh25y2pQ">physiology</a> during <a href="/facts/Pregnancy/vEKYGpep">pregnancy</a>. The placenta connects to the fetus via the <a href="/facts/Umbilical_cord/WIBgdrmF">umbilical cord</a>, and on the opposite aspect to the maternal <a href="/facts/Uterus/LIrApMXs">uterus</a> in a <a href="/facts/Species/oDg6b96r">species</a>-dependent manner. In humans, a thin layer of maternal <a href="/facts/Decidua/soTd97ml">decidual</a> (<a href="/facts/Endometrium/LcesXIXm">endometrial</a>) 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 <a href="/facts/Placental_mammal/xegVWIRG">placental mammals</a>, but are also found in <a href="/facts/Marsupials/zN5s4bMq">marsupials</a> and some non-mammals with varying levels of development.
<a href="/facts/Mammalian/phSwTzBo">Mammalian</a> placentas probably first evolved about 150 million to 200 million years ago. The protein <a href="/facts/Syncytin/bIreplyp">syncytin</a>, found in the outer barrier of the placenta (the <a href="/facts/Syncytiotrophoblast/ULNKTpDg">syncytiotrophoblast</a>) between mother and fetus, has a certain RNA signature in its genome that has led to the hypothesis that it originated from an ancient <a href="/facts/Retrovirus/GWAx3C3o">retrovirus</a>: essentially a <a href="/facts/Virus/mf88Bcbl">virus</a> that helped pave the transition from <a href="/facts/Oviparity/oSgSIVHY">egg-laying</a> to <a href="/facts/Viviparity/EEMn4rVx">live-birth</a>.
The word placenta comes from the <a href="/facts/Latin/pM3Kge85">Latin</a> word for <a href="/facts/Placenta_cake/UEJW0xod">a type of cake</a>, from <a href="/facts/Ancient_Greek/LIK7NlUh">Greek</a> πλακόεντα/πλακοῦντα plakóenta/plakoúnta, accusative of πλακόεις/πλακούς plakóeis/plakoús, "flat, slab-like", with reference to its round, flat appearance in humans. The classical plural is placentae, but the form placentas is more common in modern English.

Placenta open-in-new

Placenta