page_title: Placenta
section_title: Development
hierarchical_section_title: Placenta / Physiology / Development
caption_attribution_description: English: Placenta. See a full animation of this medical topic.
context_page_description: The placenta is a temporary organ that connects the developing fetus via the umbilical cord to the uterine wall to allow nutrient uptake, thermo-regulation, waste elimination, and gas exchange via the mother's blood supply; to fight against internal infection; and to produce hormones which support pregnancy. Placentas are a defining characteristic of placental mammals, but are also found in marsupials and some non-mammals with varying levels of development.
  The placenta functions as a fetomaternal organ with two components: the fetal placenta, which develops from the same blastocyst that forms the fetus, and the maternal placenta, which develops from the maternal uterine tissue. It metabolizes a number of substances and can release metabolic products into maternal or fetal circulations. The placenta is expelled from the body upon birth of the fetus.
  Placentas probably first evolved about 150 million to 200 million years ago.
context_section_description: The placenta begins to develop upon implantation of the blastocyst into the maternal endometrium. The outer layer of the blastocyst becomes the trophoblast, which forms the outer layer of the placenta. This outer layer is divided into two further layers: the underlying cytotrophoblast layer and the overlying syncytiotrophoblast layer. The syncytiotrophoblast is a multinucleated continuous cell layer that covers the surface of the placenta. It forms as a result of differentiation and fusion of the underlying cytotrophoblast cells, a process that continues throughout placental development. The syncytiotrophoblast (otherwise known as syncytium), thereby contributes to the barrier function of the placenta.
  The placenta grows throughout pregnancy. Development of the maternal blood supply to the placenta is complete by the end of the first trimester of pregnancy week 14 (DM).
caption: Placenta
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