Placenta
Temporary organ enabling nutrient and gas exchange in pregnancy.
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.
- type
- Temporary embryonic and fetal organ
- average_human_size
- 22 cm length, 2–2.5 cm thickness
- umbilical_cord_length
- Approximately 55–60 cm
- key_protein
- Syncytin (originated from ancient retrovirus)
- plural_forms
- Placentas or placentae
Lore & Background
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 tissue comes away with the placenta when it is expelled from the uterus following birth. Placentas are a defining characteristic of placental mammals, but are also found in marsupials and some non-mammals with varying levels of development. The protein syncytin, found in the outer barrier of the placenta, has a certain RNA signature in its genome that has led to the hypothesis that it originated from an ancient retrovirus.
Reader's Guide
The placenta has evolved independently multiple times, probably starting in fish, and was a critical factor in the explosive diversification of placental mammals. As a temporary organ, it has acted as a testing ground for newly acquired genes or mutations with limited consequences, since the mother is protected from harm once she sheds the placenta after birth. The placenta has substantially lower levels of gene regulation than other organs, allowing for 'leaky' expression of viral genes. This placenta-driven evolution hypothesis explains why the placenta has been a rapidly evolving organ, resulting in vast variation of reproductive strategies in mammals. Although all mammalian placentas have the same functions, there are important differences in structure and function in different groups, such as human, bovine, equine, and canine placentas.
Did You Know?
- The word 'placenta' comes from the Latin word for a type of cake, from Greek words meaning 'flat, slab-like'.
- The placenta occasionally takes a form with multiple distinct parts connected by blood vessels, called bilobed, trilobed, or succenturiate placenta.
- The protein syncytin in the placenta likely originated from an ancient retrovirus that helped pave the transition from egg-laying to live-birth.
Frequently Asked Questions
What is the placenta?
The placenta is a short-lived organ that forms from the blastocyst right after implantation and persists only through the duration of a pregnancy. It serves as the primary interface between the mother's and the fetus's separate blood supplies.
What does the placenta actually do during pregnancy?
It handles the exchange of nutrients, oxygen, carbon dioxide, and metabolic waste between maternal and fetal circulations without the two ever mixing directly. It also acts as an endocrine gland, secreting hormones that help regulate the physiology of both the mother and the developing fetus.
What is syncytin and why does it matter for the placenta?
Syncytin is a key protein found in the placental tissue that originated from an ancient retroviral infection in mammalian ancestors. It plays a central role in the formation and function of the placenta's syncytiotrophoblast layer.
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