In biology, placentation refers to the formation, type and structure, or arrangement of placentas. The function of placentation is to transfers nutrients from maternal tissue to a growing embryo. Placentation is best known in pregnant female mammals (eutheria), but also occurs in other animals, eggs (yolk sac placentation) and flowering plants.

Placentation in mammals

In placental mammals, the placenta forms after the embryo implants into the wall of the uterus. The developing fetus is connected to it via an umbilical cord. Animal placentas are classified based on the number of tissues separating the maternal from the fetal blood. The placentation types found in animals are:
* endotheliochorial placentation (e.g. in most carnivores like cats and dogs)
* epitheliochorial placentation (e.g. in ruminants, horses, whales)
* hemochorial placentation (e.g. in most primates, including humans)

During pregnancy, placentation is the formation and growth of the placenta inside the uterus. It occurs after the implantation of the embryo into the uterine wall and involves the remodeling of blood vessels in order to supply the needed amount of blood. In humans, placentation takes place 7-8 days after fertilization.

In the case of twins, dichorionic placentation refers to the presence of two placentas (in all dizygotic and some monozygotic twins). Monochorionic placentation occurs when monozygotic twins develop with only one placenta and bears a higher risk of complications during pregnancy. Abnormal placentation can lead to an early termination of pregnancy, for example in pre-eclampsia.

Placentation in plants

In flowering plants, placentation occurs where the ovules are attached inside the ovary. The ovules inside a flower's ovary (which later become the seeds inside a fruit) are attached via [funicle] , the plant equivalent of an umbilical cord. The part of the ovary where the funicles attaches is referred to as placenta.

In botany, the term placentation most commonly refers to the arrangement of placentas inside a flower or fruit. Plant placentation types include:
*Basal placentation: The placenta is at the base (bottom) of the ovary. Simple or compound carpel.
*Apical placentation: The placenta is at the apex (top) of the ovary. Simple or compound carpel.
*Parietal placentation: The placentas are in the ovary wall within a non-sectioned ovary. Compound carpel.
*Axil placentation: The ovary is sectioned by radial spokes with placentas in separate locules. Compound carpel.
*Free or central placentation: The placentas are in a central column within a non-sectioned ovary. Compound carpel.
*Marginal placentation: There is only one elongated placenta on one side of the ovary. This is conspicuous in legumes. Simple carpel.

ee also

* Carpel
* Ovule
* Placenta

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Look at other dictionaries:

  • placentation — [ plasɛ̃tasjɔ̃ ] n. f. • 1817 en bot.; de placenta 1 ♦ Embryol. Formation du placenta; manière dont il est disposé selon les espèces. Placentation diffuse, discoïde. 2 ♦ Bot. Manière dont les graines sont disposées sur le carpelle. Placentation… …   Encyclopédie Universelle

  • Placentation — Plac en*ta tion, n. 1. (Anat.) The mode of formation of the placenta in different animals; as, the placentation of mammals. [1913 Webster] 2. (Bot.) The mode in which the placenta is arranged or composed; as, axile placentation; parietal… …   The Collaborative International Dictionary of English

  • Placentation — Placentation, s. Fruchtknoten …   Meyers Großes Konversations-Lexikon

  • placentation — [plas΄ən tā′shən] n. 1. Anat. Zool. a) the formation or structure of a placenta b) the manner in which the placenta is attached to the uterus 2. Bot. the manner in which the placentas are arranged in an ovary …   English World dictionary

  • placentation — noun 1. the formation of the placenta in the uterus • Hypernyms: ↑bodily process, ↑body process, ↑bodily function, ↑activity 2. arrangement of the ovules in the placenta and of the placentas in the ovary • Hypernyms: ↑reproductive structure …   Useful english dictionary

  • placentation — (pla sin ta sion) s. f. Terme de botanique. Manière dont les graines sont attachées au placenta.    Placentation pariétale, situation des placentas adhérents aux parois ovariennes …   Dictionnaire de la Langue Française d'Émile Littré

  • placentation — noun Date: 1760 1. the arrangement of placentas and ovules in a plant ovary 2. a. the development of the placenta and attachment of the fetus to the uterus during pregnancy b. the morphological type of a placenta …   New Collegiate Dictionary

  • placentation — /plas euhn tay sheuhn/, n. 1. Anat., Zool. a. the formation of a placenta. b. the manner of placement or construction of a placenta. 2. Bot. the disposition or arrangement of a placenta or placentas. [1750 60; < F; see PLACENTA, ATION] * * * …   Universalium

  • placentation — noun the local fusion of the embryonic stage of an animal to its parent for physiological exchange to promote the growth and development of the young; involves a placenta in non egglaying mammals …   Wiktionary

  • placentation — The structural organization and mode of attachment of fetal to maternal tissues in the formation of the placenta. Types of p. are defined under placenta. * * * pla·cen·ta·tion .plas ən tā shən, plə .sen n …   Medical dictionary

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