Placenta - taiban (English spelling)

Japanese: 胎盤 - たいばん（英語表記）placenta

In viviparous animals, the placenta is a tissue complex in which tissues derived from the embryo and fetus and tissues derived from the maternal endometrium are in close contact with each other, creating a physiologically important relationship between the two, including the exchange of substances.It is an organ rich in blood vessels.

The shape of the placenta varies from animal to animal. The human placenta is pancake-like, fitting for the word placenta, which means "flat cake" from the roots, but cats and dogs have a band-like shape, while sheep and cows have a rosette-like shape, and so on.

[Masao Arai]

Human placenta

The placenta is a disc-like structure consisting of the chorion frondosum and decidua basalis, and is fully formed by the end of the fourth month of pregnancy. At the end of pregnancy, it is circular or elliptical, and appears as a flattened disk, 15-20 cm in diameter, 2-3 cm thick at the center, and weighs about 500 grams, which is about one-sixth of the fetal weight. The surface of the placenta that is attached to the uterine wall is called the maternal surface, and the surface that faces the fetus is called the fetal surface. The maternal surface is dark red and is divided into lobes of various sizes, resembling a stone wall. The fetal surface is smooth and pale gray, and its surface is covered with the amniotic membrane. In a fresh placenta, a thin grayish-white membrane covers the surface of each lobe and also penetrates into the grooves between the lobes, forming a somewhat thick partition (placental septum). The microstructure of the placenta is complex. The villous tissue forms a villous tree, the trunk of which is in the center of the intervillous space and connects at right angles between the chorionic plate and the basal plate. The fetal blood flows through the villous tree, while the intervillous space is filled with maternal blood. Gases are exchanged between the two, and various organic and inorganic solutions, pigments, and immune systems pass through, and the placenta functions as the digestive system, lungs, kidneys, and liver for the fetus. It also secretes placental hormones (mainly gonadotropic hormones), which are used to diagnose pregnancy early.

Placental blood circulation consists of fetal blood circulation and maternal blood circulation. Fetal blood circulation enters the capillary network in the villi from the umbilical artery, where it absorbs oxygen and nutrients from maternal blood in the intervillous space, releases unnecessary substances such as carbon dioxide, becomes arterial blood, and collects in the umbilical vein. Maternal blood circulation sends arterial blood from the uteroplacental artery to the intervillous space, where it provides oxygen and nutrients to the fetal venous blood, receives unnecessary substances such as carbon dioxide, becomes venous blood, flows into the marginal venous sinus at the edge of the placenta, and returns to the mother's heart via the uteroplacental vein.

The completed placenta consists of the mother's decidua and the chorion and amnion from the fertilized egg. Until the placenta is completed, the bond between the fertilized egg and the uterus is weak, making miscarriage likely, but once completed, the bond becomes considerably stronger and the risk of miscarriage decreases. The placenta also has a lifespan, and if the pregnancy continues for too long it will lose function and age. If this ageing occurs before delivery, the fetus will be at risk due to a lack of oxygen supply. Therefore, in cases of prolonged pregnancies where delivery is delayed by more than two weeks from the expected delivery date, delivery is induced artificially before the placenta begins to age.

The fetus and placenta must be considered as a single unit functionally, and are called the fetal-placental system. Placental function tests related to the prognosis of the fetus, such as tests for urinary and blood chorionic gonadotropin and amniotic fluid tests, are also performed.

[Masao Arai]

[Reference] | Fetus | Embryonic

Structure of the Placenta
©Shogakukan ">

Structure of the Placenta

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

胎生動物において胚(はい)および胎児由来の組織と母体子宮内膜由来の組織が緊密な接触を保って両者間に物質交換など生理的に重要な相互関係を生じている組織複合体を胎盤といい、血管に富んだ臓器である。

　胎盤の形態は動物によって異なる。ヒトの胎盤はパンケーキ状で、語源的に平らなケーキの意味をもつplacentaにふさわしいが、ネコやイヌでは帯状、ヒツジやウシはロゼット状などいろいろな形態がみられる。

［新井正夫］

ヒトの胎盤

繁生絨毛(はんせいじゅうもう)膜と基底脱落膜からなる盤状胎盤で、妊娠4か月末に完成する。妊娠末期には円形または楕円(だえん)形で、扁平(へんぺい)な円盤状を呈し、直径15～20センチメートル、厚さは中央部で2～3センチメートル、重さは約500グラムで胎児体重の約6分の1に相当する。胎盤の子宮壁につく面を母体面といい、胎児に対する面を胎児面という。母体面は暗赤色で、大小不同の分葉に分割されて石垣状を呈する。胎児面は平滑で淡灰色を呈し、表面は羊膜で覆われている。新鮮な胎盤は灰白色の薄膜が各分葉の表面を覆い、分葉間の溝の中にも侵入し、やや厚い隔壁（胎盤中隔）を形成する。胎盤の微細構造は複雑である。絨毛組織は絨毛樹を形成し、その幹の部分は絨毛間腔(くう)の中央にあり、絨毛膜板と基底板との間を直角に連ねている。絨毛樹の中には胎児の血流が通っているが、一方、絨毛間腔内には母体血が充満している。この両者間でガス交換が行われるほか、各種の有機物や無機物質溶液、色素、免疫体などが通過し、胎盤は胎児にかわって消化器をはじめ、肺・腎(じん)・肝臓の作用をするわけである。また、胎盤ホルモン（おもに性腺(せいせん)刺激ホルモン）を分泌するが、これは妊娠の早期診断に応用される。

　胎盤血行には、胎児血行と母体血行がある。胎児血行は臍(さい)動脈から絨毛内の毛細管網内に入り、絨毛間腔の母体血から酸素や栄養素をとって炭酸ガスなど不要物質を放出して動脈血となり、臍静脈に集まる。母体血行は、子宮胎盤動脈から動脈血を絨毛間腔へ送り、胎児の静脈血に酸素や栄養素を与え、炭酸ガスなど不要物質を受けて静脈血となり、胎盤の辺縁にある辺縁静脈洞内に流入して子宮胎盤静脈を経て母体の心臓に戻る。

　完成した胎盤は、母体の脱落膜と受精卵からの絨毛膜・羊膜とからなる。胎盤が完成するまでは受精卵と子宮との結合は弱く、流産をおこしやすいが、完成すると結合が相当に強くなって流産の危険も少なくなる。また、胎盤にも寿命があり、妊娠があまり長く続くと機能を失って老化してくる。分娩(ぶんべん)前に老化がおこると、酸素の供給不足などで胎児が危険になる。したがって、分娩予定日より半月以上も出産が延びる遷延妊娠の場合には、胎盤の老化が進行しないうちに人工的に分娩をおこさせるようにする。

　なお、胎児と胎盤は機能的には一つの単位としてみなければならず、胎児‐胎盤系とよばれている。また、尿中・血中絨毛性ゴナドトロピンの検査や羊水検査など、胎児の予後に関連する胎盤機能検査が行われる。

［新井正夫］

[参照項目] | 胎児 | 胎生

胎盤の構造

出典　小学館　日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について　情報 | 凡例

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