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The placenta (also known as afterbirth) is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply, fight against internal infection and produce hormones to support pregnancy. The placenta provides oxygen and nutrients to growing babies and removes waste products from the baby's blood. The placenta attaches to the wall of the uterus, and the baby's umbilical cord develops from the placenta. The umbilical cord is what connects the mother and the baby. Placentas are a defining characteristic of placental mammals, but are also found in some non-mammals with varying levels of development.〔Pough ''et al.'' 1992. Herpetology: Third Edition. Pearson Prentice Hall:Pearson Education, Inc., 2002.〕 The homology of such structures in various viviparous organisms is debatable and, in invertebrates such as Arthropoda, is analogous at best. The word ''placenta'' comes from the Latin word for ''cake'', from Greek πλακόεντα/πλακοῦντα ''plakóenta/plakoúnta'', accusative of πλακόεις/πλακούς ''plakóeis/plakoús'', "flat, slab-like",〔(Henry George Liddell, Robert Scott, "A Greek-English Lexicon", at Perseus ).〕〔("placenta" ). ''Online Etymology Dictionary''.〕 in reference to its round, flat appearance in humans. The classical plural is ''placentae'', but the form ''placentas'' is common in modern English and probably has the wider currency at present. Prototherial (egg-laying) and metatherial (marsupial) mammals produce a choriovitelline placenta that, while connected to the uterine wall, provides nutrients mainly derived from the egg sac. The placenta functions as a fetomaternal organ with two components: the fetal placenta (Chorion frondosum), which develops from the same blastocyst that forms the fetus, and the maternal placenta (Decidua basalis), which develops from the maternal uterine tissue.〔(Definitions of placental- related terms. )〕 ==Structure== In humans, the placenta averages 22 cm (9 inch) in length and 2–2.5 cm (0.8–1 inch) in thickness, with the center being the thickest, and the edges being the thinnest. It typically weighs approximately 500 grams (just over 1 lb). It has a dark reddish-blue or crimson color. It connects to the fetus by an umbilical cord of approximately 55–60 cm (22–24 inch) in length, which contains two umbilical arteries and one umbilical vein.〔(Examination of the placenta )〕 The umbilical cord inserts into the chorionic plate (has an eccentric attachment). Vessels branch out over the surface of the placenta and further divide to form a network covered by a thin layer of cells. This results in the formation of villous tree structures. On the maternal side, these villous tree structures are grouped into lobules called cotyledons. In humans, the placenta usually has a disc shape, but size varies vastly between different mammalian species.〔(Placental Structure and Classification )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「placenta」の詳細全文を読む スポンサード リンク
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