Amniotic sac

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Human ten week old fetus in the amniotic sac

The amniotic sac , and amniotic sac or fetal sac (ger .: amniotic sac called), a liquid-filled membrane sac in which the embryo or fetus amniotic land vertebrates developed. It is either located in an egg enclosed with a relatively firm shell ( reptiles , birds , monotrems ) or in the uterus of the mother animal ( "real" mammals ).

What exactly is called the "amniotic sac" is something that the literature does not define consistently. Sometimes only the amnion (amniotic cavity) filled with amniotic fluid , the inner (st) membrane surrounding the embryo or fetus , is referred to as the amniotic sac , but sometimes the amniotic sac is also defined as the totality of the membranes , i.e. it closes at least also the chorion (serosa), an outer membrane, with one.

This fuzziness may be due to the different development of the membranes in the oviparous (reptiles and birds) and viviparous ( "real" mammals ) amniotes. Although the extraembryonic coelom , the fluid-filled space between the amnion and chorion, shrinks as a result of the growth of the embryo and the amniotic cavity in both groups, in placental mammals the amnion fuses over a large area with the chorion, which is also covered by a layer of uterine mucous membrane (decidua capsularis) , so that structurally the amniotic cavity forms the only large cavity within the membrane and outside of the fetus at the latest shortly before birth or hatching. In reptiles and birds, on the other hand, the allantois , the “embryonic urinary bladder”, fuses over a large area with the chorion and finally almost completely surrounds the fetus in the egg.

In placental mammals, the amniotic sac (bursts rupture ) at the start of the birth process by the effect of contractions . The amniotic fluid now makes it easier for the unborn to slide out. The artificial opening of the amniotic sac is called an amniotomy . The amniotic sac often remains in the uterus and is only shed together with the placenta after the actual birth process ( afterbirth ).

Web links

Wiktionary: amniotic sac  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. Winfried Ahne, Horst Erich König, Hans-Georg Liebich, Manfred Stohrer, Eckard Wolf: Zoology - textbook for students of veterinary medicine and agricultural sciences. Schattauer, Stuttgart 2000, ISBN 3-7945-1764-4 , p. 261
  2. ^ Amniotic sac in the Spektrum.de lexicon of biology
  3. Walter Schmidt: The habitat of the embryo (vertebrates and humans). Reports from the scientific and medical association in Innsbruck. Vol. 70, 1983, pp. 281–289 ( PDF 830 kB)
  4. Kirsten Ferner, Andrea Mess: Evolution and development of fetal membranes and placentation in amniote vertebrates. Respiratory Physiology & Neurobiology. Vol. 178, No. 1, 2011, pp. 39-50, doi: 10.1016 / j . or 2011.03.029 (alternative full-text access : ResearchGate ).