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| Amniotic sac | |
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| Anatomical terminology |
Theamniotic sac, also called thebag of waters[1][2] or themembranes,[3] is the sac in which theembryo and laterfetus develops inamniotes. It is a thin but tough transparent pair ofmembranes that hold a developing embryo (and later fetus) until shortly beforebirth. The inner of these membranes, theamnion, encloses theamniotic cavity, containing theamniotic fluid and the embryo. The outer membrane, thechorion, contains the amnion and is part of theplacenta. On the outer side, the amniotic sac is connected to theyolk sac, theallantois, and via theumbilical cord, theplacenta.[4]
The yolk sac, amnion, chorion, and allantois are the fourextraembryonic membranes that lie outside of the embryo and are involved in providing nutrients and protection to the developing embryo.[5] They form from theinner cell mass; the first to form is the yolk sac followed by the amnion which grows over the developing embryo. The amnion remains an important extraembryonic membrane throughout prenatal development. The third membrane is the allantois, and the fourth is the chorion which surrounds the embryo after about a month and eventually fuses with the amnion.[6]
Amniocentesis is a medical procedure where fluid from the sac is sampled duringfetal development, between 15 and 20 weeks of pregnancy, to be used inprenatal diagnosis ofchromosomal abnormalities and fetal infections.[7]
The amniotic cavity is the closed sac between the embryo and the amnion, containing the amniotic fluid. The amniotic cavity is formed by the fusion of the parts of the amniotic fold, which first makes its appearance at thecephalic extremity and subsequently at thecaudal end and sides of theembryo. As the amniotic fold rises and fuses over the dorsal aspect of the embryo, the amniotic cavity is formed.[citation needed]
At the beginning of the second week, a cavity appears within theinner cell mass, and when it enlarges, it becomes the amniotic cavity. The floor of the amniotic cavity is formed by theepiblast. Epiblast migrates between the epiblastic disc and trophoblast. In this way the epiblastic cells migrate between the embryoblast andtrophoblast. The floor is formed by the epiblast which later on transforms toectoderm while the remaining cells which are present between theembryoblast and trophoblast are called amnioblasts (flattened cells). These cells are also derived from epiblast which is transformed into ectoderm.[citation needed]
The amniotic cavity is surrounded by anextraembryonic membrane, called the amnion. As the implantation of the blastocyst progresses, a small space appears in the embryoblast, which is the primordium of the amniotic cavity. Soon, amniogenic (amnion-forming cells) calledamnioblasts separate from the epiblast and line the amnion, which encloses the amniotic cavity.[8]
The epiblast forms the floor of the amniotic cavity and is continuous peripherally with the amnion. The hypoblast forms the roof of theexocoelomic cavity and is continuous with the thin exocoelomic membrane. This membrane along with hypoblast forms the primary yolk sac. The embryonic disc now lies between the amniotic cavity and the primary yolk sac. Cells from the yolk sacendoderm form a layer ofconnective tissue, the extraembryonicmesoderm, which surrounds the amnion and yolk sac.[citation needed]
If, after birth, the complete amniotic sac or big parts of the membrane remain coating the newborn, this is called acaul.
When seen in the light, the amniotic sac is shiny and very smooth, but tough.
Once the baby is pushed out of the mother'suterus, theumbilical cord,placenta, and amniotic sac are pushed out in the afterbirth.
The amniotic sac and its filling provide a liquid that surrounds and cushions the fetus. It is a site of exchange of essential substances, such as oxygen, between the umbilical cord and the fetus.[9] It allows the fetus to move freely within the walls of theuterus.[citation needed]Buoyancy is also provided.
Chorioamnionitis is inflammation of the amniotic sac (chorio- +amnion +-itis), usually because ofinfection. It is a risk factor forneonatal sepsis.
Duringlabor, the amniotic sac must break so that the child canbe born. This is known asrupture of membranes (ROM). Normally, it occurs spontaneously at full term either during or at the beginning of labor. Apremature rupture of membranes (PROM) is a rupture of the amnion that occurs prior to the onset of labor. Anartificial rupture of membranes (AROM), also known as an amniotomy, may be clinically performed using an amnihook or amnicot in order to induce or to accelerate labour.
The amniotic sac has to be punctured to performamniocentesis.[10][11] This is fairly routine procedure, but can lead to infection of the amniotic sac in a very small number of cases.[12] Infection more commonly arises vaginally.[12][13]
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