The amnion first makes its appearance in the second week of development as a cleft in the cells of the epiblast layer. It is one of the “two’s” of week two. It eventually develops into a thin membrane that covers the dorsal aspect of the embryo and then the fetus. It is a fluid filled sac forming a hydrostatic cushion around the fetus. Eventually, the lining cuboidal epithelium also covers the umbilical cord.

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  • Later Development

    During early development, the embryo is embedded in the uterine wall. The portion of the chorion adjacent to the embryo will begin to form placental villi and take on a feathery appearance. It is known as chorion frondosum. The part of the chronic membrane opposite the embryo is smooth and is called the chorion leave. The endometrium located in the region of the developing embryo is called the decidua basalis and will form the maternal component of the placenta. That in contact with the chorion leave is the decidua capsularis. The lining of the uterus in the area other than where the embryo is located is the decidua parietalis.
    By 4 months gestation, the amnion has fused with the chorion leave and the decidua capsularis to form the amniochorionic membrane. It is completed as a fluid filled sac in which the fetus is developing. At birth, rupture of the amniochorionic membrane results in the expulsion of the amniotic fluid through the birth canal. At this time the mother may report that her "water broke", thus signaling the imminent birth of the baby. Samples of amniotic fluid, obtained by aspiration, contain cells that have sloughed off from the fetus and which can be subjected to chromosomal analysis if necessary. This procedure is called amniocentesis and can provide valuable information about the any potential genetic problems of the developing fetus.

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  • Amniotic Fluid

    Amniotic fluid is responsible for enlargement of the amniotic sac.

    • It is a clear watery fluid that is largely derived, initially, from maternal blood plasma. Later, the fetus contributes to the volume.
    • The volume increases during gestation.
    • Turnover rate is about 3hrs.
    • From the 5th month on the fetus swallows 400ml/day. An overabundance of amniotic fluid, polyhydramnios, may indicate maldevelopment of the fetal nervous system and the inability of the fetus to swallow.
    • Fetal urine contributes to the volume. A lower than normal volume of amniotic fluid, oligohydramnios, may indicate agenesis of the fetal kidneys as one cause.
    • At birth it acts as a hydrostatic wedge to help dilate the birth canal.

  • After Birth

    After delivery of the fetus, the placenta usually detaches from the uterine wall and passes to the outside through the birth canal along with the attached membranes. This is commonly referred to as the “after birth”. Here one can see the fused amnion and chorionic membranes as part of the "after birth". The umbilical cord goes into the sac and is still attached to the placenta which is in the bottom of the pan.

    Note that the fetal side of the placenta is characterized by a smooth surface of simple cuboidal epithelium derived from the amnion. The umbilical cord is attached to its center and large fetal blood vessels radiate into it. The maternal side of the placenta is “bloody” and has a cobblestone appearance due to the presence of “cotyledons” formed by decimal septa that grow into it.

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