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Amnion & Amniotic Cavity

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    Folding of the embryo

    Folding of theembryo is due torapid growth of the

    embryo specially thenervous system.

    The head folds first

    then the tail . At thesame time, side toside folding occurs.

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    The amniotic cavity enlarged.The Yolk sac smaller & divided

    into (intraembryonic Y.S, Yolk stalk&extra embryonic Y.S).Allantois& connecting stalkshifted caudally.S.T Shifted anterior toCardiogenic plate.

    The amniotic cavitymoreenlarged.

    Allantois& connecting stalk

    shifted ventrally and form theumbilical cord which contains theextra embryonic Y.S and stalk.S.T Shifted caudal to Cardiogenicplate.* Placenta will face the umblical cord.

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    Results of folding

    1- Embryo change intocylinderical embryo.2-Transposition between septumtransversum and cardiogenicplate( S.T lies cranial then

    ventral and lastly caudal).3- Yolk sac reduced in size&divided into:

    a- intraembryonic ( gut).

    b- extraembryonic( atrophies).c- yolk stalk (degenerates).

    4- Allantois& connecting stalkbecome dorsal then caudal

    then ventral.

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    THE FETAL MEMBRANES

    Definition:Fetal membranes are all the structures that develop

    from the zygote and do not share in the formation of the

    embryo (extraembryonic structures from the primitiveblastomeres).

    Fetal membranes are:

    a. Chorion.

    b. Amnion.

    c. Yolk sac.

    d. The umbilical cord including allantois and body stalk.

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    - It is a membrane which bounds the amniotic

    cavity.

    - It is continuous with the ectoderm of the embryo.

    - It contains about 800-1000 ml of watery and clearfluid at full term.

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    Formation & structureBefore folding:- It begins as small vacules in theepiblast.

    -The vacules coalease together forminga single cavity.-The cells of the epiblast (ectoderm)adjacent to the cytotrophoblast arecalled amnioblasts.-The amnioblasts form the roof of theamniotic cavity.--This cavity has a floor formed of

    ectoderm.-As the amniotic cavity increases in size,the layer of the amnioblast loses itscontact with the inner surface oftrophoblast and become known as theamnion.

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    After folding:

    -The amniotic cavityincrease in size, atexpense of the extra-embryonic ceolom

    which decreases in size &finally disappears.

    -The amniotic cavitysurrounds the embryo

    in all directions. It liescranial, caudal, dorsal& ventral to theembryo.

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    Composition of amniotic fluid- 98% water, 2% solid substances like inorganic &

    organic salts, fetal epithelium, protein &enzymes.

    Origin: The following forms the amniotic fluid:

    1- Amniotic membrane2- Maternal tissue (interstitial) fluid by diffusion

    across the amnio-chorionic membrane from thedeciduas parietalis.

    3- Filtrated from maternal blood.

    4- Fluid is also secreted by the fetal respiratory tract(300 400 ml daily) and enters the amniotic cavity.

    5-Fetal urine.

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    Circulation- The amniotic fluid, formed by amnioticmembrane & filtrated from maternal bloodaccumulates in the amniotic cavity,

    - Then, it is swallowed by the embryo.- Lastly, it passes as fetal urine to accumulateagain in the amniotic cavity.

    Volume of the amniotic fluid:The volume of amniotic fluid increases slowlyfrom 30 ml at 10 weeks gestation to 350 ml at

    20 weeks to 700

    1000 ml by 37 weeks.

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    FunctionBefore labour:1-It forms an isolating bag around the embryo protectinghim from external trauma, shock & temperature.

    2-It prevents adhesion of the embryo to its membranes.

    3-It allows homogenous media needed for the growth ofthe embryo.

    4-It permits the free movement of the embryo needed

    for muscular exercise.5-It allows the embryo to practice suckling.

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    FunctionDuring labor:

    1- It forms the bags of fore water and hind water.

    2-The bag of fore water allows regular dilatation of thecervix.

    3-After rupture of membrane the amniotic fluid servesas a lubricant for fetus descent.

    4-Also the amniotic fluid is bacteriostatic.

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    Uses of the amniotic fluid

    - The cells of amniotic fluid can be used for fetalchromosomal studies

    (to diagnose chromosomal aberrations as Downsyndrome, Edward syndrome, and others), and for DNA

    studies for diagnosis of some single gene disorders.- Some proteins and enzymes in the amniotic fluid canbe estimated and its level (whether normal or deviated)is diagnostic for some genetic diseases and congenitalanomalies. These proteins are alpha fetoprotein (AFP), ,unconjugated estradiol( E3), pregnancy associated

    protein A (PAP-A), inhibin A, and human chorionicgonadotrophin (hCG).

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    Abnormalities1-Polyhydramnios: increase in the amount ofamniotic fluid (2 liters). It is due to fetal cause(oesophagial atresia) or maternal cause (diabetesof the mother). It leads to premature labour.

    2-Oligohydramnios: decrease in the volume ofamniotic fluid (0.5 liter). It is due to failure ofdevelopment of the fetal kidneydiminish the

    volume of fetal urine, decrease the amount of

    amniotic fluid adhesion of the fetal skin with itsmembrane.

    3-Cul du sac: the baby will be born in a closed sac.

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    Thank You


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