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بسم الله الرحمن الرحيم. Folding of the embryo. Folding of the embryo is due to rapid growth of the embryo specially the nervous system. The head folds first then the tail . At the same time, side to side folding occurs. The amniotic cavity  enlarged .

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Folding of the embryo l.jpg
Folding of the embryo

  • Folding of the embryo is due to rapid growth of the embryo specially the nervous system.

  • The head folds first then the tail . At the same time, side to side 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 stalk  shifted caudally.

  • S.T  Shifted anterior to Cardiogenic plate.

  • The amniotic cavity  more enlarged.

  • Allantois& connecting stalk  shifted ventrally and form the umbilical cord which contains the extra embryonic Y.S and stalk.

  • S.T  Shifted caudal to Cardiogenic plate.

  • * Placenta will face the umblical cord.


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Results of folding

1- Embryo change into cylinderical embryo.

2-Transposition between septum transversum and cardiogenic plate( 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 stalk become 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 primitive blastomeres).

Fetal membranes are:

a. Chorion.

b. Amnion.

c. Yolk sac.

d. The umbilical cord including allantois and body stalk.


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Amnion & Amniotic cavity

- 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 clear fluid at full term.


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Formation & structure

Before folding:

- It begins as small vacules in the epiblast.

  • The vaculescoalease together forming a single cavity.

    -The cells of the epiblast (ectoderm) adjacent to the cytotrophoblast are called amnioblasts.

    -The amnioblasts form the roof of the amniotic cavity.-

    -This cavity has a floor formed of ectoderm.

    -As the amniotic cavity increases in size, the layer of the amnioblast loses its contact with the inner surface of trophoblast and become known as the amnion.


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After folding:

-The amniotic cavity increase in size, at expense of the extra-embryonic ceolom

which decreases in size & finally disappears.

-The amniotic cavity surrounds the embryo in all directions. It lies cranial, caudal, dorsal & ventral to the embryo.


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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 membrane

2- Maternal tissue (interstitial) fluid by diffusion across the amnio-chorionic membrane from the deciduas 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 amniotic membrane & filtrated from maternal blood accumulates in the amniotic cavity,

- Then, it is swallowed by the embryo.

- Lastly, it passes as fetal urine to accumulate again in the amniotic cavity.

Volume of the amniotic fluid:

The volume of amniotic fluid increases slowly from 30 ml at 10 weeks gestation to 350 ml at 20 weeks to 700 – 1000 ml by 37 weeks.


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Function

Before labour:

1-It forms an isolating bag around the embryo protecting him from external trauma, shock & temperature.

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

3-It allows homogenous media needed for the growth of the 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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Function

During labor:

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

2-The bag of fore water allows regular dilatation of the cervix.

3-After rupture of membrane the amniotic fluid serves as 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 fetal chromosomal studies

(to diagnose chromosomal aberrations as Down syndrome, Edward syndrome, and others), and for DNA studies for diagnosis of some single gene disorders.

- Some proteins and enzymes in the amniotic fluid can be estimated and its level (whether normal or deviated) is diagnostic for some genetic diseases and congenital anomalies. These proteins are alpha fetoprotein (AFP), , unconjugated estradiol ( µE3), pregnancy associated protein A (PAP-A), inhibin A, and human chorionic gonadotrophin (hCG).


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Abnormalities

1-Polyhydramnios:increase in the amount of amniotic fluid (2 liters). It is due to fetal cause (oesophagial atresia) or maternal cause (diabetes of the mother). It leads to premature labour.

2-Oligohydramnios:decrease in the volume of amniotic fluid (0.5 liter). It is due to failure of development of the fetal kidney diminish the volume of fetal urine, decrease the amount of amniotic fluid  adhesion of the fetal skin with its membrane.

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



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