Derivativies of the endodermal germ layer : While folding the

Asem Ayman Embryology Sheet #4 13/5/2012

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Derivativies of the endodermal germ layer :

While folding the endoderm is the layer which forms the internal

tube in the emrbyo .

The main organ produced by the endoderm is the digestive system

from the oral cavity to the anal canal , also the endoderm gives the

epithelium of respiratory and urinary systems .

Ectoderm forms the external large tube and the endoderm forms the

internal tube and between them there are intra embryonic cavities .

Around the internal tube there is a layer called visceral mesoderm

and the layer lining the ectoderm is the parietal mesoderm .

NOTE : lateral mesoderm divides into two parts

1st The parietal mesoderm covering the ectoderm tube .

2nd The visceral mesoderm covering the endoderm tube .

*The gastrointestinal tract is the main organ system derived from the

endodermal germ layer


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*This germ layer covers the ventral surface of the embryo

* With development the embryonic disc begins to bulge into the

amniotic cavity and to fold cephalocaudally and Lateral folds also

form and move ventrally to assist in body wall closure .

As a result of cephalocaudal folding, a continuously larger portion of

the endodermal germ layer is incorporated into the body of the

embryo to form the gut tube. The tube is divided into three regions:

FOREGUT : from mouth to middle of duodenum .

MIDGUT : middle of duodenum to middle of transverse colon .

HINDGUT : middle of the transverse colon to the upper half of anal

canal .


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At its cephalic end, the foregut is temporarily bounded by an

ectodermal-endodermal

VIMP (No Mesoderm) VIMP

membrane called the OROPHARYNGEAL membrane

In the fourth week, the oropharngeal membrane ruptures,

establishing an open connection between the oral cavity

and the primitive gut .

The hindgut also terminates temporarily at an ectodermal-

endodermal membrane, THE CLOACAL membrane . The

membrane breaks down in the seventh week to create the opening

for the anus .

Notes :

1- these two membranes forms at the time of bilaminar disc , and

start to fold forming mouth and anal canal .

2-when cells migrate from epiblast forming mesoderm they reach in

the emrbyo except these two areas oral cavity and the anus (the

trilaminar disc is trilaminar except on these two places ) .

Everything that lines organs is derived from endoderm !!!


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ANS. : Because there is no need to reopen the oral cavity and the

anal canal , by getting rid of mesoderm , notice that a mesoderm

(that gives rise to muscles) surrounds each of these openings .

ENDODERM GIVES RISE TO:

•The epithelial lining of the respiratory tract .

•The parenchyma of the thyroid, parathyroids, liver, and pancreas .

•The reticular stroma of the tonsils and thymus .

•The epithelial lining of the urinary bladder and urethra .

•The epithelial lining of the tympanic cavity and auditory Tube .


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Placenta

Embryo needs food which has only one source that’s the mother

blood and the embryo must reach it .

The embryo is located in the uterus , and the uterus han an uterine

artery which is a branch of the internal iliac artery and in the uterus

the uterine artery gives many small branches which is called spiral

arteries , each one of them enters the endometriam which is the

thinnest layer of the uterus .

The embryo should make his own bid also the mother should do so ,

to form an organ in which the embryo blood meets mother blood

without mixing , this organ is defined as the placenta .

NOTE : placenta of the mother should not be physically attached to

the placenta of the baby .

Note that Decidua means falling

Part so the endometrium is

called decidua even fertilisation

next page occurs or not .


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Chorionic plate consist of extra embryonic mesoderm +

trophoblast { syncytio. + cyto. }

It divides into two parts chorionic laeve and chorionic frondosum ,

which is the fetal part of placenta .

Parts of the decidua :

Decidua basalis：under the implantation site in which the

placenta is formed .

Decidua capsularis： between the implantation site and the

uterine lumen , formed via budging

Decidua parietalis： remaining endometrium .


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Note : at the end of pregnancy both parts of the decidua the

capsularis and the paraietalis get firmly adherent to each other ,

uterine cavity will be inclosed .

Parts of chorionic plate (choirion) :

1.chorion laeve smooth , adjacent to decidua capsularis .

2.chorion frundosum finger-like shape and its adjacent to the

decidua basalis .

At the eighth day of development : The trophoblast differentiates

into two layers:

(1) an inner layer the CYTOTROPHOBLAST

(2) an outer zone the SYNCYTIOTROPHOBLAST

Note cyto. ‘Still waiting’ with the embryo for the syncytio. To bring

food ( the job of syncytio. Is to find a food supply by penetrating the

maternal sinusoids by secreting specific enzyme that cause the

sinsiud wall to degenerate . , also note that the embryo don’t require

a large amount of blood supply ( nutrition )

blood is concentrated here

this is the

placenta

to be .


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at DAY 9 :

*At the trophoblast vacuoles appear in the syncytium.

*These vacuoles fuse and form large lacunae, and this phase of

trophoblast development is known as the LACUNAR STAGE

Note when the syncytium attacks the sinsiuds which is full of blood ,

the lacunae will be full of blood .

At Day 11 & 12 :

*At the trophoblast : The syncytiotrophoblast start to penetrate deeper into the stroma and eroding the maternal capillaries known as sinusoids , then by diffusion the blood will pass through Syncytio. - cyto. the mesoderm to reach the embryo . *The syncytial lacunae become continuous with the sinusoids, and maternal blood enters the lacunar system , Thus establishing the

UTEROPLACENTAL CIRCULATION .

At Day 13 :

Cells of the cytotrophoblast proliferate locally and penetrate into the syncytiotrophoblast, forming cellular columns surrounded by syncytium. Note : mesoderm pushing cyto. And the cyto. Will push on the

syncytio. Forming the villi . the main job of the villi is to increase the

surface area .

Cellular columns with the syncytial covering are known as PRIMARY VILLI .


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( The villi )from where did they come ?

3 layers :

1.extra embryonic mesoderm .

2. cytotrophoblast .

3.syncytiotrophoblast .

*the villus has many stages ( jay 3nhom so2al bel exam )

FIRST Stage { The primary villus where the villus is only formed by 2

layers the

a. cytotrophoblast

b. syncytiotrophoblast

SECOND Stage { The secondary villus where the extra embryonic

mesoderm penetrate between the cytotrophoblast and

syncytiotrophoblast , so its formed of 3 layers

a.extra embryonic mesoderm .

b. cytotrophoblast .

c.syncytiotrophoblast .


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THIRD Stage { The tertiary villus it’s the last stage where the extra

embryonic mesoderm those cells that have the ability to do

migration , proliferation and differentiation . form arteries the baby

side , so allow diffusion to take place faster . (explanation : diffusion

takes place between

syncytio. cyto. artery to baby directly

, rather than transferring the food and oxygen via diffusion using

the long path

syncytio. cyto. extra embry. Mesoderm connecting stalk

baby

its formed by 3 layers :

a. Syncyto.

b. Cyto.

c. Extra embryonic mesoderm which is differentiated to give

blood vessels

All the capillaries from the mesoderm get attached to a larger artery

in the chorionic plate that give rise to an artery in the connecting

stalk .

So the food is transfered from maternal blood to embryo capillaries

via diffusion , then via embryo blood in the capillaries to the artery in

the umbilical cord to the embryo .


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Note that the vacuoles in the syncytio. Had become lacunae then the

inter villus spaces is formed space between the 2 villi , the blood is

accumulated from the spiral arteries in these spaces .

The blood comes to inter villus space and then by diffusion the food

goes to the capillaries across the borders :

a.syncytio.

b.cyto.

c.extra emberyonic mesoderm .

d.artery wall .

Until the age of four months its acceptable for the embryo that the

placenta has those barriers but after that the embryo’s needs

increase , so the embryo require a larger amount of food so some

changes occur to the placenta after 4 months .

So after 4 months of pregnancy the placenta barrier is not accepted

to contain 4 walls ( syncytio , cyto, extra emberyonic meso. , artery

wall . some of the placenta walls strat to degenerate and only 2 walls

are left the ( syncytio. And the endotheilial lining of the artery . ) .

Why this is so ??

1st : before 4 months the lady may not be noticing her pregnancy so

she might me uptaking some drugs and other chemicals so the 4

walls placenta may protect the baby from allowing those drugs from

diffusing to the embryo’s body easily .


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2nd : before 4 months the size of the embryo is so small weight about

500 grams but after 4 months the size get considerably larger weight

about 3 kilograms ( the organs are growing ) , so larger amount of

food must be supplied for the fetus .

The placenta membrane after 4 months is composed of ( syncytio &

artery wall ) is not considered as a true barrier because it allows

most of the substances to diffuse freely .

The cytotrophoblast starts migration towards the endometrium and

form a continuous shell around the whole emberyo .

So two main functions for the trophoblast cells :

Syncytio. finding a food supply .

Cyto. protection by forming a shell .

After 4 month of pregnancy the decidua basalis give rise to septums

that divide the placenta to cotyledons ( small compartments ) , these

cotyledons are formed in the maternal side of placenta , non of them

is formed in the embryo side .

NOTE : the birth delivery is composed of three stages :

1st the directation of the uterus .

2nd delivery of the baby .

3rd removal –falling – of the placenta

So after we cut the umbilical cord we must check that all cotyledons

on the decidua basalis ( mother side ) are present and no spaces

between them , if a space is found between any of them, then we


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must remove the missing part manually form the uterus or

otherwise it will cause internal bleeding death .

So we must be able to differentiate between maternal & emberyonic

side of placenta , in the MATERNAL side we can see cotyledons and

the surface of the placenta is rough BUT @ the emberyonic side the

placenta surface is smooth because its covered by the amniotic

cavity .

After 2 weeks the placenta ( the syncytio. Cells ) secrets hCG

hormone which is detected by urine test .


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THE DOCTOR DIDN’T MENTION SOME SLIDES AT ALL , AND AS WE

KNOW HE WILL WRITE THE QUESTIONS ACCORDING TO HIS SLIDES

SO YOU MUST REFER TO THE SLIDES IF SOMETHING IS NOT

WRITTIN HERE .

DONE BY : ASEM AL QADDOUMI

THANK U ALL LOVE U ALL …

Iam sorry for any mistake in the sheet , I have tried my best although

most of you wont read it , w I promise me myself won’t read it .

Documents

Derivativies of the endodermal germ layer : While folding the