Embryology Lec 2

Functional endometrium The part of the endometrium

responsible for the reaction to the hormones liberated from the both developing follicle

and the remaining of the follicle after the liberation of

the ovum

By the name of God Last time we have seen the fate of the fertilize the ovum when it travels through the uterine tube until it reaches the uterine cavity, and then formation of collections of cells we called it - at the beginning – morula, and then we call it blastocyst (in the uterine cavity), the mass of cells which are located on one pole are called inner cell mass, they will be directed toward the point of contact of the whole blastocyst with the functional endometrium.

(note: el doctor 7aka 2nno 27na met25reen OMG etha heek mt25reen keef law mo mt25reen allah estor :P )

That was in the last lecture

Now, let's remember these things: the outer covering of the whole mass was made of special cells which we named it trophoblast.

The trophoblast is capable of engulfing, during the end of the first week and the beginning of seconed week this trophoblast will find it way through the functional endometrium to be implanted … this is what we called it implantation.

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When the dividing zygote reaches the proximal end of the uterine tube and approaches the uterine cavity, a stage of 16 cells (16 divisions) and more is reached. In this stage, the whole structure is

called the morula.

(el doctor be7ke l 27mad tameme ma t8a6e3ne w etha 3endak 2e so2al bs 27kelak btes2lne l2nne 7afed el maw9'oo3 w ma bdde 25arbe6 OMG :0)

After the whole mass of blastocyst enters the endometrium, it becomes totally implanted.

The surface of the functional endometrium will seal the point of entry, so actually the wall of the lining of the uterus will become continuous, no more perforation or an opening for the site of implantation.

Note in the picture: the uterine cavity (1), covering of the lining of functional endometrium (2).

The whole mass which was outside (in the first week ) it will be totally outside in the second, and of course at the beginning when it was outside the whole collections of cells here( look at the figure: inner cell mass) will be inside the endometrium. The whole inner cell mass will show another cavity (remember we have a single cavity at the beginning we called it the yolk sac), there is

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another cavity when two layers of cell are formed the columnar one we call it epiblast, and the cuboidal one (facing the yolk sac (5)) we call it hypoblast. The epiblast will start also to surround a cavity (small cavity) we call it amniotic cavity (4), so the whole mass in the second week is: - Inside the functional endometrium. - Two germ layers are present (epibplast and hypoblast). - Two cavities one of them is the amniotic cavity facing the epiblast and the other one the large one is the yolk sac facing the hypoblast. Remember: that the whole process of implantation was actually made by the trophoblast.

- Note: the endomertrium is just a site ( like a bed) for implantation.

Now look at the diagram abobe

The whole trophoblast is going to continue to find( sorry; I couldn't hear it) because this mass is going to enlarge, and it is going to be held by the endometrium itself,( look at the diagriam) this will be the site of contact between the embryonic tissues and the maternal tissues, so the endometrium is in the mother, and the whole mass of blastocyst - at the beginning - is the embryo, when they become in contact, the point of contact will be in this region ( look at the diagram) the region of the outer covering ( the trophoblast).

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There will be an interaction between these two sites (the site surrounding the embryo and the site of the endometrium itself), the whole tropoblast area will be divided into two different regions, one of them is a proliferation of a large cellular layer which we call it the cytotrophoblast, and the endometrium itself is going to bring in irregular spaces blood vessels towards this implanted mass.

- Note: blood vessels are required for providing nutrients and oxygen to the developing embryo.

Because of the share or the part of endometrium approaching this cytotrophoblast (the cellular one) has no cellular boundaries we call this part as syncytiotrophoblast - that mean the tissue without cell boundary(cell membrane) - so the whole area in contact between this part of the embryo and this part of the endometrium is named by special name we call it chorion, this is actually made by acellular part (syncytiotrophoblast) and the cellular part (cytotrophoblast).

The chorione this is the two type of trophoblast one from the embryonic part and one from the maternal part this is actually going to be the placenta in the future. So, the chorione is the earliest foundation of the placenta, we know that the embryo is going to depend on the mother in nutrition and oxygenation, and this is the early foundation of the site which is responsible for nutrition of this growing embryo, the chorione.

- What's the chorione? - It’s the two layer of trophoblast.

- What are these two layers? - Of course the cellular and acellular (cytotrophoblast and syncytiotrophoblast)

- Is the syncytotrophoplast from the maternal site??

A: yes, from functional endometrium.

At this period which is the second week we mentioned it earlier, the whole mass of cells in the embryonic site are divided into two layers (

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Pregnancy testIt is a test for hormones

)Human chorionic gonadotropin(hCG)(

circulating in the blood and the urine of the

pregnant woman, used for expecting of the

pregnancy

epiblast and haypoblast) facing the two cavity, these two layers are also going to be transformed ( named) later on into ectoderm and endoderm. - Remember: we cannot detect pregnancy unless we reach the third week because the third layer must be added to these two layers, so that the three germ layer in human being will be established, and this will appear or will establish itself when the third week is reached.

In the beginning of third week - remember we have seen epiplast and the hypoplast ( endoderm and ectoderm) - a third layer will be formed between them and this is the mesoderm, so the three germ layers of the human embryo are formed. The process of formation of three germ layers is named by special name we call it gastrulation.

- It's something related to the formation three germ layers.- It's not related to stomach.

(take care : el doctor 7aka 2nno had so2al l2eem momken eege bel exam 3an el gastrulation allah esam7ak ea doctor laish hae el7arakat :’(

So the three layers are formed during the third week.

So, as the formation of the three germ layers (the process of gastrulation is completed), some of the mesodermal cell are located outside the mass of the embryo in continuous contact with the chorione (syncytiotrophoblast and cytotrophoblast), these mass of cellular connection of the developing embryo with the wall is called the connecting stalk (body stock), this actually marks the beginning of formation of the umbilical cord.

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The whole embryo and fetus will be connected to the maternal site by a cord - later on is going to be found within umbilicus - to the placenta and this will be the source of nutrition and oxygenation for the growing fetus.

So, the embryo appears like this, three germ layers, two cavities, one of them we called yolk sac facing the deepest layer of cell – the endoderm - , and amniotic cavity facing the surface layer of cells – the ectoderm - , and the third layer is mesoderm ( between the endoderm and ectoderm), that's what we call embryonic disc. Now, during the third week outside the whole embryonic disc and outside both cavities there is another large cavity which surround the whole mass of embryonic disc and the two cavities, we call it extraembryonic coelom ( extra; because it's outside).

-Note: if we examine any figure we can determine if it is in the third, that's if we find the three germ layers and the three cavities.

Q: what are these cavities filled with?- All of these cavities are filled with fluid.

Q: what layer builds the extraembryonic coelom?- It's only made by the mesoderm

Q: which cavity of them was in the blastocyst?- It's now only the yolk sac.

Q: Related to what we call the layers endoderm, ectoderm, and mesoderm?- relative to superficial and deep we will see that later but at this stage we don't know, but gradually we will see how we'll decide which one is dorsal and which one is the ventral.

Q: what are the cells inside the yolk sac?- They are derived from hypoblast (they are actually the periphery of the hypoblast).

So, at this stage a third week embryo will appear like this:

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- Three germ layers:

1- Ectoderm 2- Endoderm 3- Mesoderm

- Three cavities:

1- Amniotic: facing the ectoderm 2- Yolk sac: facing the endoderm 3- extraembryonic coelom: surrounding the whole embryonic mass- connecting stalk: is the condensation of mesoderm connects the whole embryonic disc to the outside.

- chorion: is the modified trophoblast both the cellular and acellular, the cellular which is derived from the embryonic part, and the acellular which is syncytiotrophoblast derived or it's the share of the maternal part.

Now, let's move toward ( still in the third week) the outer surrounding, the chorion, which is going to proliferate rapidly and become very rich in blood vessels ( within the syncytiotrophoblast) to form the future site of nourishing and oxygenating the growing embryo. ---- So the whole germ layers are formed and established by the third week.

So now we know what happened inside the embryonic disc, now if we open the amniotic cavity and looked at the dorsal of the embryonic disc, this is what we see……..

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Look at the figure

Now when we look at the dorsal of the embryonic disc (ectoderm), it will appear more or less oval in shape (A) (it will remain actually oval), in the book it's mentioned that it will become a bean shape and later toward the end of third week it becomes a slipper – like shape (D).

- Note: we are now following the embryo step by step until it reaches what we call the end of embryonic period.

In the human being, we divide pregnancy into two parts:

- The embryonic period: the period by which the fertilized ovum will become human embryo, this actually last about 8 weeks.

- The fetal period: from week 9 until week 36 (the end of the pregnancy), we call like that because the growing structure is fetus not embryo, so there is a difference between embryo and fetus.

So, we need 8 weeks to form the complete embryo, What decide this embryonic period? - In the end of this period, normally it should have the shape of a human being.

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All animals will have more or less similar early development of their embryos, but by the end of their embryonic period they should have the shape of that special animal.

REMEMBER: during this 8 weeks of embryonic period, we have two weeks the first two which are not considered a stage of development, so actually the embryonic period is including the week of fertilization and implantation, the second week of the two germ layers and the epiblast appear, and the third week which is actually a human embryo, so the whole process of development and formation of this human embryo will take place in the period between the third week and the end of the 8th week .

Now, let's return to the last figure.

Now in the dorsal of the embryo we will start to see certain modifications or certain thickening of the cellular element ( all this changes happens during third week), there is a thickening in the caudal end(figure A), and another thickening in the cranial end(figure A), the caudal one is in the axon of embryonic disc called primitive streak (Figure A), and the cranial one is called the prechordal plate (Figure A), there is actually a fusion between the surface facing the amniotic cavity (ectoderm) and the surface facing the yolk sac ( endoderm), this marks the two extremes.

If you follow the figure, you can see that the primitive streak elongate (Figure A-C) toward the cranial end until it reaches a certain thickening called the primitive node (Figure B), from this node at about day 17 (Figure B) a rapid growth of a new plate which is going cranially and we call it notochordal process, this actually grows very rapidly until it reaches the prechordal plate.

Now at this age (about the middle of the third week) we are going to call the prechordal plate the oropharyngeal membrane, and the caudal end of the primitive streak the cloacal membrane, so we have two areas we call them membranes, actually they are opining in the future embryo one of them(oropharyngeal) marks the oral cavity, and the cloacal membrane marks the anus.

At about the end of the 3rd week ( day 21), the whole center or axis of this plate (notochordal process) will become thickened and this is what we call it the notochord ( Figure D), notochord actually plays a very

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important role as becoming a rigid axis for the whole embryo, it acts as a guide for the developing embryo, but it will disappear later, the site of the notochord is the site where the axial skeleton will start to be formed, only remnant of the notochord remains in the adult, what remains of the original notochord is only the center of the intervertebral disc the nucleus pulposes ( thick fluid inside the intervertebral disc).

Q: depending on that the cloacal membrane is the distal opining of the digestive system and the oropharyngeal membrane is the proximal one, Is the digestive system the earliest system to be formed?

- No, the earliest system to be formed is the central nervous system and followed immediately by the cardiovascular system.

THE END

تعالى الله بحمد تم

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االهداااااااااااااااااااااء صفحه واخيراKHALED:PLEASE; FORGIVE ME FOR ANY MISTAKE

Ehda2 2la kol 25wane w25wate fe dof3et HOPEEhda2 ela kol el shabab eldof3ah w27la shabab bel uni kolha Wmashallah law bdde 23mal ehda2 w2thkor kol wa7ad bdde 100 saneh 9'aw2eeh ma b5alesSoEl2hda2 lalkol bela estethna2 w5o9a9an :

3emad 3babneh (sadee8 el3omer ); m7amad eldardoor(9a7eb el2btesameh el sa7erah ) ; 5aled 3'araibeh ( my bro (wa7d thane msh 2na ) ); 7mzeh domidat(my bro ) ; 7mzeh 3babneh(sde8eeeee w 25oee); ma7mood totanji(methel ma wa3dtak ); m7amad rshedat(:*)

Wehda2 mn el8aleb ela 7abebe w sade8e w 25oeee

m7ammad banydome ):*:*

w fe 2nsan 3'alee 3lle ktheeeeer ma ra7 27ke 2smo l2ne 2na 7or :P:P

:عماد عبابنة -.اعتذر عن أي خطأ.......... واعتذر عن قلة الصور

..........وبالنسبة لألهداء االهداء موجه لجميع طالب الدفعة.............

بالتوفيق للجميع

الى به تصعد سلما منها واصنع .. اجمعها بالحجارة تتعثر ال النجاح

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