8th embryology lecture – dr. shireen...1 8th embryology lecture – dr. shireen 12/4/2012 the...

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8th Embryology Lecture – Dr. Shireen 12/4/2012

The Story of Embryology – Continue

Dr. Shireen began with a very quick revision for the last part we took with Dr. Maher al-Heidi…

For each stage, we will consider changes in the trophoblast and the embryonic cell mass…

After fertilization, the zygote undergoes many mitotic divisions to transform into the Morula.

When the morula changes to become the blastocyst (the blastocyst contains a cavity, the morula

doesn’t).

The blastocyst is formed of two types of cells:

a) The large condensed group of cells (كتلة الخلايا المتقاربة) is called the inner cell mass or embryonic

cell mass. This will later on form the baby.

b) The layer of cells around the inner cell mass and the cavity is called the trophoblast.

The blastocyst travels in the uterine tube, reaches the uterus, and (the blastocyst) undergoes

implantation.

Day 8: changes in trophoblast and embryonic cell mass.

- The trophoblast differentiates into two layers (at this stage, the trophoblast differentiation only

occurs at the embryonic pole of the blastocyst):

a) Cytotrophoblast (cyto=cell): columnar cells, mono-nucleated, cell walls are distinguishable.

b) Syncytiotrophoblast: multinucleated, non-distinguishable cell walls.

Ladies and gentlemen of duf3et 6ibb 2011-2017, Dr. Shireen announced that: 1- All her embryology lectures will be copy-paste (text and photos) from Langman’s. 2- There will be no lecture on Sunday 15/4/2012.

لقد تجنبت إضافة أي شرح من طرفي قدر المستطاع تجنبا للوقوع في الخطأ، فخير الكلام ما قل ودل.

ئدة مع الأمثال العربية والانجليزية التي أضفتها إلى هذه الورقة هي بعض مما قاله لي والداي وأرشدني في مرحلة التوجيهي، فوددت مشاركة الفا

الجميع.

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- The embryonic cell mass differentiates وإترصت into two rows, and forms the bilaminar embryonic

plate:

a) Epiblast (epi = فوق): dorsally (towards the baby’s back), columnar cells.

b) Hypoblast (hypo = تحت): ventrally (towards the baby’s face), cuboidal cells.

- Small cavities form in the middle of the epiblast, and then they group together in one larger cavity,

the amniotic cavity (amnion)… which is dorsal to the embryonic plate.

Day 9:

9 days

- Trophoblast:

a) At day 8, the cytotrophoblast and syncytiotrophoblast are only present on the embryonic pole.

In day 9 they grow (the cells divide) and form a border around the whole blastocyst.

b) The syncytiotrophoblast now contains small cavities, which group into empty lacunar spaces

(trophoblastic lacunae). Therefore, we call this stage the Lacunar Stage.

- Embryonic plate: as we said, it has two layers, the epiblast (dorsal) and the hypoblast (ventral).

a) The hypoblast produces new cells, which travel more ventrally and line the inner aspect of the

cytotrophoblast.

This forms Heuser’s membrane, which encloses (يحيط) a cavity called the primitive yolk sac.

** Therefore, dorsal to the embryonic plate is the amnion, and ventral is the yolk sac.

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Day 10, 11, 12:

11, 12 days

- Trophoblast:

a) The lacunar spaces enlarge (تزداد حجما) and communicate with the maternal sinusoids

(endometrium blood vessels). Then, the mother’s blood enters syncytiotrophoblast.

Uteroplacental circulation is formed (this part is the first stage in forming the placenta (المشيمة) ).

- The Embryonic Plate and the two cavities:

a) From the hypoblast and Heuser’s membrane, new cells will be produced and grouped together

=> a new tissue.

This tissue will be placed between Heuser’s membrane and the cytotrophoblast; now forming

the mesoderm (but this mesoderm isn’t included in the baby, so it is called extra-embryonic

mesoderm).

b) Small cavities will be formed in the extra-embryonic mesoderm, and these cavities will group in

one bigger cavity, the extra-embryonic coelom (coelom = cavity), which will separate the e.e

(extra-embryonic) mesoderm into two layers:

The Extra-embryonic Somatopleur mesoderm: (somato = related to body surface) lines the

cytotrophoblast.

The Extra-embryonic Splanchopleur mesoderm: (splanchno = related to the internal organs

or visceral parts) lines the yolk sac, the amnion, and the embryonic plate.

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Day 13:

a) The e.e coelom will separate the wall (trophoblast and e.e mesoderm) on the outside from the

embryonic disk and the two cavities on the inside.

The only communication between this wall and the embryonic disk is the connecting stalk, which

will be the future umbilical cord (الحبل السري) connecting the baby to the mother.

b) The cells in the endometrium will undergo a decidual reaction.

They (the uterus endometrium cells) are swollen with glycogen and lipid. Why? Because the

trophoblast depends for nutrition on absorption from the endometrium, and the bigger it becomes

the more nutrition it needs. It takes its nutrition through the syncytioblast by absorption from the

endometrium.

13 days

** In this stage, the whole structure is called the chorionic vesicle, no more blastocyst. It

consists of the 1- syncytiotrophoblast 2- cytotrophoblast 3- somatopleur 4- e.e coelom

5- Embryonic plate with its two cavities, محوطين by the splanchnopleur.

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Summary so far:

We started after fertilization and the formation of the morula. A cavity forms in it, and now it is called

the blastocyst. It consists of the embryonic cell mass and the trophoblast. The blastocyst will continue

with a single cavity. The moment the extra-embryonic coelom is formed, it is not called a blastocyst; it is

called a chorionic vesicle.

The embryonic plate ONLY AND ALONE will form the future embryo. The other layers and cavities form

the fetal membranes, which play a very important role in the development of the baby. These

membranes are (they are not part of the baby):

a- The chorion, which is formed of the syncytioblast, trophoblast, and e.e somatopleur. This forms part

the placenta.

b- The amnion, which is dorsal to the embryonic plate.

c- The yolk sac, which is ventral to the embryonic plate.

d- The connecting stalk, which is the future umbilical cord.

e- The allantoenteric diverticulum, to be in the urinary system. Fingerlike projection from the yolk sac

into the connecting stalk. معجزة! حتة مش مطلوووووبة!!!!!!! الحمد لله!!!!أنا مش مسدأ! دي

The placenta consists of two parts, partially chorion, and partially decidua. The sinusoids are not in the

chorion, they are in the endometrium.

وعندما سأل زيد الكايد عن "العصاي" اللي تحت الإمبريونيك ديسك، أجابت الدكتورة:

is a part of the yolk sac. The rest of والبالون اللي لتحت شوية ,is part of the mesoderm العصاية دي اللي تحت الديسك

the yolk sac (the part exactly under the embryonic disk/plate is called the secondary yolk sac. It used to

be named primitive yolk sac when it was surrounded by Heuser’s membrane.

Afterwards, the rest of implantation and the abnormal sites, which we took with Skilled the Iron Man والله

بعرف إنها دبشة، بس حاليا أنا جدا رايق

Implantation site at

end of 2nd week

Shoot for the moon… If you fail, you

will land between the stars

انطلق نحو القمر، فإن فشلت، سترسو بين النجوم

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، وهو تقريبا نهاية الاسبوع الثاني، ولحد هلا31كووووول الكلام ده، كان لحد نهاية اليوم

the embryonic plate, or disk, is bilaminar.

THIRD WEEK:

The embryo is formed of 3 layers, so it’s called the Trilaminar embryonic disk.

Now, what changes?

** At this stage, we must define the embryo into cephalic (cranial, superior, head) end, and caudal

(inferior, feet) end.

Left=inferior, Right=superior posterior view of embryonic plate

a) On the caudal (inferior) end, a groove is formed on the epiblast, called the primitive streak. It

lengthens تمتد cranially (superiorly).

Proliferation تزايد، انقسام of the cells on the superior end of the groove produces a bulge called the

primitive node (photo to the right).

On the cranial end, some epiblast cells change into tall columnar cells, then…

…These cells become firmly adherent متلاصق to the hypoblast cells, and this layer (epiblast and

hypoblast cells highly adherent to each other) is called the

buccopharyngeal membrane, which is important in the

development of the oral cavity (on the cranial end of the baby).

ذوواق... فمحب التقنية والعمل اليدو،، يدلل الجراحةالطب فيه ما يشبع جميع الأ

محب الوحدة والعزلة... يعمل في الملتبر مع العينات

ومحب التكنولوجيا والآلات... يعمل في قسم الأشعة

ومحب البشر... يدلل في تلصصات الباطنية

ومن أراد وقت حرا بعد الظهر... يتلصص بالجلدية أو العيون

النفاق والاحتيال والكذوب، وتذوكروا أنكم ليرة العقول، ومقصد الباحثين عن الحنان.لكن، احذوروا و

Once it is begun,

half of it is

done…

بمجرد بداية العمل،

تقترب النهاية...

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We take a cross section of the embryonic disk.

b) Epiblast cells undergo migration towards the primitive streak.

They detach تنفصل from the epiblast and become invaginated على نفسها وتشكل طبقةتلتف between the

epiblast and the hypoblast.

They become flask-shaped.

Then these cells start distributing تنتشر between the epiblast and the hypoblast and form a third

layer INSIDE the embryonic plate. This layer forms the intra-embryonic mesoderm (remember

extra-embryonic mesoderm between Heuser’s membrane and the cytotrophoblast).

c) Now, we forget the words epiblast and hypoblast. The layers of the embryonic disk are: ventrally

endoderm / dorsally ectoderm / in the middle the intra-embryonic mesoderm.

Quick revision:

=> On the caudal end of the embryonic plate the primitive streak was formed and the primitive

node on its superior end.

=> On the cranial end, epiblast cells become columnar w ma5zoo2a min ta7t w bte3mal the

buccopharyngeal membrane.

/ين فيه ليس أحقر من قولك بعد الاختبار ، "هذا ليس قصار جهدي"تفشل

، لتشعرك بالإنجازوليس أجمل من شعور النتيجة المفرحة بعد التعب

المتاعب، فهي التي تشعرك بجمال /ينبقيمة الحياة إن لم تواجه /ينولن تشعر

الراحة

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We take a cross-section of the embryonic disk (image in previous page).

Not all the disk is formed of 3 layers, only 3 parts…

The mesoderm spreads laterally and cranially, but doesn’t spread in all of the embryonic plate.

1- Formation of the notochord: The notochord is center of the embryo, and the vertebral column

will fit at this site.

We take a longitudinal section of the plate. (Images in next page)

2- On the midline, at the center of the primitive node, the primitive pit (حفرة صغيرة) will appear.

This pit will stretch and the formed line in the longitudinal section is called pre-notochordal

plane.

Primitive pit cells will undergo proliferation and the pre-notochordal plane will stretch in the

midline forwards between the epiblast and the hypoblast cranially, until it reaches the

buccopharyngeal membrane (where the epiblast and the hypoblast are firmly adherent).

In the end, by an unneeded process if we take a cross section, the notochord will be exactly in

the middle.

So the embryo, at this point, has 3 layers:

Ventrally the endoderm / dorsally and ectoderm / in the middle the mesoderm

There are 3 areas where the mesoderm is not present:

The midline is the notochord, which is the ax of the body where the vertebral column will form / second is the

buccopharyngeal membrane (superiorly, cranially) where NOTHING separates the epiblast from the hypoblast /

third is the cloacae membrane caudally (which is formed the same way the buccopharyngeal membrane was

formed).

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Longitudinal section

Made by Laith hayel al-ejeilat… والسلام الختام

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