Embryology Review

8/6/2019 Embryology Review

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Development and Stuff

By: Abe


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At about 3 days= MORULA


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DAY 4:

The zygote is divided into 2 parts

Oucer cells = Trophoblast (Future Placenta!!)

INNER CELL MASS OR EMBRYOBLAST

This is the BLASTOCYST

Chills for 2 days before implanting, zona pellucida needs to break off!


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SUMMARY OF THE FIRST WEEK

1. Ovulation1. Fertilization

2. Cleavage, morula, blastocyst

3. Zona pellucida disappears

4. Trophoblast invades superficial


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THE SECOND WEEK of Development


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The 7th to 8th day: INNER CELL MASS becomes 2 layers:

1. Top is Columnar layer: EPIBLAST

2.Cuboidal layer underneath: HYPOBLAST

These two cell layers form the BILAMINAR GERM DISC.


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The 7th to 8th day:THE INVASIVE TROPHOBLAST DIFFERENTIATES INTO 2 LAYERS:

SYNCYTIOTROPHOBLAST: aka badass layer, it just takes what it wants!

CYTOTROPHOBLAST(continues with characteristics similar to original

trophoblast, divides and provides new baby-cells to become syncytio-cells)


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DAY 8: Amnioblasts form between the cytotrophoblast and germ

layer. Push the Germ layer (Epi/Hypoblasts) and cytotrophoblast

layer apart. Gab in between is amniotic cavity.


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By the 9th to 10th

Complete invasion!!!!

NATIONALFIBRIN

PLUG DAY

(HUGE DEAL!! Know

the day fibrin plug

forms.)

Epiblast/Hypoblast iscurrently sandwiched. Above

is amniotic cavity and below

is primitive yolk sac-


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TROPHOBLAST

Syncytio layer aka

(Mr.badass) continues to

eat into the endometrium.

Lacunar spaces begin to

form within the Syncytio

layer (You can't have too

much awesome in contact

with itself).

HEUSER'S

(Exocoelomic)MEMBRANE

This guy detaches fromtrophoblast layer and forms

primitive yolk sac.

9

th

to 10th

Day


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The 11th to 12th Day :

Uterine lining grows

over fibrin plug

Embryoblast is reallyjust chilling, letting

the cytotrophoblast

and Mr. Badass do

most of the work.

Day 12: Hypoblast has

grown over Heuser's

Membrane.


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As the Syncytio layer eats its

way into the endometrium,

eventually it hits blood vessels.

The blood that leaks out gushesinto the lacunar spaces and

forms the intialUTEROPLACENTAL

circulation.

Some blood may leak out

(gross) and cause improper

pregnancy dating!!!!!

Day 13:

The Hypoblasts that grew

over Heuser's membrane

proliferate and form

Secondary Yolk sac. 11th to 12th Day


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CYTOTROPHOBLAST Little nubs form, these are PRIMARY STEM

VILLI will become important later.

13th Day


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The extraembryonic coelom

expands into a large cavity

known as the CHORIONIC

CAVITY. Extraembryonicmesoderm Covers inner part

ofcytotrophoblasts as well as

outside of yolk sac!

Extraembryonic mesodermcrosses the cavity only at the

CONNECTING STALK, which

connects the embryo to the

cytotrophoblast. This stalk will

become part of the UMBILICAL

CORD.


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The embryo is still a

bilaminar germ disc, but

in the future head

[cephalic] region, thehypoblast is thickened.

These columnar cells are

firmly attached to the

overlying epiblast. This

region is called the

PRECHORDAL PLATE.


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Implantation sites: Where shit can go wrong.

Fallopian is most common (90 percent of ectopics).

If implantation is near cervix, placenta previa!!!


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(B) Epiblast invaginate and some cells detach and form a new

layer. This process requieres FGF which is made by epiblast

cells .

Some invaginated cells displace the hypoblast forming the

endoderm , others tend to lie between the epiblast and

endoderm thus forming the mesoderm.Cells remaining in the

epiblast then form the ectodermal layer.

Know what each cell line develops into!!!!

Formation of the Germ Layers (B)

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A) A groove forms with epiblasts bulging on either side

The cephalic end of streak contains the primitive pit

Formation of the Primitive streak (A)

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Formation of the Notochord

Notochord is made up of theinvading mesodermal cells!

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Mesodermal cells invade

Between epiblast and

endoderm except where

cloacal and oropharyngeal

membranes form.

High yield!


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Allantois Formation. outpouching ) of the yolk sac by day 16 that

extends into the connecting stalk.

Development of the urinary bladder.

Blood vessels of the allantois become the umbilical arteries and vein.

With bladder enlargement, allantois becomes the urachus which

isrepresented as the median umbilical ligament in adults

Sagittal section,

17th day

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Body asymmetry

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Ectodermal Germ Layer -

Neurulation (Week 3)

Notochord induces the

ectodermal layer to get

thicker/proliferate and form

neural tube.

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Stages of

Neurulation

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Closure of the cranial neuropore occurs at day 25-26 ,

whereas closure of the caudal neuropore occurs at day 27-28.

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Fail to close at the head

is anencephaly

Failure to close at the

bottom is Spina Bifida

Clinical

correlation

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Neurulation also produces some

extra cells. These are Neural

Crest cells and do other

wonderful crap.

The surface ectoderm will

then form the epidermis of

the skin

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With neural tube formation, ectodermal thickenings

become visible in the cephalic region as otic and lens

placodes

Otic placode invaginates to form otic vesicles which willdevelop into structures forhearing and equilibrium

Lens placode invaginates during 5th week to form the lens of

the eyes

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Derivatives of the Ectoderm (memorize)

This germ layer forms structures and organs that

maintain contact with with the external world

1) Central nervous system

2) Peripheral nervous system

3) Sensory epithelium of ear , eye and nose

4) Epidermis of the skin including hair, nails

and mammary glands

5) The anterior portion of the Pituitary gland

6) Enamal of teeth

7) Derivatives of Neural crest cells29


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Mesodermal Germ Layer (also memorize)

By 17 days, mesodermal cells close to the midline form

a) Paraxial mesoderm Forms somitomeres. These break

apart and form scleratome (ribs), myotome(muscle) anddermatome (Dermis, NOT EPIDERMIS.

b) Lateral plate mesoderm 2 outcomes. Parietal (body

walls) vs Visceral (surrounds GI tube)

c) Intermediate mesoderm - Kidneys and Genitals

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Derivatives of Mesoderm (Memorize)

1)C.T., cartilage and bones ( except in the head and neck

region)

2)Striated, smooth and cardiac muscles

3) Blood and lymph vessels, blood cells, and heart4) Kidneys

5)Gonads and ducts

6)Cortex of adrenal gland NOT MEDULLA!!! That's

Neural Crest!!7)Spleen

8)Serous membranes lining the body cavities

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Endodermal germ Layer

Folding happens Cephalocaudally and then laterally. This folding

process forms the gut tube as the endoderm gets

swallowed in the folding process.

By the head will be foregut

By the tail will be hindgut

The midgut temporarily communicates with the yolk sacby the vitelline duct.

The oropharygeal membrane (composed of ectoderm and

endoderm ) ruptures in week 4. Amiotic sac is not in direct

communication.

The cloacal membrane (ectoderm and endoderm) ruptures

week7. ANUS

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Sagittal and Transverse sections

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Derivatives of Endoderm (memorize)

1)Epithelial lining of the gastrointestinal tract

2)Epithelial lining of the respiratory tract

3)Parenchyma of the tonsils, thyroid and parathyroid

glands, thymus, liver and pancreas4)Epithelial lining of the urinary bladder and most

of the urethra

5)Epithelial lining of the tympanic cavity and

auditory tube

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We had a question on this: In What range

is embryo the most sensitive to organ

damage? (Weeks 3-8)

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FGF2 VEGF

FGFfibroblast

growth factor

VEGF- vascular endothelial

growth factor

(blood islands)

Blood vessel Formation-by 2 ways :

1)Vasculogenesis - vessels arising from blood islands

2) Angiogenesis - new vessels arising from existing vessels

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21 Day Embryo--heart tubes fuse into a single

tube that starts to beat at day 21-22

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