8/3/2019 4 the Second Week of Human Development E-learning

http://slidepdf.com/reader/full/4-the-second-week-of-human-development-e-learning 1/19

The Second Week of HumanThe Second Week of HumanDevelopmentDevelopment

8/3/2019 4 the Second Week of Human Development E-learning

http://slidepdf.com/reader/full/4-the-second-week-of-human-development-e-learning 2/19

22ndnd Week of Human DevelopmentWeek of Human Development

Continuation of Implantation

Formation of Amnion, Embryonic disc & Yolk sac

Development of the Chorion

8/3/2019 4 the Second Week of Human Development E-learning

http://slidepdf.com/reader/full/4-the-second-week-of-human-development-e-learning 3/19

ImplantationImplantation

Starts at the end of 1st week (6th /7th day)

I. Blastocyst attachment to endo. Wall

II. Trophoblasts proliferate (?) & differentiate:

1. Cytotrophoblast:

mononucleated cells that surround the blastocyst

mitotically active

2. Syncytiotrophoblast:

expanding multinucleated mass of tissue

cells from cytotrophoblast join & loose their boundaries

8/3/2019 4 the Second Week of Human Development E-learning

http://slidepdf.com/reader/full/4-the-second-week-of-human-development-e-learning 4/19

Day 10

Day 12

8/3/2019 4 the Second Week of Human Development E-learning

http://slidepdf.com/reader/full/4-the-second-week-of-human-development-e-learning 5/19

SyncytiotrophoblastSyncytiotrophoblast

Originates from trophoblasts that fuse to each other & invade the

endometrium

Fxn.:Fxn.:

1. Secrets proteolytic enzymes:digest endo. C.T.

blastocyst can invade & embeds itself in the endo.

2. Produces Human Chorionic Gonadotropin H. (HCG):

maintains the activity of corpus luteum

to secret progesterone

maintenance of fxnal layer & pregnancy phase

8/3/2019 4 the Second Week of Human Development E-learning

http://slidepdf.com/reader/full/4-the-second-week-of-human-development-e-learning 6/19

HCGHCG

F orms the basis for the pregnancy test 

Secreted by syncytiotrophoblast into lacunae where

it enters the maternal bld.

Produced by start of 2nd week

¼    Cannot be detected until end of 2nd week

Enough amount of HCG is produced at

end of 2nd week & can be detected in either 

bld. or urine

8/3/2019 4 the Second Week of Human Development E-learning

http://slidepdf.com/reader/full/4-the-second-week-of-human-development-e-learning 7/19

Chronology of ImplantationChronology of Implantation

Day 6/7:attachment to endometrial wall & division of trophoblasts

Day 8:

Formation of lacunae within syncytiotrophoblast,

Continuation of invasion,Start secretion of HCG

Day 10:

blastocyst is completely embedded in endometrium

bld. Clot (closing plug) is formed

Day 12:

uterine epith. Is completely regenerated

lacunae fuse to form lacunar networks

8/3/2019 4 the Second Week of Human Development E-learning

http://slidepdf.com/reader/full/4-the-second-week-of-human-development-e-learning 8/19

Day 10

Day 12

Day 8

8/3/2019 4 the Second Week of Human Development E-learning

http://slidepdf.com/reader/full/4-the-second-week-of-human-development-e-learning 9/19

Amnion & Embryonic DiscAmnion & Embryonic Disc

Cells within inner cell mass separated & a cavity appears Amniotic cavity

Cells of inner cell mass that forms the roof of amniotic cavity

Amnion

Remaining cells of inner mass arrange in 2 layers

Forming a flattened circular bilaminar plate

³The Embryonic Disc´

8/3/2019 4 the Second Week of Human Development E-learning

http://slidepdf.com/reader/full/4-the-second-week-of-human-development-e-learning 10/19

Amniotic CavityAmniotic Cavity

Starts to be formed by 8th day

Initially: above the embryonic disc

Later: entirely surrounds the embryo

Filled with amniotic fluid:

initially: from maternal filtrate

later: fetus urine

Fxn.: shock absorber & prevents dry out of the fetus&

& adhesions with surrounding tissue

Amnion ruptures just before birth & amniotic fluid flows out

³bag of water´

8/3/2019 4 the Second Week of Human Development E-learning

http://slidepdf.com/reader/full/4-the-second-week-of-human-development-e-learning 11/19

8/3/2019 4 the Second Week of Human Development E-learning

http://slidepdf.com/reader/full/4-the-second-week-of-human-development-e-learning 12/19

Embryonic DiscEmbryonic Disc

A bilaminar circular plate of embryoblasts

2 layers:

Epiblast:

high columnar cellsfloor of amniotic cavity

ectoderm

Hypoblast:

small cuboidal cells

facing blastocyst cavity

endoderm

8/3/2019 4 the Second Week of Human Development E-learning

http://slidepdf.com/reader/full/4-the-second-week-of-human-development-e-learning 13/19

 Yolk Sac Yolk Sac

At 8th day also:

Cells from hypoblast start to migrate &

surround the blastocyst cavity

Blastocyst cavity = exocoelomic cavity

Migrating hypoblasts exocoelomic

memb.

8/3/2019 4 the Second Week of Human Development E-learning

http://slidepdf.com/reader/full/4-the-second-week-of-human-development-e-learning 14/19

 Yolk Sac Yolk Sac

Exocoelomic memb. (cells) start to separate from the

cytotrophoblast layer 

Once seperated:

exocoelomic cavity 1o

yolk sac

At this stage (12 th day ):

Embryonic disc lies between

Amniotic cavity & 1o

yolk sac

L.C.T. layer between yolk sac

& cytotrophoblast formed

extraembryonic mesoderm

8/3/2019 4 the Second Week of Human Development E-learning

http://slidepdf.com/reader/full/4-the-second-week-of-human-development-e-learning 15/19

 Yolk Sac Yolk Sac

Functions:

supplies nutrients to

embryoduring 2nd & 3rd weeks

Prevents adhesions

* It decreases in size and as

development progress

8/3/2019 4 the Second Week of Human Development E-learning

http://slidepdf.com/reader/full/4-the-second-week-of-human-development-e-learning 16/19

Extraembryonic CoelomExtraembryonic Coelom

Extraembryonic mesoderm & isolated spaces appear within it

³Extraembryonic Coelom´

- Fluid filled space

- Surrounds Amnion & Yolk sac

- Splits extraemb. Mesoderm

into 2 layers:

Ex traemb. Somatic mesod.

Ex traemb. Splanchnic mesod.

8/3/2019 4 the Second Week of Human Development E-learning

http://slidepdf.com/reader/full/4-the-second-week-of-human-development-e-learning 17/19

Extraembryonic MesodermExtraembryonic Mesoderm

Splits by extraemb. Coelom into:

1. Extraembryonic Somatic

mesoderm:

lines trophoblasts

2. Extraembryonic splanchnic

mesoderm:

surrounds yolk sac

3. Connecting stalk:

connects embryonic disc to

trophoblast

Future umbilical cord

8/3/2019 4 the Second Week of Human Development E-learning

http://slidepdf.com/reader/full/4-the-second-week-of-human-development-e-learning 18/19

The ChorionThe Chorion

T he membrane that surrounds the embryo & will become later asthe principal part of placenta

It is composed of:

Extraemb. Somatic mesoderm

Cytotrophoblast

Syncytiotrophoblast

8/3/2019 4 the Second Week of Human Development E-learning

http://slidepdf.com/reader/full/4-the-second-week-of-human-development-e-learning 19/19

The chorion develops by the end of 

2nd week (day 14):

Extraembryonic coelom

chorionic cavity

Fxn. Of Chorion (future placenta):

1. Protection

2. Exchange of materials between

mother & fetus

3. Secretion of hCG