EMBRYOLOGY
Lecture 3 & 4 Prof.Dr.Alhubaity
GENERAL EMBRYOLOGY
SECOND WEEK DEVELOPMENT
Is being considered as the week of ''TWOS'' and always
remember 2 in 2
nd
wk as:
1-The trophoblast becomes 2 layers
syncytiotrophoblast& inner cytotrophoblast.
2-Embryoblast becomes Bilaminar disc ,an outer
Ectoderm &innerer Endoderm.
3-Two cavities are formed during this wk an Amniotic
cavity & Yolk sac.
At Day 8 the Blastocyst is partially embedded in the
Endometrium & the part of Trophoblast that lies over the inner
cell mass(Embryonic pole)is differentiated into an outer dark
zone which takes the responsibility of implantation by opening
a tin y gap in the endometrial mucosa with the aid of
proteolytic enzyme(Syncytiotro..)&an inner pale(lighter) zone
having a clear cellular boundaries known as Cytotrophoblas
which directs the nutrients materials coming to it from the
outer Syncytiotrophoblast toward the inner cell
mass(Embryoblast).
The EMBRYONIC DISC is differentiated in to what is called
as Bilaminar germ disc consists of inner small rounded and
polyhedral cells facing the Blastocyst cavity known as
Endoderm(Hypoblast)& outer tall columnar layer Ectoderm
(Epiblast).
The amniotic cavity starts to form at this time whose roof is
formed by Amnioblast( derives from inner surface of
cytotrophoblast),while it's floor is the outer surface of the
Ectoderm(Epiblast).
DAYS 9 & 10:t of Development
1-Implantation is about to finish &fibrin coagulum
appears at implantation site.
2-Developmental changes in the Trophoblast is more
progress compared to that of the Embryonic disc which
is relatively slow& the trophoblast differentiate into 2
layers surrounding the Blastocyst completely at this
time.At the mean time Vacules& lacunae appears in the
Syncytiotrophoblastlayer,thus this stage is called the
Lacunar stage.
3-The Amniotic cavity becomes much larger.
4-Some cells from the Endoderm layer creeps on the
inner surface of the cytotrophoblast forming what is
called Heuser's membrane which becomes continuous
with the edges of the Endoderm& forming the primary
yolk sac(it's roof is by Endoderm & it's floor by
Heuser's membrane).
Days 11
th
& 12
th
Development
1-Implantation is completed at 12
th
day& fibrin clot is
replaced by endometrial cells.
2-The 2 cavities(Amniotic & primary yolk sac) show no
definitive changes.
3-The Trophoblast shows the following changes:
a-Lacunae increases in number &size,and some of them
communicates & open to other lacunae forming what is
called the Trabecular appearance in the Syncytium
b-More erosion of the stromal endothelial layer of the
capillaries produces sinusoids,as a result the syncytial
lacunae become continuous with sinusoids & maternal
blood enters the lacunae thus establishing what is called
Utero-placental circulation.
4-Formation of extraembryonic mesoderm
&coelomiccavity.The extraembryonic mesoderm is
formed from the inner surface of cytotrophoblast&
doesn’t share in the formation of any tissue or structure
of the embryo.Thus the trophoblast is called now as
chorion,while the Blastocyst is called now as Chorionic
vesicle.
The extraembryonic mesoderm includes 2 parts as :
a-Somatopleural mesoderm covering the amniotic
cavity.
b-Splanchnopleuric membrane covering the yolk sac.
The extraembryonic mesoderm forms the connecting
stalk at the site of connection of the embryo with the
trophoblast.This stalk will progress latelcn into the
umbilical cord..while the 2 parts of extraembryonic
mesoderm are separated from each other by the
extraembryonic coelom(chorionic cavity).
5-Decidual reaction at the site of attachment of the
embryo to the endometrium.The cells become larger in
size,polyhedral& storing materials to supply the embryo
with nutrient materials.This will progress into what is
called as Decidua Basalis which is considered as
maternal part of the placenta.
Day 13
th
1-The primary chorionic velli form from the
trophoblast around the embryonic pole & this will
progress into the fetal part of placenta(chorionic
frondosum).
2-The major part of the primary yolk sac is pinched
off&the remaining part becomes the secondary yolk
sac (Definitive yolk sac).
3-Bleeding may occur at the site of implantation&
confuses with menstrual bleeding.
4-The connecting stalk becomes more clear at this
day.
Day 14
th
At this time a localized area of thickening part named the
prochordal plate appears in the anterior part of the
endoderm layer.At this place the endoderm becomes
very adherent to the overlying ectoderm.
Day 14 is the day of prochordal plate& is also the day
of bleedings which might confuses the woman about the
exact date of her conception ( pregnancy).
THE EMBRYONIC PERIOD
Is the period of Organogenesis,where each of the 3
layers of Trilaminar disc gives rise to a number of
tissues&organs.It extends from 3
rd
-8
th
weeks…….
The derivatives of the Ectoderm are:
1-Neuroectoderm which gives rise to C.N.S ,, P.N.S
,& autonomic N.S.
2-Sensory epithelium of the ear ,nose & eye.
3-The epidermis of skin & skin appendages as
hair,nails,sweat 7sebacous glands.
4-The pituitary gland & Medulla of suprarenal gland (
Adrenal ).
5-The breast& sweat glands.
6-The epithelium of anterior part of Oral cavity &
lower part of Anal canal.
The Derivatives of the Mesoderm are :
1-The dermis of the skin & connective tissue.
2-The muscles of trunk & skeleton( except the skull)
1 & 2 form the Paraxial mesoderm which becomes
separated from day 20 into Somites.At day 20 the first
pair of somitesappears,then 3 pairs are added daily till
day 35 where they become 42-44 somites ,which are
arranged as 4 occipital, 8 cervical,12 thoracic, 5
lumbar,5 sacral & 8-10 coccygeals.
Each somites is differentiated in to 2 parts an inner
(ventromedial part) called Sklerotome( forms vertebrae &
bones ) & an outer (dorsolateral part) as Dermomytotome(
gives muscles & strip of skin).
3-The intermediate Mesoderm ( Nephrogenic cords)
gives rise to the Urogenital&accessory glands.
4- Lateral plate Mesoderm will gives rise to the
followings:
a-Connectivetissues,smooth muscles of viscera & limbs.
b-Bloodvessels,blood cells & lymphatic tissues & lymph
cells.
c-The Cardiovascular system.
d-The Spleen.
e-The Adrenal cortex.
The Derivatives of The Endoderm are:
1-The epithelial lining of Gastrointestinal tract including
liver &pancreas.
2-Thyroid¶thyroid glands + Tonsils.
3-The Tympanic cavity & pharyngotympanic tube.
4-The epithelial lining of the Pharynx.
5-Epithelial lining of Trachea ,Bronchii& lungs.
THE FETAL PERIOD
It extends from the 9
th
week to the end of pregnancy (40 wks)
i.e from beginning of 3
rd
month until delivery time ( birth of
baby).The pregnancy extends for 40 wks from the date of the
last menstrual period(L.M.P) which is equivalent to 280 days.
Or 38 wks from the date of fertilization( 266 days).
The length of the fetus can be estimated as Crown-Rump
length(CRL)i.e sitting position early in the fetal period ,or
Crown-Heel(CHL) i.e standing height later on during the fetal
period.The length is expressed in cms,then correlated with age
of the fetus.The growth in length is a striking feature in the 3
rd
-
5
th
month with an a verage of about 5 cm per month,while the
increase in weight is a striking feature during the last two
month with an a verage increase as 500 grams per month
during the last two months.
The features of fetus by the end of the 3
rd
month ( i.e
12-13 weeks) are:
1-The face appears to be more human looking.
2-The limbs reach their relativelength & position in
comparison to rest of body.
3-Primary ossification centers can be seen in some long
bones& skull.
4-The external genitalia becomes clear& sex can be
determined at this time.
5-The head is an enormous part of the fetus& is about half
the CRL(sitting ).
The Features at the 5
th
month are :
1-The head is about one third the crown-heel
length(CHL) i.e of standing length.
2-Fine lanugo hair, eye brows & head hair are also
visible.
3-By the end of 5
th
month the body weight is about 500
grams( 1/ 7 of birth wt).
4-Fetal movement is easily recognized by the pregnant
mother.
The weight of the fetus increases slowly during the first
half of pregnancy,where as it increases rapidly during the
2
nd
half of pregnancy,specially in lthe last 2 months, where
about 50% the full term baby weight is added.
At 6
th
to early 7
th
month
1-If borne ,it has no ability to survive because both the
respiratory & nervous systems are not fully
differentiated.
2-Has rounded appearance due to deposition of some fats
under the skin& the
Skin is covered by Vernix Caseosa..
If born by the end of the 7
th
month(28 wks),then it is able
to survive but with some difficulty &needs a special care
in the Intensive Neonatal care Unit( Cubicles).
The Features at the end of the 9
th
month( at delivery or
just before delivery)are:
1-The skull (Head) has the largest circumference of all
parts of the body(12-13 cms)
,which forms about one fourth of the CHL( standing
length is about 50 cms at birth).
2-The testes are now located in the scrotum ( i.eout side
the body).
3-The weight at birth is a bout 3000-3400 grams( Average
3200 gms).
4- The CHL is about 50cms& CRL is about 36 cms.
Some investigation are needed during fetal period as
followings:
1-Ultrasonography to assess placental & fetal size,to
diagnose Twins & Multiple pregnancy ,then to detect any
congenital anomaly before birth of the baby.
2-Amniocentesis,usually done after the 14
th
week( aspirate
sample of Amniotic fluid) for some biochemical analysis
is cases of spina bifida(neural tube defect)& anencephaly
where Alpha-Feto protein is increased.Also to detect some
inborne Biochemical error.
Also to examine some fetal cells after culture & Giemsa
stain.
3-Chorionic Vellous Sampling ( CVS),in order to do cell
analysis with out culture for chromosomal & Biochemical
analysis can be done from the 9
th
week of pregnancy.
By : Ody Drd