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Third week ofdevelopment Dr. eman khammas alsadi
University of mysan
Embryology lecturer
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The third week of development
trilaminar germ disc
The most characteristic event occur in the third week
is
1-Gastrulation
2- Invagination.
3-Notochord formation.
4-Establishment of the body axes.
5-fate map established during gastrulation
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1-gastrulation :which is the process that establish
all the 3 layers in the embryo
(ectoderm ,mesoderm ,endoderm)
starts with the formation of
1. primitive streak on the epiblast
surface.
2. primitive node is a slight
elevation ,at the cephalic
region of the streak
3. the primitive pit ;there is a
small pit inside the node
The primitive streak:Is a narrow groove with a
slight bulging on either
sides. This streak becomes
visible at 15-16 day of
development
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2-invagination:
theprocess by which the epiblast cells willgive rise to 2 layers as in the followings
1-invaginated cells from the epiblast at the
region of the streak &the node will form
the mesoderm which then give rise to
endoderm
2-the remaining of epiblast cell form
ectoderm
So the epiblast gives rise
to3 layers in the embryo
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The mesodermcontinue to migrate
between theepiblast & theendoderm until itestablish a contactwith the extra
embryonicmesoderm
In the cephalic regioncells of themesoderm fromeach side will formthe cardiogenic
plate
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3-Development of thenotochord
prenotochordial cells :
i.e invaginated Epiblast cells nearthe prechordial plate whichbecome intercalated with thehypoblastic cells& form thenotochordial plate
with further development the
plate detaches from the
endoderm & from the solid cord
known asdefinitive notochord.The notochord form s a mid-
line axis which will serve as
the base of the axial
skeleton.
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4-Establishment of the body axes:-1-anteroposterior
2- dorsoventral3- left-right
takes place (before and during) the period ofgastrulation.
The anteroposterior axis is signaled by cells at theanterior (cranial) margin of the embryonic disc.
expresses genes essential for head formation establish
the cranial end of the embryo before gastrulation.
The primitive streak itself is initiated first then thedorsal and ventral mesoderm and head and tailstructures.
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fate map established during gastrulation
Regions of the migrating epiblast have beenmapped, and their ultimate fates have been
determined ,For example,1-(origin):cells that ingress through the cranial region
of the node become (fate ): prechordal plate andnotochord;
2-(origin)lateral edges of the node and from thecranial end of the streak (fate): paraxial mesoderm;
3- (origin)cells migrating through the midstreak regionbecome (fate) intermediate mesoderm;
4-(origin) migrating through the more caudal part ofthe streak form (fate) lateral plate mesoderm;
5-(origin) cells migrating through the caudalmost partof the streak contribute to(fate) extraembryonic
mesoderm (the other source of this tissue is theprimitive yolk sac [hypoblast]).
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Thebeginning of the third week of development,
when gastrulation is initiated, is a highly sensitive
stage for teratogenic insult.For example, high doses of alcohol at this stage kill
cells in the anterior midline of the germ disc,
producing
a deficiency of the midline in craniofacial structures
and resulting in holoprosencephaly .In such a child, the forebrain is small, the two lateral
ventricles often merge into a single ventricle, and the
eyes are close together (hypotelorism)
Clinical correlate
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Gastrulation itself may be disrupted by
1- genetic abnormalities and
2- toxic insults.
A- In caudal dysgenesis (sirenomelia), insufficient
mesoderm is formed in the caudalmost region of theembryo. Because this mesoderm contributes toformation of
A. the lower limbs,
B. urogenital system (intermediatemesoderm),
C. and lumbosacral vertebrae,
They exhibit a variable range of defects, including
1. hypoplasia and fusion of the lower limbs,
2. vertebral abnormalities,
3. renal agenesis,4. imperforate anus,
5. and anomalies of the genital organs .
is associated with maternal diabetes
B- Situs inversus is a condition in which transposition ofthe viscera in the thorax and abdomen occurs.
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1-left-right patterning
abnormal cilia syndrom (Kartagenersyndrome). situs inversus, chronic sinusitis, and
bronchiectasiscilia are normally present on the ventral surface of the
primitive node and may be involved in left-rightpatterning during gastrulation
laterality sequences. Patients with these conditions do not have
complete situs inversus but appear to bepredominantly bilaterally left-sided or right-sided.The spleen reflects the differences;
1. those with left-sided bilaterality havepolysplenia,
2. and those with right-sided bilaterality haveasplenia or hypoplastic spleen.
3. Patients with laterality sequences are also likelyto have other malformations, especially heart
defects.
T A i d Wi h
http://emedicine.medscape.com/article/232791-overviewhttp://emedicine.medscape.com/article/296961-overviewhttp://emedicine.medscape.com/article/296961-overviewhttp://emedicine.medscape.com/article/232791-overview7/27/2019 7th Lecture the Third Week of Development
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Tumors Associated WithGastrulation
sacrococcygeal teratomas:
1-This is the most commontumor in newborns.
2-frequency of one in 37,000.
3-contain tissues derived fromall three germ layers
4-Sometimes, remnants ofthe primitive streak persistin the sacrococcygealregion.
These clusters of cellsproliferate and form thetumor or may also arisefrom primordial germ cellsthat fail to migrate to the
gonadal ridge
f th d l t f th t h bl t
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further development of the trophoblast
1-By the beginning of the 3rdweek, the trophoblast is characterized by primary villithat consist of a cytotrophoblastic core covered by a syncytial layer
2-During further development, mesodermal cells penetrate the core of primary villiand grow toward the decidua. The newly formed structure is known as a secondaryvillus
3-By the end of the third week, mesodermal cells in the core of the villus begin todifferentiate into blood cells and small blood vessels, forming villous capillary system
,The villus is now known as a tertiary villus or definitive placental villus.
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4- Capillaries in tertiary villi make contact with capillaries developing inthe mesoderm of the chorionic plate and in the connecting stalk ,Thesevessels establish contact with the intraembryonic circulatory system,connecting the placenta and the embryo.
when the heart begins to beat in the 4th week of development, the villoussystem is ready to supply the embryo proper with essential nutrients andoxygen
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anchoring villi The villi contact withneighboring villous stems, attaches thechorionic sac firmly to the maternalendometrial tissue).
(terminal) villi: Villi that extend from thechorionic plate to the decidua basalis
Other branches from the sides of stem villi arefree , through which exchange of nutrients
and other factors will occur.
The chorionic cavity, meanwhile,
becomes larger, and( by the 19th or20th day), the embryo is attached to
its trophoblastic shell by a narrowconnecting stalk ,
The connecting stalk later develops into theumbilical cord,
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Thank you
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