7th lecture the third week of development

7/27/2019 7th Lecture the Third Week of Development

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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-overview


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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

7th lecture the third week of development

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