Explanation of development of human being in third week

1. General Embryology Topics
1. Subdivisions of the prenatal and postnatal periods
2. Male genital system and spermatogenesis
3. Female genital system. Oogenesis and folliculogenesis
4. Ovulation and formation of Corpus luteum. Structure of Uterus and Menstruel cycle.
5. Transport of gametes, fertilization and first week of human development
6. Second week of human development
7. Third week of human development (1st part)
8. Embryonic period and fetal period
9. Structure and functions of placenta and extraembryonic/fetal membranes: Chorion,
Amnion, Yolk Sac, Allantois and Umblical Cord
10. Multiple pregnancies

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Embryonic period: This period extends from beginning of the third week to the end of the eight week.
This period is also known as organogenetic period (All organs and systems are formed in this period).
The cells of each germ layer (Ectoderm, Endoderm and Mesoderm) divide, migrate, aggregate, and
differentiate in rather precise patterns as they form the various organ systems.
During this period, embryo has taken on recognizable human shape.
Embryonic period is also most risky and dangerous period of most affected by teratogens (Alcohol,
cigarette, some drugs, radiation and infections-TORCH).
Teratology is a branch of embryology that is concerned with the congenital anomalies or birth defects.
It deals with abnormal embryonic and fetal development.

3. Important Events of Third and Fourth Week of Human Development
I. Formation of Primitive Streak and then development of Trilaminar
Embryonic Disc (Ectoderm, mesoderm and endoderm)
II. Development of Notochord and function and importance of Notochord
III. Neurulation: Formation of Neural Tube (Neural tube will become brain and
spinal cord)
IV. Beginning of the development of chorionic villi of placenta
V. Early development of Cardiyovascular System
VI. Embryonic Folding
VII. Further development of trilaminar embryonic disc (Ectoderm, mesoderm
and endoderm)

4. The most characteristic event occurring during the third week of gestation is gastrulation.
Gastrulation is a process by which the bilaminar embryonic disc is converted into a
trilaminar embryonic disc.
Gastrulation is the beginning of morphogenesis (development of body form).
Epiblast
Hypoblast
Ectoderm
Mesoderm
Endoderm

5. Formation of primitive
streak/primitive groove and
primitive node/primitive pit:
At the beginning of the third week
(day 15), a longitudinal ridge
appears in the midline at the caudal
end of the dorsal aspect of the
bilaminar embryonic disc.
This longitudinal cellular ridge is
called primitive streak.
It is formed due to proliferation of
the Epiplast cells.
It becomes visible on the dorsal
surface of embryonic disc as a
narrow groove (Primitive groove)
flanked by a slight bulge on either
side.
At the cranial end of the primitive
streak, the cells proliferate and
form a rounded elevation called
primitive node (Hensen’s node)
surrounding a small primitive pit.

6. Primitive streak and Primitive
groove
Primitive Node
and Primitive Pit
Caudal Region of
bilaminar disc

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The epiblast cells on both sides of the primitive streak are detached from each other and migrate to the primitive
groove.
Upon arrival in the region of the streak, they become flask-shaped, detach from the epiblast, and slip beneath it.
This inward and downward movement of epiblastic cells is known as invagination.
The epiblast cells enter from the primitive groove between the epiblast and the hypoblast layers.
First targets of invaginated epiblast cells are hypoblastic cells.
Hypoblast cells that see the epiblast cells begin to die with apoptosis.
Hypoblast is removed from here and replaced with new epiblast cells.
This new cell layer is now called the “Intraembryonic Endoderm Layer”.
* Other epiblast cells come to lie between the epiblast and the newly formed endoderm to form the ‘’intraembryonic
mesoderm layer’’.
* The remaining cells of the epiblast now form the ‘’intraembryonic ectoderm layer’’.
* all the three primary germ layers are derived from epiblast cells

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Mode of spread of intraembryonic
mesoderm:
The intraembryonic mesoderm
spreads in cranial, caudal, and lateral
directions into all parts of the
embryonic disc, except in the
following regions:
* Region of prechordal plate where
the ectoderm and endoderm are in
firm contact with each other and it
will become the buccopharyngeal
membrane and mouth.
* Region of cloacal membrane
(circular area at the caudal end of the
disc): Here also the ectoderm and
endoderm are in an intimate contact
with each other.
It will become anus.

10. Important Events of Third and Fourth Week of Human Development
I. Formation of Primitive Streak and then development of Trilaminar
Embryonic Disc (Ectoderm, mesoderm and endoderm)
II. Development of Notochord and function and importance of Notochord
III. Neurulation: Formation of Neural Tube (Neural tube will become brain and
spinal cord)
IV. Beginning of the development of chorionic villi of placenta
V. Early development of Cardiyovascular System
VI. Embryonic Folding
VII. Further development of trilaminar embryonic disc (Ectoderm, mesoderm
and endoderm)

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Cranial end
Oropharyngeal
membrane (Mouth)
Caudal end
Cloacal membrane (Anus)
Formation of Notochord:
The notochord is a midline
epiblastic cellular cordon that
develops in the region
between the primitive node
and the prechordal plate
(buccopharyngeal membrane).
Primitive Node and pit

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Formation of Notochord:
The notochord is a midline epiblastic cellular
structure that develops in the region between the
primitive node and the prochordal plate
(buccopharyngeal membrane).
Understanding of development of notochord is
essential because;
* it forms the central axis of the embryonic disc and
gives it some rigidity.
* It induces the ectodermal cells and then formation
neural tube and nervous system.
* It induces the further development of
intraembryonic mesodermal layer cells.
Fate of Notochord:
In human beings, it appears only in embryo.
In later life, it disappears but its remnants are seen
in the form of nucleus pulposus of the intervertebral
discs.
Remnants of notocord can lead to Chordoma
tumors.

14. Notochord
Primitive node
and pit
Cranial end
(Buccopharyngeal
membrane)

15. Important Events of Third and Fourth Week of Human Development
I. Formation of Primitive Streak and then development of Trilaminar
Embryonic Disc (Ectoderm, mesoderm and endoderm)
II. Development of Notochord and function and importance of Notochord
III. Neurulation: Formation of Neural Tube (Neural tube will become brain and
spinal cord)
IV. Beginning of the development of chorionic villi of placenta
V. Early development of Cardiyovascular System
VI. Embryonic Folding
VII. Further development of trilaminar embryonic disc (Ectoderm, mesoderm
and endoderm)

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Notochord cells
induces and
promotes
ectodermal cells to
development of
nervous system
and also induces
further
development of
mesodermal layer.

17. Neurulation (16-22 days): The process of formation of neural tube
1. The notochord cells induce the ectoderm cells
on it and thicken the edges of this layer to form
the neural plate
2. In the middle of the neural plate, a trough
called neural groove and neural folds occurs
3. During the process of neural plate formation, a
group of cells on the most surface of ectoderm
layer are differentiated into neural crest cells
4. Neural folds converge by approaching midline,
and form neural tube surrounded by
neuroepithelial cells around the neural cavity
5. After the neural tube has been formed, the
ectoderm layer is identified by 3 different names:
I. Outermost: Surface Ectoderm (Outer ectoderm)
II. In the innermost: the neuroepithelial layer of
the neural tube (Neuroectoderm)
III. In the middle: Neural Crest cell layer which
moved to migrate to different regions of body

19. Neuroepithelial layer of the
neural tube (Neuroectoderm)

20. Important Events of Third and Fourth Week of Human Development
I. Formation of Primitive Streak and then development of Trilaminar
Embryonic Disc (Ectoderm, mesoderm and endoderm)
II. Development of Notochord and function and importance of Notochord
III. Neurulation: Formation of Neural Tube (Neural tube will become brain and
spinal cord)
IV. Beginning of the development of chorionic villi of placenta
V. Early development of Cardiyovascular System
VI. Embryonic Folding
VII. Further development of trilaminar embryonic disc (Ectoderm, mesoderm
and endoderm)

21. Development of Chorionic Villi of Placenta
1. Primary villus, 12-13 days:
The cytotrophoblast forms fingerlike projections that invade the
syncytiotrophoblast.
This finger-like projection of cytotrophoblast surrounded by a layer of
syncytiotrophoblast is called the primary villus.

22. Primary Chorionic Villus
(Inner cytotrophoblast
Outer syncytiotrophoblast)
Extraembryonic Somatic
Mesoderm
Cytotrophoblast
Syncytiotrophoblast
Desidua
Basalis

23. Development of Chorionic Villi of Placenta
2. Secondary villus, 15-16 days:
The extraembryonic somatopleuric mesoderm lying deep to the cytotrophoblast
now invades the center of each villus.
As a result, now each villus consists of three layers.
From inside to outside these are mesoderm, cytotrophoblast, and
syncytiotrophoblast.
This villus is now termed secondary villus.

24. Secondary Chorionic Villus
Extraembryonic mesoderm
Cytotrophoblast
Syncytiotrophoblast

25. Development of Chorionic Villi of Placenta
3. Tertiary villus, 20-21 days:
The blood vessels develop in the mesoderm of the secondary villus.
The secondary villus with blood vessels in its mesoderm is called tertiary villus.

26. Tertiary Chorionic Villus
Fetal Vessels
Mesoderm
Cytotrophoblast
Syncytiotrophoblast

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Desidua Basalis