During the third week of development, the bilaminar embryonic disk transforms into a trilaminar structure through the process of gastrulation. The primitive streak forms and epiblast cells migrate through it to form the mesoderm germ layer. Neurulation also occurs as the neural plate forms and folds in on itself to become the neural tube. This establishes the basis for the nervous system. By the end of the third week, the three germ layers—ectoderm, mesoderm, and endoderm—have formed and begun developing into tissues and organs.

1. 3rd Week of development and
derivatives of germ layers
By
Dr. Abdul Waheed Ansari
Chairperson & Prof. Anatomy,
RAKCODS.RAKMHSU.
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2. Learning outcomes of the topic
• What are the embryonic changes occurring
during 3rd week of intrauterine life ( I.U.L.)?
• Formation of primitive streak with the cranial end
of future embryo and caudal end defined
• Gastrulation and creation of the third germ layer
from the epiblast cells.
• Neurulation is the formation of neural tube from
the epiblast cells.
• Formation of neural crest cells (N.C.C.)
• Derivatives of all germ layers.
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3. The bilaminar germ disk changes into
trilaminar germ disk
• The primitive streak is formed on the surface of
the embryonic disk.
• The ectoblast cells migrate through the primitive
streak and form the mesoblast.
• At this stage the embryonic disk is oval -shaped
and the ectoblast is bathed in amniotic fluid.
• From the 17th day a thickening of the embryonic
disk begins around the median line along the
rostro-caudal axis.
• This median structure (primitive streak) lengthens
until it occupies roughly half the embryo.
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4. At the anterior end, a groove forms in the ectoblast (primitive groove).
The cranial region is strengthened by the epiblast cells and so forms the
primitive pit with the primitive node (Hensen's node in birds).
• The head of the embryo will form at the extremity of the
embryonic disk near the primitive pit.
• The dorsal germinal layer becomes the ectoblast/-derm.
• The middle germinal layer (3rd germinal layer) becomes the
mesoblast/-derm.
• The ventral germinal layer receives the name endoblast/-
derm.
• The epiblast cells form the ectoblast, the mesoblast and the
intraembryonic endoblast.
• The hypoblast cells give rise to the extra embryonic
endoblast (the umbilical vesicle and the allantois).
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5. The embryonic disk 3rd week of I.U.L.
• 1. Primitive groove
• 2. Primitive pit
• 3. Primitive node
• 4. Oropharyngeal membrane
• 5. Cardial plate
• 6. Sectional edge of amniotic membrane
• 7. Mesoderm
• .
• 8. Endoderm
• 9. Future cloacal membrane
• NB -1+2+3 primitive streak 9/24/2014 5

6. Gastrulation is the emergence of
mesoderm from the epiblast cells
• Third germinal layer, the
intraembryonic mesoblast.
• The mesoblast cells wander
in all directions: laterally,
cranially and caudally.
• This middle germinal layer
lies between the definitive
endoblast and epiblast.
• Exceptions are the cloacal
membrane as well as the
pharyngeal membrane,
where the ectoderm and
endoderm lie directly
opposite each other.
1. Primitive groove
2. Epiblast
3. Extraembryonic mesoblast
4. Definitive endoblast
5. Invading epiblastic cells forming the
intraembryonic mesoblast.
6. Hypoblast
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8. Notochord is formed during III week of
development
• From the epiblasts that
make up the floor of the
amnion cavity.
• The notochord forms the
first longitudinal midline axis
around which the vertebral
bodies are organized and is
the basis for the axial
skeleton.
• The notochord induces the
formation of the neural plate
(neurulation), synchronizing
the development of the
neural tube.
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9. Neurulation
• The embryonic ectoderm
over the developing
notochord thickens to form
a neural plate .
• The ectoderm of the plate is
called neuroectoderm and
eventually gives rise to the
central nervous system
(brain and spinal cord).
• Neuroectodermal cells at
the lateral edge of the
neural plate -do not become
part of the tube but form a
neural crest over the neural
tube and give rise to the
neural crest cells.
• By the end of week 3, the
neural folds move together,
fuse, and convert the neural
plate into the neural tube.
• Closure begins in the middle
of the embryo and
progresses toward both
cephalic and caudal ends. It
begins on day 21.
• The anterior or cranial
neuropore closes in week 4
(day 26), whereas the
posterior or caudal
neuropore closes near day
28.
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10. Defects in the closure of neural pore
will give rise to
• Anencephaly – when
the anterior neuropore
does not close, the
brain will not develop
and the fetus will be
born with out brain
inside the cranial cavity.
• Spina bifida – there will
be a wide open gap at
the lower end of spinal
cord and it will be
exposed out as a
vesicle, as the posterior
neuropore doesn't
close.
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11. Neural tube formation and neural crest
cells migration
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12. The allantois
• Appears on day 16 as a
small, fingerlike out
pouching or diverticulum
from the caudal wall of
the yolk sac.
• It remains small in the
human embryo, is
involved with early blood
formation, and is related
to the development of
the urinary bladder.
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13. The primary germ layers
• endoderm, mesoderm, and ectoderm are formed
and organized in their proper locations during
gastrulation.
• Endoderm, the most internal germ layer, forms
the lining of the gut and other internal organs.
• Ectoderm, the most exterior germ layer, forms
skin, brain, the nervous system, and other
external tissues.
• Mesoderm, the middle germ layer, forms muscle,
the skeletal system, and the circulatory system.
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14. The neural crest cells
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15. Ectoderm derivatives
• The central nervous system;
• The peripheral nervous system;
• The sensory epithelium of the ear, nose and eye;
• The epidermis, hair and nails; and
• The subcutaneous, mammary and pituitary gland;
• The enamel of teeth.
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17. References
• http://labspace.open.ac.uk/mod/resource/vie
w.php?id=285192
• http://cro.sagepub.com/content/14/2/78.figu
res-only
• http://www.embryology.ch/anglais/hdisquee
mbry/triderm01.html
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