3rd week of embryo development
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3rd week of embryo development
Notochord
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Gastrulation • Established 3 germ
layers (trilaminar embryonic disc)
• Epiblast migrate from primitive streak to:
1. hypoblast-----endoderm.
2. Inbetween epiblast and hypoblast------intraembryonic mesoderm.
3. Remainder of epiblast-----ectoderm
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Intraembryonic mesoderm
• Epiblast remain fused with hypoblast (NO mesoderm) at:
1. cranially: buccopharyngeal membrane (future site of mouth).
2. Caudally: cloacal membrane (future site for anus)
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Notochord • Solid extension
epiblast• Lie in midline.• Extends from
primitive node to prochordal plate.
• Induces changes in:1. ectoderm-----neural
plate (CNS).2. Mesoderm---
vertebral bodies
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Steps of Notochord Formation1. Epiblast cells invaginate
from primitive node to prochordal plate (buccopharyngeal membrane)-------solid cord = NOTOCHORDAL PROCESS
2. Primitive pit extends in the process---------NOTOCHORDAL CANAL
Notochordal process
Notochordal canal
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B- the floor of the notochordal canal fuse with the endoderm and degenerate------------- Notochordal plate.
Notochordal plate
Neurenteric canal
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C- infolding of the notochordal plate------- solid rod-like structure (Notochord).
ectoderm
mesoderm
endoderm
Notochordal plate
Notochord
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NOTOCHORD• Form the basis for the
axial skeleton• Induces the
development of vertebrae.
• Its remainder forms the nucleus pulposus of the intervertebral disc.
Bodies of vertebra
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The first indication of gastrulation in the embryo is:
• Formation of extraembryonic mesoderm.• Formation of chorionic villi.• Formation of notochord.• Formation of primitive streak.• Formation of prochordal plate.
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For a woman the first indication that she is pregnant is a missed period. In which week
of embryonic development will she experience her first missed period?
• Start of week 1• Start of week 2.• Start of week 3.• Start of week 4.
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Development of trophoblast
Chorionic villi
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Primary chorionic villi
1
2
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Secondary chorionic villi
• Syncytiotrophoblast.• Cytotrophoblast.• Mesodermal core
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Angiogenesis
Formation of blood vessels and blood cells occur in extraembryonic mesoderm during 3rd week------ extend in the secondary villi--------tertiary villi
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Tertiary chorionic villi
1
23
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• cytotrophoblast penetrate syncytiotrophoblast and make contact with the endometrium.
• proliferate forming outer cytotrophoblastic shell-----------
1. Firmly attaches chorion to endometrium.
2. Prevents further invasion by syncytiotrophoblast.
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Mesoderm
Extraembryonic• Formed during
week 2.• Involved in
chorionic villi and placenta formation
Intraembryonic • Formed during
week 3.• Forms various
tissues and organs.