3rd week of embryo development
DESCRIPTION
embryo,anatTRANSCRIPT
3rd week of embryo development
Notochord
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
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)
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
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
B- the floor of the notochordal canal fuse with the endoderm and degenerate------------- Notochordal plate.
Notochordal plate
Neurenteric canal
C- infolding of the notochordal plate------- solid rod-like structure (Notochord).
ectoderm
mesoderm
endoderm
Notochordal plate
Notochord
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
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.
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.
Development of trophoblast
Chorionic villi
Primary chorionic villi
1
2
Secondary chorionic villi
• Syncytiotrophoblast.• Cytotrophoblast.• Mesodermal core
Angiogenesis
Formation of blood vessels and blood cells occur in extraembryonic mesoderm during 3rd week------ extend in the secondary villi--------tertiary villi
Tertiary chorionic villi
1
23
• 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.
Mesoderm
Extraembryonic• Formed during
week 2.• Involved in
chorionic villi and placenta formation
Intraembryonic • Formed during
week 3.• Forms various
tissues and organs.