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.