3rd Wk of embryonic development

3rd week changes : germ disc changes and trophoblast changes

Germ disc development

1. Shape

2. Structure → gastrulation, spread and differentiation of intraembryonic mesoderm and notochord formation

Changes in trophoblast

1. Formation of chorionic villi

2. Formation of outer cytotrophoblastic shell

Change in germ disc shape

initially germ disc shape flattened and rounded → oval → pear shaped

Gastrulation (Def , Formation of primitive streak)

Def: Process where bilaminar embryonic disc → trilaminar embryonic disk.

(Forms primitive streak, 3 germ cell layers, forms prechordal plate)

Forms primitive streak Day 15 - 16

• Proliferation & migration of some epiblast cells in midline region at caudal end of embryonic disc

• Appars on epiblast surface as narrow groove with raised margins

• Cephalic end of streak becomes primitive node represented on epiblast surface by depression called primitive pit

Gastrulation (Formation of 3 germ cell layers + importance)

Forms 3 germ layers

• Some epiblast cells proliferate and migrate through primitive streak replace hypoblast → endoderm layer

• other epiblast cells proliferate and migrate through primitive streak between epiblast and endoderm → Intraembryonic mesoderm

• Remaining epiblast cells → Ectoderm

Importance: Epiblast source of the 3 germ layers → all embryo tissues and organs

Formation of prechordal plate and buccopharyngeal membrane

Formation of prechordal plate and buccopharyngeal membrane:

1. Endodermal cells near cranial end of embryonic disc enlarge to form the prechordal plate

2. Firmly attach to overlying ectodermal cells → form buccopharyngeal membrane (ectoendodermal membrane)

Germ disc development: Spread and differentiation of intraembryonic disc

(Spread)

Spread

Cells of intraembryonic mesoderm spread in all directions between ectoderm and endoderm besides:

1. buccopharyngeal membrane

2. cloacal membrane (caudal to primitive streak where ectoderm and endoderm firmly attached)

3. Middle region of embryonic disc between primitive node and the buccopharyngeal membrane (where notochord develops)

Germ disc development: Spread and differentiation of intraembryonic disc

(Differentiation incl Intraembryonic coelom)

Differentiation: 17th day intraembryonic mesoderm → paraxial, intermediate and lateral plate mesoderm.

Intraembryonic coelom:

• Small cavities in lateral plate mesoderm → cavities fuse form intraembryonic coelom ( u shaped cavity)

• The intraembryonic coelom divides lateral plate mesoderm into two layers → somatic and splanchnic (visceral).

Formation of notochord ( development stages)

1. Notochordal process - Day 17

• 17th day primitive node cells grow cranially in midline between ectoderm and endoderm till the prechordal plate

2. Notochordal canal - Day 18

• 18th day primitive pit invaginate notochordal process transforming it into canal

3. Neurenteric canal - Day 19

• Floor of notochordal canal fuses with underlying endoderm

• Degeneration of notochordal-endodermal junction → Neurenteric canal (temporary communication between amniotic cavity and yolk sac)

4. Definitive notochord - Day 20

• Roof of notochordal canal forms notochordal plate.

• Day 20 notochordal plate detach from endoderm & folded around longitudinal axis → Definitive notochord

• Endodermal cells proliferate to repair yolk sac roof

• Notochord forms temporary midline axis (will be replaced by permanent axial skeleton

Formation of notochord (Fate)

Fate of notochord:

Cranial part: included in the base occipital bone and body of sphenoid.

Vertebral part:

in body of vertebrae → degenerates and disappears

in vertebral disc → forms nucleus pulposus

Formation of chorionic villi in trophoblast (def of chorionic villi,

time and structure of stages )

Chorionic villi def: Finger like processes extending from chorion into endometrium

Development stages

1. Primary chorionic villi

• appears by end of 2nd week of embryonic development

• 1ry chorionic villus consists of cytotrophoblastic core + syncitiotrophoblastic (covering) layer

2. Secondary chorionic vill

• 3rd week 1ry villi invaded by extra embryonic mesoderm core

• 2ry chorionic villus formed of mesoderm core + cytotrophoblast then syncytiotrophoblast layer

3. Tertiary chorionic villi:

• End of 3rd week, mesodermal cells core invaded by chorionic blood vessels.

• Tertiary chorionic villus formed of : core of mesoderm (containing chorionic blood vessels) covered by cytotrophoblast then syncytiotrophoblast layer

Formation of outer cytotrophoblastic shell

Formation of outer cytotrophoblastic shell

Cytotrophoblastic cells in villi extend through the syncytiotrophoblast to reach the endometrium → come in contact with neighbouring villi → forms outer cytotrophoblastic shell.