Third Week of Development and Gastrulation

What key change happens to the shape of the embryonic disc in week 3?

It becomes pear-shaped because the cranial part grows faster than the caudal part.

What are the five major processes in the embryonic disc during week 3?

1. Gastrulation
2. Formation of the primitive streak (Day 15)
3. Invagination (cell migration)
4. Formation and differentiation of intraembryonic mesoderm
5. Formation of the notochord and neurulation (neural tube) + beginning of folding

What is the chorion and its layers?

Outer wall of chorionic vesicle
• Outer: Syncytiotrophoblast
• Middle: Cytotrophoblast
• Inner: Extraembryonic somatic mesoderm

What are chorionic villi and what fills the space between them?

Finger-like projections from chorion that develop into primary, secondary, and tertiary villi; the spaces (lacunae) are filled with maternal blood from eroded uterine vessels → future intervillous spaces

What distinguishes primary, secondary, and tertiary villi?

• Primary: Core = cytotrophoblast; outer = syncytiotrophoblast
• Secondary: Core = mesoderm; outer = cytotrophoblast + syncytiotrophoblast
• Tertiary: Core mesoderm differentiates into fetal blood vessels

What fills the intervillous spaces?

Maternal blood

What is the cytotrophoblastic shell?

Cytotrophoblast penetrates the syncytium and reaches the maternal endometrium; cells from neighboring villi connect to form a continuous outer shell anchoring the chorionic vesicle

What are the three parts of tertiary villi and their roles?

• Stem villi – attached to chorionic plate
• Anchoring villi – fix chorion to decidua basalis
• Floating villi – branch off and float in maternal blood for nutrient/gas exchange

What are the two parts of the chorion and what do they become?

• Chorion frondosum – part with enlarged villi near decidua basalis → fetal part of placenta
• Chorion laeve – part with villi near decidua capsularis → atrophies

What is gastrulation and what does it result in?

Process where the bilaminar embryonic disc becomes trilaminar (ectoderm, mesoderm, endoderm) — all derived from epiblast

What two regions remain bilaminar?

1. Prochordal plate → future oropharyngeal membrane (mouth)
2. Cloacal membrane → future anal region

Where and how does the primitive streak form?

In the midline near the caudal end → epiblast cells migrate toward midline, forming the streak

What are the primitive groove, node, and pit?

• Groove: longitudinal depression along streak
• Node: bulge at cranial end
• Pit: depression at node – site where cells invaginate to form notochord

What is invagination and what does it produce?

Epiblast cells move toward and beneath the primitive streak:
a) Replace hypoblast → endoderm
b) Remaining epiblast → ectoderm
c) Some invaginated cells → intraembryonic mesoderm

Where does mesoderm not form?

Prochordal plate and cloacal membrane (remain ectoderm + endoderm only)

What is the notochord?

Rod-like midline structure → temporary axial skeleton → replaced by vertebral column

How does the pre-notochordal process form?

Cells from primitive node invaginate and migrate cranially between ectoderm and endoderm

What is the pre-notochordal canal and what does it connect?

Temporary canal from primitive pit through midline → connects amniotic cavity and yolk sac

What is the neurenteric canal and how is it formed?

Formed by degeneration of pre-notochordal floor + underlying endoderm → connects yolk sac with amniotic cavity

What is the notochordal plate?

Formed when roof of pre-notochordal canal fuses with endoderm and displaces laterally

What is the definitive notochord and its function?

A solid rod of cells extending from primitive pit to prochordal plate → acts as axial skeleton until vertebral column forms