week 3

Chorion

Choruonic vesicle wall -Chorion is chorionic vesicle wall, formed of somatic extraembryonic mesoderm,

cytotrophoblast & syncytiotrophoblast (from internal to external). It secretes

chorionic gonadotropin which maintains corpus luteum (progesterone source

in ovary) for 4 month

Chorionic Viking definition time of formation

Projections from chorion (chorionic vesicle wall).

*Time of formation: Start at 2nd gestational week end then continues during 3rd week.

Type

*Types: a)Primary villi: Cytotrophoblast cells proliferate & push syncytio-

trophoblast forming primary villi, which are separated from each other by

lacunae filled with maternal blood.

b)Secondary villi: Formed when somatic extraembryonic mesoderm

enters primary villi core, at middle of 3rd week.

c)Tertiary villi: Formed when fetal blood vessels appear in secondary

villi mesoderm at 3rd week end. Separated by intervillous spaces filled with

maternal blood. Cytotrophoblastic shell is formed when cytotrophoblast cells

penetrate syncytiotrophoblast at stem villi apices & extend to surrounds

chorionic villi & intervillous space

Parts of 3 v

Parts of tertiary villous: Each villous is composed of;

1-Stem (anchoring) villous: Villous extending between chorion & decidua basalis.

2-Free (floating or absorbing) villi: Side branches from stem villous,

floating in maternal blood inside intervillous spaces. Responsible for nutrient

& gases exchange with maternal blood .

Part of chorionic

Parts of chorion: a)Chorion frondosum (chorionic plate): Part

carrying well developed tertiary villi & facing decidua basalis.

b)Chorion laeve: Rest of chorion carrying degenerating tertiary villi &

covered with decidua capsularis.

-Fate of chorion: Chorion frondosum (chorionic plate) persists sharing in

placenta formation, while chorion laeve degenerates

Gastrulation

Transformation of bilaminar embryonic disc into trilaminar disc,

starting by development of primitive streak & primitive node in epiblast layer