Chapter 4: Third Week Of Development Flashcards
Third week of development
The most characteristic events that occur during the third week of gestation are the:
1. Development of chorion.
2. Gastrulation of the embryonic disc.
What is a chorion?
The wall of the chorionic vesicle.
What is the chorion composed of?
It is composed of:
1. somatic extraembryonic mesoderm.
2. Cytotrophoblast.
3. Syncytiotrophoblast (from internal to external).
What are chorionic villi?
Projections from the wall of chorionic vesicle (chorion).
What is the time of formation of the chorionic villi?
They start forming at the end of the 2nd gestational week, then continues during the 3rd week.
What are the types of chorionic villi?
- Primary villi.
- Secondary villi.
- Tertiary villi.
Primary villi
Cytotrophoblast cells proliferate and push synctiotrophoblast to form the primary villi which are separated from each other by lacunae filled with maternal blood.
Secondary villi
They are formed when somatic extraembryonic mesoderm enters the core of the primary villi. They are formed at the middle of the third week.
Tertiary villi
They are formed when fetal blood vessels appear in the mesoderm of the secondary villi at the end of the third week. They are separated by intervillous filled with maternal blood. Cytotrophoblast she’ll is formed when the cytotrophoblast cells penetrate syncytiotrophoblast at the apices of the stem villi and extend to surround chorionic villi and intervillious spaces.
Parts of the tertiary villus
Each villous is composed of:
1. Stem (anchoring) villus.
2. Free (floating or absorbing) villi.
Stem (anchoring) villus
Stem of the villous that extends between the chorion and decidua basalis.
Free (floating or absorbing) villi
They are side branches from the stem villous that float in the maternal blood inside the intervillous spaces. They are responsible for exchange of nutrients and gases.
What are parts of the chorion.
- Chorion frondusom (chorionic plate).
- Chorionic laeve.
Chorionic frondosum (chorionic plate)
Part of the chorion that carries well developed tertiary villi that faces the Decidua basalis.
Chorionic laeve
The rest of the chorion that carries degenerating tertiary villi that are covered with decidua capsularis.
What is the fate of chorionic frondosum (chorionic plate)?
Persist to share the formation of the placenta.
What is the fate of chorionic laeve?
It degenerates.
What is Gastrulation?
Transformation of bilaminar embryonic disc into trilaminar disc.
How does Gastrulation start?
It starts by the development of primitive streak and primitive node in the epiblast layer.
Primitive streak site
Appears in the midline of the caudal of the embryonic disc as a median narrow groove with bulging sides.
Primitive streak development
It is developed from proliferation and migration of epiblast towards the primitive groove,
How does the primitive streaks appearance help identify directions?
After it’s appearance, it is possible to identify the embryos cranio-caudal axis, ventral and dorsal aspects as well as right and left sides.
Primitive node
Rounded bulge in the cephalic end of the primitive streak with middle depression known as the primitive pit.
Invagination
Epiblast cells migrate towards the primitive groove to pass through it towards the hypoblast to form:
1. Endodermal layer that replaces the hypoblast.
2. Intraembryonic mesoderm which forms the middle layer in the embryonic disc.
3. Notochord in the median region of the embryonic disc.
The remaining epiblast layer is called ectoderm and its junction with amnion is called amnio-ectodermal junction.
What are the results of invagination?
The embryonic disc becomes formed of 3 germ layers:
1. Ectoderm
2. Mesoderm
3. Endoderm
Buccopharyngeal membrane
A rounded area of fusion between the ectoderm and endoderm at the cranial part of the embryonic disc.
Cloacal membrane
Rounded area of fusion between ectoderm and endoderm at the caudal part of the embryonic disc.
Shape change of the embryonic disc
At the beginning of the third week, the disc is oval in shape then due to the spread of intraembryonic mesoderm, the cranial part becomes broader than the caudal part, giving the disc a pear shape.
Notochord
Temporary axial skeleton of the embryonic disc.
Prenotochordal process
Formed of the invagination of solid cord of cells from the primitive pit. Prenotochordal process extends cranially in the midline between ectoderm till the buccopharyngeal membrane.
Notochordal canal
Primitive pit extends into the process transforming it into a canal. It’s roof is in contact with the ectoderm, while the floor is fused with the endoderm.
Nuerenteric canal
Temporary communication between amniotic cavity and yolk sac due to degeneration of flow of the canal together with the median endoderm fused with it.
Definitive notochord
Notochordal plate becomes folded upon itself to form a solid cord (definitive notochord). Endoderm regenerates so the amniotic cavity and yolk sac regain their separation from each other.
Importance of notochord
- Temporary axial skeleton of the embryonic disc.
- During folding, its firmness limits head fold.
- Vertebral column is formed around it.
Fate of notochord
Vertebral column and invertebral discs are formed around the notochord, which persists as nucleus pulposus inside the invertebral disc.
Intra-embryonic mesoderm formation
Cells of the intra-embryonic esoteric originate from the epiblast that migrate to the primitive node. Then they slip through the primitive groove and pit to invaginate between the ectoderm and endoderm.
- The intra-embryonic mesoderm forms the middle layer of the embryonic disc, transforming it into a trilaminar disc (Gastrulation).
Intra-embryonic mesoderm is not present at the follow sites:
- Buccopharyngeal membrane.
- Cloacal membrane.
- Site of developing of notochord and neural tube (median region between primitive node and prochordal membrane).
When is the intro-embryonic mesoderm divided?
At the 17th day. It is divided into:
1. Paraxial-intermediate.
2. Lateral plate-mesoderm.
