Chapter 4: Third Week Of Development

Question 1

Third week of development

Answer 1

The most characteristic events that occur during the third week of gestation are the:
1. Development of chorion.
2. Gastrulation of the embryonic disc.

Question 2

What is a chorion?

Answer 2

The wall of the chorionic vesicle.

Question 3

What is the chorion composed of?

Answer 3

It is composed of:
1. somatic extraembryonic mesoderm.
2. Cytotrophoblast.
3. Syncytiotrophoblast (from internal to external).

Question 4

What are chorionic villi?

Answer 4

Projections from the wall of chorionic vesicle (chorion).

Question 5

What is the time of formation of the chorionic villi?

Answer 5

They start forming at the end of the 2nd gestational week, then continues during the 3rd week.

Question 6

What are the types of chorionic villi?

Answer 6

1. Primary villi.
2. Secondary villi.
3. Tertiary villi.

Question 7

Primary villi

Answer 7

Cytotrophoblast cells proliferate and push synctiotrophoblast to form the primary villi which are separated from each other by lacunae filled with maternal blood.

Question 8

Secondary villi

Answer 8

They are formed when somatic extraembryonic mesoderm enters the core of the primary villi. They are formed at the middle of the third week.

Question 9

Tertiary villi

Answer 9

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.

Question 10

Parts of the tertiary villus

Answer 10

Each villous is composed of:
1. Stem (anchoring) villus.
2. Free (floating or absorbing) villi.

Question 11

Stem (anchoring) villus

Answer 11

Stem of the villous that extends between the chorion and decidua basalis.

Question 12

Free (floating or absorbing) villi

Answer 12

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.

Question 13

What are parts of the chorion.

Answer 13

1. Chorion frondusom (chorionic plate).
2. Chorionic laeve.

Question 14

Chorionic frondosum (chorionic plate)

Answer 14

Part of the chorion that carries well developed tertiary villi that faces the Decidua basalis.

Question 15

Chorionic laeve

Answer 15

The rest of the chorion that carries degenerating tertiary villi that are covered with decidua capsularis.

Question 16

What is the fate of chorionic frondosum (chorionic plate)?

Answer 16

Persist to share the formation of the placenta.

Question 17

What is the fate of chorionic laeve?

Answer 17

It degenerates.

Question 18

What is Gastrulation?

Answer 18

Transformation of bilaminar embryonic disc into trilaminar disc.

Question 19

How does Gastrulation start?

Answer 19

It starts by the development of primitive streak and primitive node in the epiblast layer.

Question 20

Primitive streak site

Answer 20

Appears in the midline of the caudal of the embryonic disc as a median narrow groove with bulging sides.

Question 21

Primitive streak development

Answer 21

It is developed from proliferation and migration of epiblast towards the primitive groove,

Question 22

How does the primitive streaks appearance help identify directions?

Answer 22

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.

Question 23

Primitive node

Answer 23

Rounded bulge in the cephalic end of the primitive streak with middle depression known as the primitive pit.

Question 24

Invagination

Answer 24

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.

Question 25

What are the results of invagination?

Answer 25

The embryonic disc becomes formed of 3 germ layers: 1. Ectoderm 2. Mesoderm 3. Endoderm

Question 26

Buccopharyngeal membrane

Answer 26

A rounded area of fusion between the ectoderm and endoderm at the cranial part of the embryonic disc.

Question 27

Cloacal membrane

Answer 27

Rounded area of fusion between ectoderm and endoderm at the caudal part of the embryonic disc.

Question 28

Shape change of the embryonic disc

Answer 28

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.

Question 29

Notochord

Answer 29

Temporary axial skeleton of the embryonic disc.

Question 30

Prenotochordal process

Answer 30

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.

Question 31

Notochordal canal

Answer 31

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.

Question 32

Nuerenteric canal

Answer 32

Temporary communication between amniotic cavity and yolk sac due to degeneration of flow of the canal together with the median endoderm fused with it.

Question 33

Definitive notochord

Answer 33

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.

Question 34

Importance of notochord

Answer 34

1. Temporary axial skeleton of the embryonic disc. 2. During folding, its firmness limits head fold. 3. Vertebral column is formed around it.

Question 35

Fate of notochord

Answer 35

Vertebral column and invertebral discs are formed around the notochord, which persists as nucleus pulposus inside the invertebral disc.

Question 36

Intra-embryonic mesoderm formation

Answer 36

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).

Question 37

Intra-embryonic mesoderm is not present at the follow sites:

Answer 37

1. Buccopharyngeal membrane. 2. Cloacal membrane. 3. Site of developing of notochord and neural tube (median region between primitive node and prochordal membrane).

Question 38

When is the intro-embryonic mesoderm divided?

Answer 38

At the 17th day. It is divided into: 1. Paraxial-intermediate. 2. Lateral plate-mesoderm.