Gastrulation and Embryonic Development

Questions and Answers

The villous system is established

The embryo is detached from its trophoblastic shell

The villous system is ready to supply the embryo proper with essential nutrients and oxygen (correct)

The chorionic cavity becomes smaller

Chorionic villi

Free or terminal villi

Umbilical villi

Stem or anchoring villi (correct)

To anchor the embryo to the mother

To establish contact with the intraembryonic circulatory system

To develop into the umbilical cord

To supply the embryo with essential nutrients and oxygen (correct)

By the 19th or 20th day Signup and view all the answers

The umbilical cord Signup and view all the answers

To supply the embryo with essential nutrients and oxygen Signup and view all the answers

By the 19th or 20th day Signup and view all the answers

To provide space for the growing embryo Signup and view all the answers

Free or terminal villi Signup and view all the answers

The villous system is ready to supply the embryo proper with essential nutrients and oxygen Signup and view all the answers

Study Notes

Cells invaginate, displacing hypoblast to form embryonic endoderm, and some cells come to lie between epiblast and newly formed endoderm to form mesoderm.
Remaining cells in the epiblast form ectoderm.
The epiblast is the source of all germ layers, and cells in these layers will give rise to all tissues and organs in the embryo.

Cells migrate beyond the margin of the disc, establishing contact with the extraembryonic mesoderm covering the yolk sac and amnion.
In the cephalic direction, cells pass on each side of the prechordal plate and the node in a cephalic direction.

Prenotochordal cells invaginate in the primitive pit, move forward, and reach the prechordal plate.
As the hypoblast is replaced by endoderm cells, cells of the notochordal plate proliferate and detach from the endoderm, forming a solid cord of cells, the definitive notochord.
The notochord underlies the neural tube and serves as the basis for the axial skeleton.

The primitive streak shows regressive changes, rapidly shrinks, and soon disappears by the end of the 4th week.
The primitive streak at the caudal end of the disc continues to supply new cells until the end of the 4th week.
Germ layers begin their specific differentiation by the middle of the 3rd week in the cephalic part, and by the end of the 4th week in the caudal part.

The beginning of the 3rd week of development is a highly sensitive stage for teratogenic insult.
Fate maps can be made for various organ systems, which may be damaged by teratogens.
This stage is reached 2 weeks after fertilization, approximately 4 weeks from the last menses.

By the beginning of the 3rd week, the trophoblast is characterized by primary villi consisting of a cytotrophoblastic core covered by a syncytial layer.
Mesodermal cells penetrate the core of primary villi and grow toward the decidua, forming secondary villi.
By the end of the 3rd week, mesodermal cells in the core of the villus begin to differentiate into blood cells and small blood vessels, forming the villous capillary system.

The villus is now known as a tertiary villus or definitive placental villus.
Capillaries in tertiary villi make contact with capillaries developing in mesoderm of the chorionic plate and in the connecting stalk.
These vessels establish contact with the intraembryonic circulatory system, connecting the placenta and the embryo.