Chapter 22. Week 3 of Development: Trophoblast and Villus Development - Review of Medical Embryology Book

THE TROPHOBLAST is characterized by many primary stem villi, consisting of a cytotrophoblast core covered by a syncytial layer, at the beginning of week 3

With development, mesodermal cells from the extraembryonic somatopleuric mesoderm or cytotrophoblast penetrate the core of the primary villi and grow in the direction of the decidua to form the secondary stem villi which consist of a loose connective tissue core covered by a cytotrophoblastic layer which, in turn, is covered by a thin syncytial layer

BY THE END OF WEEK 3, mesodermal cells in the villus core differentiate into blood cells and small blood vessels, forming the villous capillary system, and thus create the tertiary villi

By week 4, the tertiary villi are seen over the entire surface of the chorion
The capillaries in the tertiary villi contact capillaries developing in the mesoderm of the chorionic plate and in the connecting stalk, eventually contact the intraembryonic circulatory system, and connect the placenta and the embryo. Thus, in week 4, when the heart begins to beat, the villous system is able to supply the embryo with oxygen and nutrients, whereas prior to that time it was all done by diffusion

CYTOTROPHOBLAST CELLS in the villi penetrate the overlying syncytium to reach the maternal endometrium

They establish contact with similar extensions of neighboring villous stems to form a thin outer cytotrophoblast shell

The cytotrophoblast shell is seen on the embryonic pole initially and then expands toward the abembryonic pole until it covers the entire trophoblast, thus attaching the chorionic sac firmly to the maternal endometrial tissue

Villi attached to the maternal tissues via the trophoblastic shell are called stem or anchoring villi
Villi that grow from the sides of the stem villi are called branch villi, and it is through these that the major exchange of materials between the mother and the embryo takes place

BY DAYS 19 AND 20, the extraembryonic coelom or chorionic cavity enlarges, and the embryo is attached to its trophoblast shell only by a narrow connecting stalk

The stalk is composed of extraembryonic mesoderm which is continuous with the chorionic plate and is attached to the embryo at its caudal end
The connecting stalk or body stalk later develops into the umbilical cord to connect the placenta and the embryo