First weeks of development

Question 1

How implantation occur?

Answer 1

Implantation is the process of attachment of the blastocyst to the endometrium of the uterus. The point of contact allow exchange of nutrient, gases and waste between the mother and the child.

1. The blastocyst arrive in the uterine cavity.
2. The blastocyst and the endometrium make contact.
3. I Syncytial trophoblast cells (out layer of the trophoblasts, also called syncytiotrophoblasts) divide rapidly forming several layers (syncytial layer), and synthesize hCG hormone (human chorionic gonadotropin hormone), which sends signals to the corpus luteum to increase size and secrete progesterone. Syncytiotrophoblasts destroy the endometrium glands and vessels, via enzymes that attack extracellulat matrix.
4. II At the same time the ICM grow rapidly, forming a cavity (amniotic cavity) and a new layer (hypoblasts)
5. Syncytiotrophoblasts form lacuna, which will develop in the yolk sac.
6. Hypoblasts migrate from the ICM to the lacuna, forming the primary yolk sac.
7. The extraembryonic mesoderm proliferate between the yolk sac and the cytotrophoblasts (inner layer of trophoblasts)
8. The extraembryonic mesoderm start to diviide, forming a cavity: the chorion.
9. Other hypoblast cells migrate, which push the primary yolk sac forward, forming the secondary yolk sac.
10. The chorionic cavity expands.

Question 2

How is implantation regulated?

Answer 2

Implantation is mainly regulated by Endothelial Growth Factors Receptors and Heparin-binding EGF-like factor.

Question 3

What new layers are formed? And new cavities?

Answer 3

Layers:

• Ectoderm
• Endoderm
• Mesoderm

Cavities:

• Chorion (Ecto- + Mesoderm)
• Yolk sac (Endo- + Mesoderm)
• Amnion (Meso- + Ectoderm)
• Allantois (Meso- +Endoderm)