43. Implantation. Formation of a two-layer germ disc.

Question 1

Implantation

Answer 1

Definition: Embedding of embryo in endometrium.

Timing: Starts 6th-7th day after fertilization, lasts ~40 hours.

Stages:
1. Adhesion (Adzechia):

-Trophoblast attaches to uterine epithelium.
-Forms two layers:
–Inner: Cytotrophoblast (individual cells).
–Outer: Syncytiotrophoblast (syncytium).

2. Invasion (Penetration):
   -Syncytiotrophoblast produces proteolytic enzymes, destroys endometrium.
   -Trophoblast forms primary villi penetrating the endometrium.

Question 2

Formation of the two-layer germ disc

Answer 2

Structure: Two-layer disk.

Upper Layer (Epiblast):
-Forms amniotic sac.
-Outer Layer: Develops into amnion.
-Inner Layer: Develops into embryo.

-Lower Layer (Hypoblast/Primary Endoderm):
–Forms primary yolk sac, which develops into secondary yolk sac.