Chorionic villi
Primary villi -
11-13 days
Cytotrophoblast covered by
syncytiotrophoblast
Secondary villi -
16 days
Core of extraembryonic
mesenchyme
Tertiary villi -
21 days
Embryonic blood vessels
Chorionic cavity
Anchoring villus
Stem villus
Normal implantation:
Larsen, WI, Human Embryology, 3rd ed., Churchill Livingston, NY (2001), p. 45
Mesoderm in villi form blood cells and small blood vessels.
These make contact with capillaries in chorionic plate and connecting stalk.
Embryo primitive streak - gastrulation beginning at 16 days
Note cytotrophoblastic shell.