Development
The chorion undergoes rapid proliferation and forms numerous processes, the chorionic villi, which invade and destroy the uterine decidua and at the same time absorb from it nutritive materials for the growth of the embryo.
They undergo several stages, depending on their composition.
| Stage | Description | Period of gestation | Contents |
| Primary | The chorionic villi are at first small and non-vascular. | End of 4th week | trophoblast only |
| Secondary | The villi increase in size and ramify, while the mesoderm grows into them. | 5th week | trophoblast and mesoderm |
| Tertiary | Branches of the umbilical vessels grow into the mesoderm, and in this way the chorionic villi are vascularized. | 5th - 6th week | trophoblast, mesoderm, and blood vessels |
Until about the end of the second month of pregnancy the villi cover the entire chorion, and are almost uniform in size, but after this they develop unequally.
Read more about this topic: Chorionic Villi
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