Chorionic villi
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| Chorionic villi | |
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| Diagram of a transverse section, showing the mode of formation of the amnion in the chick. The amniotic folds have nearly united in the middle line. Ectoderm, blue; mesoderm, red; entoderm and notochord, black. (Villi of chorion labeled at upper left.) | |
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| Model of human embryo 1.3 mm. long. (Villi of chorion labeled at lower right.) | |
| Gray's | subject #12 60 |
| MeSH | A10.615.284.473.200 |
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.
The chorionic villi are at first small and non-vascular, and consist of trophoblast only, but they increase in size and ramify, while the mesoderm, carrying branches of the umbilical vessels, grows into them, and in this way they are vascularized.
Blood is carried to the villi by the branches of the umbilical arteries, and after circulating through the capillaries of the villi, is returned to the embryo by the umbilical veins.
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.
The greater part of the chorion is in contact with the decidua capsularis, and over this portion the villi, with their contained vessels, undergo atrophy, so that by the fourth month scarcely a trace of them is left, and hence this part of the chorion becomes smooth, and is named the chorion læve; as it takes no share in the formation of the placenta, it is also named the non-placental part of the chorion.
On the other hand, the villi on that part of the chorion which is in contact with the decidua placentalis increase greatly in size and complexity, and hence this part is named the chorion frondosum.
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[edit] Floating Villi
These villi are found floating freely in the intervillous space. They exhibit a bi-layered epithelium consisting of cytotrophoblasts with overlaying syncytium (scyntiotrophoblasts).
[edit] Anchoring Villi
These villi act to stablise mechanical integrity of the placental-maternal interface. Anchoring villi lack a continuous syncytium, and instead they are featured as villous tips with multilayered columns of large mononuclear cytotrophoblasts.
[edit] Tissue composition and cell types
The bulk of the villi consist of connective tissues in which blood vessels are found. Most of the cells in the connective tissue core of the villi are fibroblasts. Macrophages known as Hofbauer cells are also present.
[edit] Additional images
 Section through the embryo. |
 Human embryo of about fourteen days, with yolk-sac. |
[edit] See also
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