BGDA Practical Placenta - Villi Development

Chorionic Villi

Primary villi Week 2, first stage of chorionic villi development, trophoblastic shell cells (syncitiotrophoblasts and cytotrophoblasts) form finger-like extensions into maternal decidua.
Secondary villi Week 3, second stage of chorionic villi development, extraembryonic mesoderm grows into villi, covers entire surface of chorionic sac.
Tertiary villi Week 4, third stage of chorionic villi development, mesenchyme differentiates into blood vessels and cells, forms arteriocapillary network, fuse with placental vessels, developing in connecting stalk.

• stem villi - or anchoring villi, cytotrophoblast cells attached to maternal tissue.

• branched villi - or terminal villi, grow from sides of stem villi, region of main exchange, surrounded by maternal blood in intervillous spaces.

First trimester and term chorionic villi

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  Placenta anchoring villi

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  Villi first trimester

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  Villi at term

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  Villi at term

Virtual Slide

We will now look at an example of first trimester placentation in a Virtual Slide. Please note that there are additional slides listed in the current set, only the first will be covered in the practical class.

Virtual Slides - Placenta

Chorionoic Villi Location

Originally villi cover entire chorionic surface and then become restricted to decidua basalis region forming 2 regions:

1. Frondosum - "leafy" where villi are mainly located.
2. Capsularis - smooth chorion, where villi are absent or not abundant.

Week 4 (Stage 11)	Week 7 (Stage 18)

Additional Information

Placental villi cartoon

Cytotrophoblast Layer

There is a new interpretation of the changes that are occuring in the cytotrophoblast (CTB) layer during early to full-term human placenta development. Traditionally the interpretation was that the cytotrophoblast layer thinned and became discontinuous towards term. The thinning is thought due to the epithelium surface expanding at a faster rate than its volume. Two recent studies suggest that while the cytotrophoblast layer does indeed thin, it does not become discontinuous.

Syncytiotrophoblast Layer

The syncytiotrophoblast (STB) layer forms the epithelial covering of the entire villous tree. These cells are multinucleated, terminally-differentiated syncytium formed by the fusion of the underlying progenitor cytotrophoblast (CTB) cells. The process is described as "syncytialization" and is mediated by syncytin-1, an envelope protein of a human endogenous retrovirus W (HERV-W). The differentiation is regulated by chorionic gonadotropin (hCG) and the fusion of cytotrophoblast cells is ongoing during placental development.

Cellular parts derived from the syncytiotrophoblasts (apoptotic nuclei and microparticulate debris) can be shed into the maternal blood in which they are bathed. The apototic process appears to be part of the fusion mechanism between cytotrophoblast and the overlying multinucleate syncytiotrophoblast layer.

Studies have suggested that these cells are transcriptionally inactive. A recent study using a number of different detection techniques now suggests that at least some of the cells nuclei may still be transcriptionally inactive.

Mesenchymal Villi

Mesenchymal villi generate all other villous types:

• immature intermediate villi
• stem villi
• mature intermediate villi
• terminal villi

Mesenchymal villi continuously form out of the trophoblastic sprouts throughout pregnancy and have been considered the basis for growth and differentiation of the villous trees.

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  Decidua and villi location

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  Chorionic villi

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