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. Basal region will form chorionic plate.
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 - grow from sides of stem villi, region of main exchange, surrounded by maternal blood in intervillous spaces.

• terminal villi - not active outgrowths caused by proliferation of the trophoblast. Passive protrusions induced by capillary coiling due to growth of the fetal capillaries within the mature intermediate villi (third trimester).

• chorionic plate - region of membrane at the base of the villi through which placental arteries and vein passes.

First trimester and term chorionic villi

Anchoring Villi

Floating 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.

Virtual Slides - Placenta

Please note that there are additional slides listed in the current set, only the first placenta slide and the cord cross-section will be covered in detail in the practical class.

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

Human Villi Timeline

The placental vill development data below is based upon a recent immunochemistry confocal laser scanning microscope (CLSM) study.

Note that the paper uses clinical gestational age (GA) from last menstrual period (LMP) and has been corrected for post-conception (fertilization) age, approximately 14 days later.

Fertilization Age (weeks)	Gestational Age (weeks)	Vessel Lumen Diameter (range in microns, µm)	Features
3 to 4	5 and 6	10 - 15	a complex network of cords and vessels with redundant connections network comprises mainly cords, already connected together vessels and cords are connected to each other without any interruptions chorionic villus dominated by this network of vascular elements vessels and cords are located centrally as well as peripherally and as a consequence contact the overlying trophoblastic layer
5 to 6	7 and 8	10 - 26	villi dominated by capillary network of vessels and cords capillary network contains more vessels than cords
7 to 8	9 and 10	60 - 75 two central vessels 26 - 34 capillary network	villi have two large centrally located vessels surrounded by and connected to a peripheral capillary network capillary network contains vessels with a lumen in tight contact with overlying trophoblastic layer villous projections also contain blind ending capillary sprouts
9 to 10	11 and 12	70 - 90 two central vessels 26 - 34 capillary network	immature intermediate villi characterized by two large vessels surrounded by a capillary network capillary network has few cords blind ending capillary sprouts off the capillary network

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