The chorionic villi layer (analogous to the mouse placental labyrinth layer) is the innermost of the three human placental layers, which also include the outer extravillous cytotrophoblast layer (analogous to mouse trophoblast giant cell layer) and the middle column cytotrophoblast layer (analogous to mouse spongiotrophoblast layer).
Late in the second week of embryonic development, Extensions of proliferating cytotrophoblast cells evaginate into the syncytiotrophoblast in various places. These extensions are the first stage in the development of the primary villi of the placenta. The secondary villus is characterized by appearance of a mesenchymal core within an expanding villus. The secondary villus becomes a tertiary villus when blood vessels penetrate its mesenchymal core and newly formed branches. This occurs toward the end of the third week of pregnancy.
The chorionic villi layer consists of four sub-layers: the outer multinuclear syncytiotrophoblast, the inner single layer villous cytotrophoblast stem cells (which form and maintain the syncytiotrophoblast layer by cell fusion), blood vessels and stroma. By approximately 20 weeks of gestation, the cytotrophoblast cell layer of many villi disappears. Thus, most chorionic villi membranes consists of three layers and, in some areas, becomes extremely thin, such that the syncytiotrophoblast comes in direct contact with the fetal capillary endothelium.
The chorionic plate is a term often used to describe the area of the mature placenta from which the villi project at the end of the 2nd week of human pregnancy, and most closely correlates to the chorionic villi layer of the placenta, and thus is used as a synonym.
At the end of pregnancy, six types of villi can be found in the placenta: stem villi, tertiary mesenchymatous villi, immature intermediate villi, mature intermediate villi, terminal or free villi, and trophoblast buds.
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