Starting on day 12 postconception, proliferation of cytotro-phoblast pushes trophoblast cells to penetrate into the syncytial trabeculae, reaching the maternal side of the syncytiotrophoblast by day 14. Further proliferation of trophoblast cells inside the trabeculae (day 13) stretches the trabeculae resulting in the development of syncytial side branches filled with cytotrophoblast cells (primary villi).
Shortly after, the mesenchymal cells from the extraem-bryonic mesoderm too follow the cytotrophoblast and penetrate the trabeculae and the primary villi, thus generating secondary villi. At this stage there is always a complete cytotrophoblast layer between the penetrating mesenchyme and syncytiotrophoblast.
Around day 20-21, vascularization (development of new vessels from hemangioblastic precursor cells) within the villous mesenchyme gives rise to the formation of the first placental vessels (tertiary villi). Only later, the connection to the fetal vessel system will be established.
The villi are organized in villous trees that cluster together into a series of spherical units known as lobules or placentones. Each placentone originates from the chorionic plate by a thick villous trunk stemming from a trabecula. Continuous branching of the main trunk results in daughter villi mostly freely ending in the intervillous space [5,6].
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Once your pregnancy is over and done with, your baby is happily in your arms, and youre headed back home from the hospital, youll begin to realize that things have only just begun. Over the next few days, weeks, and months, youre going to increasingly notice that your entire life has changed in more ways than you could ever imagine.