Placenta is initiated first, as cells of a fertilized egg divide and specialize — ScienceDaily

The first stages of placental development take place days before the embryo starts to form in human pregnancies. The finding highlights the importance of healthy placental development in pregnancy, and could lead to future improvements in fertility treatments such as IVF, and a better understanding of placental-related diseases in pregnancy.

In a study published in the journal Nature, researchers looked at the biological pathways active in human embryos during their first few days of development to understand how cells acquire different fates and functions within the early embryo.

They observed that shortly after fertilisation as cells start to divide, some cells start to stick together. This triggers a cascade of molecular events that initiate placental development. A subset of cells change shape, or ‘polarise’, and this drives the change into a placental progenitor cell — the precursor to a specialised placenta cell — that can be distinguished by differences

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