Imagine lots of cells melting together into one giant cell. That is the syncytiotrophoblast, the outer layer of the placenta.
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The placenta has lots of tiny finger-like branches. A special layer covers every one of them: the syncytiotrophoblast.
It forms when many builder cells, called cytotrophoblasts, join together. There are no walls between them anymore. It is one giant cell with lots of nuclei.
Near the end of a pregnancy, scientists think this one giant cell has billions of nuclei. That makes it the biggest joined-up cell in the human body!
The finger-like branches sit in a pool of the mother's blood. But the two bloods do not mix. The syncytiotrophoblast sits in between.
Press Deliver food on the 3D model and watch food and oxygen cross into the baby's blood vessel.