The first Human Cell Atlas study of early pregnancy in humans has shown how the function of the maternal immune system is affected by cells from the developing placenta. Researchers used genomics and bioinformatics approaches to map over 70,000 single cells at the junction of the uterus and placenta. This revealed how the cells talk to each other to modify the immune response and enable the pregnancy.

Published in Nature, this work presents new and unexpected cell states in the uterus and placenta, and shows which genes are switched on in each cell. Insights from this will help us to understand what leads to a successful pregnancy, and what can go wrong during miscarriages or pre-eclampsia.

Maintaining a healthy pregnancy can be difficult sometimes, with many women suffering from miscarriages or still birth, and others having problems such as pre-eclampsia. The roots of many of these problems occur in the first few weeks of the pregnancy, when the placenta is formed.

The fetus creates a placenta that surrounds it in the uterus to provide nutrients and oxygen. This is in contact with the mother where it implants into the lining of the uterus - known as the decidua - to create a good blood supply for the placenta. Research on the interface between mother and fetus could help answer many vital questions, including how the mother's immune system is modified to allow both mother and the developing fetus to coexist. However, until now this area has not been well studied.

To understand this area, researchers studied more than 70,000 single cells from first trimester pregnancies. Using single-cell RNA and DNA sequencing they identified maternal and fetal cells in the decidua and placenta, and found how these cells were interacting with one another. They discovered that the fetal and maternal cells were using signals to talk to each other, and this conversation enabled the maternal immune system to support fetal growth.

Using microscopy-based methods the researchers were also able to pinpoint the location of new cell states in the different layers of the decidua. They saw how the biological blocks of the placenta - called trophoblast cells - invade into the lining of the mother's uterus and cause the tissue to change structure, creating the blood supply for the developing fetus.

The corresponding author said: "This study was only possible due to the Human Developmental Biology Resource, which provides tissues to enable research into understanding human development to help improve health. Our single cell study has shown us the exact cellular composition of the decidua and placenta for the first time, and how the cells from the developing placenta and uterus communicate. This has huge implications for understanding what happens in a normal pregnancy, and for studying what can go wrong during conditions such as pre-eclampsia and miscarriage."

https://www.nature.com/articles/s41586-018-0698-6