A collection of human placental cells known as trophoblasts are responsible for much of what goes right in a healthy pregnancy. But growing and studying these cells in the lab has been a major challenge. Now, a team of Japanese scientists has found a way to grow trophoblasts outside of the body. Their findings, reported in Cell Stem Cell, could provide researchers with a powerful tool to investigate pregnancy complications.

“This is really the first paper that has shown really convincingly that you can grow human trophoblast cells pretty much indefinitely from the human placenta and from blastocysts,” says reproductive biologist Ashley Moffett of the University of Cambridge in the UK, who was not involved in the study.

Trophoblasts carry out a wide range of critical interactions between mother and fetus—from hormone production to the proper exchange of gases and nutrients. One of these cell types, known as cytotrophoblast cells (or CT cells) can differentiate into two other subtypes called syncytiotrophoblast (ST) and extravillous cytotrophoblast (EVT) cells. And all trophoblast cells in the placenta trace their lineages back to cells in the outer layer of the blastocyst, the hollow ball of cells that forms about a week after fertilization.

The researchers began by collecting trophoblast cells from human placentas in the first trimester. “Our main goal was to make genuine human trophoblast stem cells,” says study author Takahiro Arima, a molecular biologist and obstetrician at Tohoku University in Japan. Arima and colleagues then performed an RNA sequencing analysis to identify the collections of genes that the cells expressed. CT cells, they found, expressed many of the same genes as stem cells in the skin and intestines, which scientists already know how to culture.

Arima’s group then experimented with treating the CT cells with different combinations of proteins and small molecules known to support the growth of these other stem cell types. They settled on a combination that enabled the CT cells to continuously proliferate, without differentiating, for at least five months. By tweaking the ingredients further, the researchers were able to differentiate the cells into the ST and EVT cells.

The team then extracted cells directly from human blastocysts and cultured them under the same conditions. These too proliferated for at least five months and could also be prompted to differentiate into ST and EVT cells.

The findings have potentially important implications for reproductive biology studies. Scientists want to better understand the biological conditions that enable trophoblast cells to properly proliferate and differentiate to form a healthy placenta. But they can’t study these cells in an actual womb because of clear ethical and logistical barriers. And they can’t study them in vitro without first figuring out how to grow them in a dish. Growing CT cells with ease could lead to research breakthroughs.

Prior to this work, researchers had conducted in vitro studies using mouse trophoblast cell lines. These studies have limited application since the mouse placenta differs greatly from the human one. “It’s been a major stumbling block in the field that we have not had reliable human trophoblast cell lines,” says Moffett.

Other scientists have attempted to culture human trophoblast cells, including perinatal pathologist Mana Parast of the University of California, San Diego, who was not involved in the recent study. But growing cells taken directly from the tissue proved vexing. Her team and others, she says, “could only isolate them and differentiate them.” Researchers could culture human trophoblast cells that model the already differentiated forms. But these are not stem cells that can be differentiated into the ST and EVT subtypes, explains Parast.

Moffett sees applications for her own work. “These cells will give us an in vitro tool to look at how placental interactions with the uterus will affect how trophoblast cells differentiate,” she says.

Arima hopes that these trophoblast stem cell lines offer a powerful tool for understanding the pathology of trophoblastic diseases such as miscarriage, preeclampsia, and intrauterine growth restriction.