With innovative bioreactor as womb, mouse stem cells transform into organ-filled embryos

What happens in embryonic development is one of nature’s best guarded secrets, unfolding deep in the mother’s body. Now, researchers have opened a new window on the process. They’ve made artificial mouse embryos from stem cells—no sperm or eggs required—and used an innovative bioreactor to nurture their creations for longer than any previous embryo models. The simulated embryos developed anatomy that matched the real thing and “very impressive similarities at the cellular level. The right cells arise at the right time,” says stem cell biologist Niels Geijsen of the Leiden University Medical Center, who was not involved in the work.

The feat, reported this week in Cell , may allow biologists to delve deeper into developmental mechanisms and better understand what goes wrong in birth defects. And the team’s leader, stem cell biologist Jacob Hanna of the Weizmann Institute of Science, says that next, he hopes to do the same with comparable human stem cells.

Researchers have already reprised parts of early development with embryo mimics made from an assortment of mouse or human stem cells, including embryonic stem (ES) cells, which are derived from normal embryos and can form all of a body’s tissues. They’ve mimicked the blastocyst, the simple developmental stage that implants in the uterus, and recreated gastrulation, when embryos become multilayered. These simulated embryos hit a developmental wall, however. Their cells begin to specialize but do not coalesce into organs.

One obstacle has been keeping the ersatz embryos alive for more than a few days. Last year, Hanna and colleagues unveiled a nurturing procedure that allowed them to grow standard mouse embryos outside of the mother’s body for a record 11 days. (Typical mouse gestation is about 20 days.) A key step involves placing the embryos in an incubator outfitted with a Ferris wheel–like device, which rotates the embryos inside bottles of liquid filled with nutrients and growth factors. The setup enables the team to precisely control growth conditions such as oxygen levels.

Those embryos came from fertilized mouse eggs, however. To determine whether the same procedure would allow stem cells to transform into full-fledged embryos, Hanna’s team mingled basic mouse ES cells with ES cell lineages genetically altered to spawn tissues outside the embryo that shape and support its growth. After initially rearing the cell congregations on culture plates, the team shifted them to rotating bottles on the fifth day.

By the eighth day, the “embryoids” were very similar to 8.5-day-old natural embryos and boasted a beating heart, distinct head and tail ends, the blocklike segments that become skeletal muscles, a developing brain and spinal cord, and the beginnings of other organs. The researchers also measured gene activity in more than 40,000 embryoid cells, finding all of the expected cell types in the correct locations, Hanna says.

“This is an important study as they demonstrate ES cells alone can generate whole embryolike structures containing all the early organs completely in vitro,” says cell biologist Jun Wu of the University of Texas Southwestern Medical Center.

Synthetic embryo growth on Day 8 (top) is compared to a natural embryo growth (bottom) during the same time period
Tissue staining of normal 8.5-day-old mouse embryos (top) and 8-day-old “embryoids” derived from mouse stem cells shows comparable organ growth and placement. Jacob Hannna Lab/Weizman Institute

For unknown reasons, the artificial embryos stalled at the eighth day of development. The researchers hope to overcome this barrier and extend development even further. Still, stem cell–derived embryos have an advantage over normal mouse embryos for research because the cells are available in larger numbers and scientists can more easily manipulate them, says stem cell biologist Nicolas Rivron of the Institute of Molecular Biotechnology of the Austrian Academy of Sciences.

The current procedure for making the simulated embryos fails most of the time—less than 1% of the initial cell aggregations form embryo mimics. But, Hanna notes, “The advantage of this technique is that we can make millions of aggregates in one batch.”

Achieving the same feat with human ES cells might avoid some of the controversies of research on human embryos. “This is providing an ethical and technical alternative to the use of embryos,” Rivron says.

Hanna has co-founded a company that will investigate whether the approach will work with human induced pluripotent stem cells, which are derived from adult cells rather than embryos. Cells and tissues in an embryo release factors that orchestrate the correct development of their neighbors. So growing stem cells into artificial embryos first may provide a better way of producing cell types that can be transplanted to treat human diseases. It is “more physiological,” Hanna says.