The model demonstrates never-before-seen potential in human pluripotent stem cells
Key takeaways
- For the first time, stem cells typically considered restricted to forming body tissues spontaneously formed a yolk-sac-like structure in a model of the human embryo.
- Stem cell models of human embryos have the potential to shed light on early pregnancy loss.
- The models were grown at University of Michigan Engineering.
For the first time, a stem cell model has produced a structure resembling an early human embryo with a yolk-sac-like structure, from a single starting stem cell population and without direct genetic manipulation.
The models were made at University of Michigan Engineering. Researchers at the Chinese Academy of Sciences provided monkey embryo data to help confirm that the Michigan team was indeed seeing a yolk-sac-like structure in their models.
"According to canonical knowledge of human development, the yolk sac should come from hypoblast cells. We know our system can't produce hypoblasts or their derivatives, so we thought we couldn't see a yolk sac structure," said Jianping Fu, U-M professor of mechanical engineering and corresponding author of the study in Nature Cell Biology.
Study: A transgene-free, human peri-gastrulation embryo model presents trilaminar embryonic disc-, amnion- and yolk sac-like structures (DOI: 10.1038/s41556-026-01930-y)
Embryologists have captured still images of most human development stages, but these cannot answer many questions in human development. How do different cells and tissues emerge in the early human embryo? Which signaling molecules are at play? Which genes are important during early human development? And why do so many potential pregnancies end within the first few weeks after fertilization?
To better understand human development and help more families have healthy pregnancies, researchers are developing stem cell models of these early weeks. This study was funded primarily by the University of Michigan.
Mechanical confinement to study gastrulation
The yolk sac has been difficult to replicate in stem cell models of human development. The early embryo builds up this energy store while the placenta is under construction. The yolk sac is also the organ that forms the first blood circulatory system in the human body. Labs that have produced yolk sac-like structures in stem cell models have forced the cells down this path through genetic manipulation.
Fu's team is known for using mechanical signals to guide embryo-like development in human pluripotent stem cells. This type of stem cell mimics epiblasts, the foundational cells that can become any tissue in the body. This time, Fu and his team were trying to recreate gastrulation, during which epiblast cells in the embryo start organizing into the body's basic structure and give rise to major organ rudiments.
The team patterned human pluripotent stem cells into a single layer, forming a disc 0.8 millimeters in diameter. At the onset of gastrulation, the epiblast in the human embryo forms a disc of roughly the same size.
"The first important step in our approach is to establish the initial geometric confinement for the stem cells. This circular pattern provides geometrical confinements that promote the cells to interact and self-organize," said Shiyu Sun, a Ph.D. student in mechanical engineering at U-M and first author of the study.
To kickstart gastrulation, the team exposed the cells to a signaling molecule called BMP-4. That molecule is normally produced by a shell of cells that surround the embryo during normal human development but is missing in this model's initial state. Sun noted the presence of other signaling molecules in the cell culture medium, which also help guide the cells' transformation into different types. This process is called differentiation.
Stem cells over-deliver, producing yolk-sac-like structures
The team hoped to see that disc become three layers thick, with distinct cell types in each layer, roughly corresponding to the precursors of the outer body and nervous system, the gut and the tissues in between. In the embryo, this begins with the formation of the "primitive streak," a structure that helps establish the body's head-to-tail axis.
In the models, the cells that began sending primitive streak signals were not organized into a line. Instead, the cells seemed to form concentric circles, which then arranged into a three-layer disc.
And it wasn't just the disc. On the top side, where the precursors of the outside of the body and nervous system formed, a cavity lined with amnion cells emerged-a structure like the beginning of an amniotic sac. And on the gut side, the structure resembling a yolk sac appeared.
"It was pretty surprising to find these yolk-sac-like structures," Sun said. "At first, I didn't think it was a yolk sac."
The yolk sac is thought to arise from hypoblasts, a set of cells that normally appear alongside epiblast cells rather than descending from them. Until now, researchers did not know that during gastrulation, epiblast cells have extra options, able to build structures outside the embryo proper.
As those cavities began to form, the 3D cell cultures detached from the plate. The cells continued to develop in an embryo-like way at first but then began to diverge, becoming more disorganized. Similarity to human embryos peaked at day eight of the cell culture, mimicking human development at around 16-21 days after fertilization. About 15%-20% of the cultures formed these structures, depending on the cell line, which is highly efficient in comparison to similar models, the researchers say.
Confirming the observation of the yolk sac
Because this level of development is beyond the 14-day rule for culturing human embryos, the Michigan team relied on colleagues in China, who have access to post-implantation monkey embryos, to confirm the findings from their models. Together, they identified a definitive marker for yolk sac development: the activation of the gene HNF4A, also associated with the development of the liver, kidneys and intestines.
The team used lineage tracing to identify the pathway that turned epiblast cells into yolk sac cells, finding that they indeed arose through gastrulation. They did this by splicing in a bit of genetic code to a gene that activates during gastrulation, causing the cells to produce a fluorescent green protein.
While useful for showing some of the dynamics of human development, the models cannot grow further. Even at peak organization, the three layers of the body plan were thicker than normal. Also, the model lacks trophoblast cells, which form the placenta.
The culture plates with micropatterned circles were made in the Lurie Nanofabrication Facility. Analysis of the stem cell models relied on the Michigan Medicine Microscopy Core, Michigan Orthopedic Research Laboratories Histology Core, Michigan Advanced Genomics Core, and the Michigan Flow Cytometry Core. These facilities are supported in part by indirect cost allocations in federal grants.
The team has applied for patent protection with the assistance of U-M Innovation Partnerships and is seeking partners to bring the technology to market.
Fu is also a professor of cell and developmental biology and of biomedical engineering.
