For the first time, a stem cell model has generated structures resembling early human embryos with yolk sac-like structures from a single starting stem cell population without direct genetic manipulation.
The model was created at the University of Michigan’s College of Engineering. Researchers at the Chinese Academy of Sciences provided data from monkey embryos to confirm that the Michigan research team was indeed seeing a yolk sac-like structure in their model.
“According to standard knowledge of human development, the yolk sac should arise from subblast cells. We know that our system cannot generate subblast cells or their derivatives, so we thought we would not be able to see the structure of the yolk sac,” said Jianping Fu, professor of mechanical engineering and corresponding author of the study. Cell Biology of Nature.
Embryologists capture still images of most stages of human development, but these cannot answer many questions about human development. How do different cells and tissues emerge in early human embryos? Which signaling molecules are involved? Which genes are important during early human development? And why do so many potential pregnancies end within the first few weeks after fertilization?
Researchers are developing stem cell models of these early weeks to better understand human development and help more families have healthy pregnancies. This research was primarily funded 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. Early embryos accumulate this energy reserve during the construction of the placenta. The yolk sac is also the organ that forms the first blood circulation system in the human body. The lab that created the yolk sac-like structure in a stem cell model used genetic engineering to force the cells down this pathway.
Hu’s team is known for using mechanical signals to induce embryonic-like development in human pluripotent stem cells. This type of stem cell mimics the epiblast, the basic cell that can become any tissue in the body. This time, Fu and his team were trying to recreate gastrulation. During gastrulation, the embryo’s epiblast cells begin to organize into the basic structures of the body and the primitives of major organs are formed.
The research team patterned human pluripotent stem cells into a monolayer, forming disks 0.8 millimeters in diameter. At the beginning of gastrulation, the epiblast of the human embryo forms discs of approximately equal size.
“The first critical step in our approach is to establish the initial geometric confinement of the stem cells. This circular pattern provides a geometric confinement that facilitates cell interaction and self-organization,” said Shiyu Sun, Ph.D. student in mechanical engineering at UM and lead author of the study.
To initiate gastrulation, the researchers exposed the cells to a signaling molecule called BMP-4. This molecule is normally produced by the shell of cells surrounding the embryo during normal human development, but is missing in the initial state of this model. Sun pointed out that there are other signaling molecules in the cell culture medium that also help guide the transformation of cells into different types. This process is called differentiation.
Stem cells deliver in excess and produce yolk sac-like structures
The researchers expected that the disc would be three layers thick, with each layer containing a different type of cell, roughly corresponding to the precursors of the outside of the body and the nervous system, and the intestines and the tissues in between. In the fetus, it begins with the formation of the “primitive streak,” a structure that helps establish the head-to-tail axis of the body.
In this model, the cells that started transmitting primitive streak signals were not organized in a single line. Instead, the cells appeared to form concentric circles, which were arranged into three-layered discs.
And it wasn’t just the discs. On the outside of the body and on the upper surface, where the precursors of the nervous system are formed, a cavity lined with amniotic cells, a structure similar to the beginning of an amniotic sac, appeared. A structure resembling a yolk sac appeared on the intestinal side.
“It was very surprising to find this yolk sac-like structure,” Sun said. “I didn’t think it was a yolk sac at first.”
The yolk sac is not derived from the epiblast cells, but is thought to arise from the sublayer cells, a set of cells that usually appear alongside the epiblast cells. Until now, researchers didn’t know that during gastrulation, epiblast cells have additional options that allow them to build structures outside the embryo’s native body.
Once these cavities began to form, the 3D cell culture detached from the plate. The cells initially continued to develop in an embryonic manner, but then began to branch and become more disorganized. Similarity to human embryos peaked at day 8 of cell culture, mimicking human development approximately 16 to 21 days after fertilization. Depending on the cell line, about 15-20% of the cultures formed these structures, which the researchers say is highly efficient compared to similar models.
Check the observation of the yolk sac
Because this level of development exceeds the 14-day rule for culturing human embryos, the Michigan team turned to colleagues in China, who have access to post-transplant monkey embryos, to confirm the model’s results. Together, they identified activation of HNF4A, a gene also associated with liver, kidney and intestine development, which is a crucial marker of yolk sac development.
Using lineage tracing, the researchers identified the pathway by which epiblast cells transform into yolk sac cells and found that they did indeed arise through gastrulation. They accomplished this by inserting a piece of genetic code into a gene that is activated during gastrulation, causing the cells to produce a green fluorescent protein.
Although useful for showing some of the dynamics of human development, the model cannot grow any further. Even in peak tissue, the three layers of the body plan are thicker than normal. This model also lacks the trophoblast cells that form the placenta.
Culture plates with micropatterned circles were fabricated at the Lurie Nanofabrication Facility. Analysis of the stem cell model relied on the Michigan Medical Microscopy Core, the Michigan Orthopedic Institute Histology Core, the Michigan Advanced Genomics Core, and the Michigan Flow Cytometry Core. These facilities are supported in part by indirect cost allocations from federal grants.
The team has applied for patent protection with support from UM Innovation Partnerships and is looking for partners to bring the technology to market.
Hu is also a professor of cell biology, developmental biology, and biomedical engineering.
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DOI: 10.1038/s41556-026-01930-y
