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Researchers at the Francis Crick Institute have developed a new stem cell model of the mature human amniotic sac, which replicates development of the tissues supporting the embryo from two to four weeks after fertilization. This is the first model of amniotic sac development after two weeks.

As described in research in Cell, the new model can be used to study the origin and function of the human amnion and help identify previously unknown ways the amniotic sac might support embryonic development. It also holds promise for medical procedures that make use of the amniotic membrane.

The amnion is a protective membrane that balloons into a sac containing amniotic fluid, which leaves the body when the mother's waters break before labor. Until recently, its major role was thought to be surrounding and protecting the baby, cushioning it against shocks and allowing nutrients to pass before the placenta forms.

Researchers haven't been able to study the amnion in detail because current stem cell embryo models don't typically capture later stages of human development and, for ethical reasons, human embryos can't be studied after fourteen days.

Replicating the amniotic sac

The team at the Crick created a new 3D model—called a post-gastrulation amnioid (PGA)—which closely resembles the human amnion and other supportive tissues after gastrulation (when the embryo's cells organize into layers that will form tissues and organs).

They did this by culturing human embryonic stem cells in a series of steps with just two chemical signals over 48 hours, after which the cells organized themselves into the inner and outer layers of the amnion.

A sac-like structure formed by day 10 in over 90% of the PGAs. These gradually expanded in size over 90 days, without being given any further signals. The cell composition in the models showed remarkable similarity to the human amniotic sac, and the fluid within PGAs mimicked the contents of human amniotic fluid.

Crosstalk between the amnion and the embryo

Using genetic manipulation techniques, the researchers found a transcription factor (a gene which switches other genes on or off) called GATA3 caused impaired growth of amnion tissue if switched off in PGAs.

Conversely, when they boosted GATA3 in embryonic stem cells, the cells developed an amniotic sac-like structure without being given any other signals. These experiments showed that GATA3 is necessary to kick-start amnion development.

Finally, the team asked if the amniotic tissues help the embryonic cells develop and specify rather than just protect them. They mixed PGA cells with embryonic stem cells that had not been treated, and saw that untreated cells developed amniotic sac-like structures, showing that signals from the amnion might indeed communicate with embryonic cells..

A new approach for medical procedures

Due to its regenerative, anti-inflammatory and antimicrobial properties, the amniotic sac membrane can be donated by people who have had elective c-sections, to be used for medical procedures like reconstructing the cornea in the eye, repairing the uterus lining or treating burns and ulcers.

Because transplanted tissue relies on donations, the team believes PGAs could provide an alternative source of amniotic membranes, which could even be grown from a patient's own cells.

The researchers are now working with Crick's Translation team to explore the potential of using PGAs in the clinic, as well as delving further into understanding the communication between the amniotic sac and the embryonic cells.

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Shifting our view of the amnion

Silvia Santos, Group Leader of the Quantitative Stem Cell Biology Laboratory at the Crick and senior author, said, "Early human development is still a black box due to ethical and technical constraints, but our new model gives some visibility during this critical time, without needing to use human embryos.

"This work shifts our view of the amnion as just a protective structure: it's actively talking to the embryo and promoting its growth. We're also excited about the potential of PGAs as a fast, cheap and scalable way to provide amniotic membranes for medical use."

Borzo Gharibi, Principal Laboratory Research Scientist in the Quantitative Stem Cell Biology Laboratory at the Crick and first author, said, "We were originally investigating how embryonic cells form their identity, and then our research took a very interesting turn, leading to the development of PGAs.

"This model replicates the mature human amnion for the first time, closely mimicking the structural and functional traits of the human amniotic sac with high reproducibility. This gives researchers a tool to investigate later stages of human development and the functions of tissues supporting the embryo."

This project was made possible through close collaboration with the Genomics, Metabolomics, and Proteomics teams at the Crick.