Babraham Institute collaboration reveals reason for cell fate decision in human embryonic development

In the earliest days of human development, what is it that prompts some cells to form the surface layer of skin, hair and nails and others to create the amniotic membrane that surrounds the embryo?

As scientists increasingly turn to stem cell-based embryo models for their research, it is a question that has so far puzzled them.

But now collaborative work between the Babraham Institute and the Wellcome-MRC Cambridge Stem Cell Institute has found the answer - and it relates to cell crowding.

It should lead to the creation of more successful stem cell-based models to help us probe the black box of human development.

Human pluripotent stem cells exist in the earliest stage of embryonic growth and are able to differentiate into nearly any type of cell in the body.

The researchers developed a new cell culture system that differentiated these cells into amniotic ectoderm and surface ectoderm based on cell density.

Amniotic ectoderm is a single cell layer that forms the membrane surrounding the developing embryo and provides essential signals for its development. But we do not fully understand how it comes about.

Meanwhile, the surface ectoderm is a dense sheet of epidermal progenitors that gives rise to the surface covering of the body - that is, the skin, teeth, hair and fingernails.

Its artificial production would be of medical interest if the technologies to generate it could be improved.

At an early stage, amniotic and surface ectoderm cells are specialised and they share many biological features but the reason for that has been a mystery - and nor have we understood, until now, how they differentiate during development.

Dr Shota Nakanoh, a postdoctoral researcher in Dr Teresa Rayon’s lab at the Babraham Institute, in a collaboration with Professor Ludovic Vallier’s lab at the Wellcome-MRC Cambridge Stem Cell Institute learned that the degree of cell crowding in the early human embryo influences whether cells develop as extra-embryonic cells or become a part of the embryo itself.

They developed a cell culture system that was able to differentiate the human pluripotent stem cells into amniotic ectoderm and surface ectoderm based on cell density and used single cell RNA sequencing analysis to understand the differentiation pathway.

Dr Nakanoh said: “Amnion protects the embryo and provides key developmental cues while surface ectoderm contributes to a substantial part of the adult body.

“Both cell types are of high clinical interest and are important elements for successful in vitro models for human embryos.

“Although there are protocols to differentiate cells to either amniotic or surface ectoderms, researchers were not able to make a clear distinction between these cell types. Our findings about cellular density as a key regulator fills this knowledge gap and thus facilitate us to be more certain about generating the cell types of interest.”

The researchers tested variations in the culture media but found cell density was the only factor influencing the cell fate choice between amniotic ectoderm and surface ectoderm.

Cells became amniotic ectoderm in sparse conditions, while high density culturing led to cells that expressed markers for surface ectoderm.

In the pre-gastrulation embryo - that is, before the third week - the researchers believe amniotic ectoderm arises as a loose sheet with relatively small numbers of cells, while surface ectoderm is formed as a continuous dense sheet of cells.

Dr Teresa Rayon, tenure-track group leader in the Institute’s Epigenetics research programme, said: “We have only recently begun to explore the generation of amnion during human development. These findings advance our understanding on how to generate extra-embryonic cells in the lab in vitro, and sheds light on the mechanisms that drive the formation of cell types at the stages that correspond to the ‘black box’ of human development.

“Given the growing interest in using stem cell embryo models as proxies of human embryos, this work provides more knowledge for the generation of successful integrated models.”

Prof Vallier, now based at the Berlin Institute of Health at Charité (BIH), added: “Our culture system also generates extra-embryonic mesoderm, another tissue not studied well in human embryos. It will provide better understanding of human development and could improve our knowledge about diseases affecting the first step of foetal life. This work also opens the door for new studies regarding the role of cellular density in cell fate decision.”

The research was published in Science Advances.