Mammalian development is a wonder of nature; it seems like an almost spontaneous process in which a high degree of organization is on display, driven by unknown forces. If an embryo gets cut in two, two complete embryos will form from them. Researchers wanted to know more about the regulation of development in mammals, which seems like it could be incredibly complex. Their work suggests, however, that it can be boiled down to four simple decisions a cell makes while taking its neighbors into account.
During the development of a mammalian organism, illustrated with a human embryo in the video above, a fertilized egg grows into a structure with a head, trunk, and limbs. Scientists at the Faculty of Health and Medical Sciences and the Niels Bohr Institute at the University of Copenhagen published a report in PLOS Biology investigating the rules governing this process.
"Cells are much smarter than we give them credit for. We have shown that they can build an embryo just by making four simple decisions. The most prominent of these is counting their neighbors. It almost sounds too simple to be true, but by counting their neighbors, the cells can determine whether they are placed on the outside or on the inside of the cell group. When they have made their decision, they adjust their properties, start to specialize and begin to form an embryo," explained Professor Josh Brickman of The Danish Stem Cell Center (DanStem).
For their investigation, the researchers utilized a computer model in which they could determine the minimal needs for proper embryonic development. After whittling down the rules, the investigators settled on four choices cells make, choices that are based on how cells are arranged with their neighbors. Cells must choose lineage or identity, cells adopt polarity, and they must alter their adhesion, or, die. When confronted in the model with only these four choices, cells properly formed the embryo.
There have been many theories about what drives development. Some have suggested it is maternal material providing the directions, while others have suggested master genes turn on at random until the correct ones are expressed. This work indicates it doesn’t depend on mom and is not as chaotic as random gene activation.
"We have shown that by following the four simple rules, the cells will develop themselves. They only need a little bit of information from the genome, which allows evolution to play with the genes as much as it likes. Basically, it provides the robustness to ensure that development will always work, but then it gives evolution the room to play with the genome. This gives rise to the diversity of the mammalian species," commented the first author of the work, Silas Boye Nissen, a doctoral student at the Niels Bohr Institute.