Five days elapse from the time a sperm fertilizes an egg until it is implanted in the secure environment of the uterus. During this time, the egg develops from one cell into a cell cluster surrounded by a layer of cells that specialize in penetrating the uterine membrane. Researchers from the University of Copenhagen have now cracked the code that explains how life can develop during these first 5 days without any intervention or support from the mother.
How the complex human organism, including the brain, heart and all the other organs, develops from one original cell is still slightly mysterious, especially since all the cells in the body contain identical genetic codes. Researchers have now attempted to reveal the mechanism that explains the journey toward the uterus a human being takes in solitary splendour in the first 5 days of life while one cell divides into many – creating life’s first structures.
“We have developed a computer simulation system based on four very simple principles that can describe the processes occurring from the time the egg is fertilized to when the embryo penetrates the uterine membrane. This simulation accurately reflects the processes we can observe directly in the embryos of mice,” explains Joshua Brickman, Professor, Novo Nordisk Foundation Center for Stem Cell Biology, University of Copenhagen.
Salt and pepper
The researchers’ computer simulation is based on many years of research on stem cells by their laboratory and other laboratories. Based on this, the researchers have established four basic principles covering the phases in the first 5 days of embryonic development. The first principle describes the process after about 3 days, when only a cluster of 16 cells exists.
At that time, the outermost cells in the cluster specialize into trophoblast cells comprising the outer layer and subsequently create the foetal membrane and the placenta. At this stage, the cells only need to determine whether they have four or fewer neighbouring cells because, if they do, then they are on the outside and therefore must develop a special polarity and other special properties that prepare them to penetrate the uterine membrane later.
Once the outside cells establish polarity, creating the morula, the inner cells make a second decision. The cells establish a pattern. During this next phase of development the inner cells must choose between making the later embryo itself or another support lineage, the primitive endoderm, associated with placental development. Some of the descendants of these cells will also help make the internal surfaces such as the lungs or intestines. Once again, simple principles can explain how the cells are helped on their way.
The most important thing about this early phase is that the cells select their direction, but not all cells differentiate in the same way. The key principles are that the cells must make choices and communicate their decisions to their neighbours. As a result, if a cell is surrounded by too many cells moving in one direction, it then has to choose another direction and a balance of these two choices is maintained. In this phase, the centre of the embryo looks like a salt and pepper pattern of two different cell types.
Figure out which way is up
The essential element in the next phase is that the future endoderm cells surround themselves with other future endoderm cells. According to the researchers’ model, this takes place because the endoderm cells bind less to other cells. They thus move slowly but surely towards the bottom surface on the inside of the embryo and subsequently become a new internal surface.
“Put another way, in this phase the embryo determines directionality, indicating which cells should go to the top, and which to the bottom. The last and fourth principle simply rectifies the errors that have occurred. This principle exploits the pre-programmed cell death or apoptosis, meaning that cells commit suicide if they arrive at the wrong location by mistake.”
The fertilized egg has now developed into a blastocyst – a minute fluid-filled bubble of almost 200 cells that is ready to penetrate the uterine membrane and become lodged in the uterus, in which the structures created in the first 5 days are developed during the following 9 months.
The key element in creating the blastocyst is ensuring functioning communication between cells. Cells send and receive many signals to and from each other that help them both to develop correctly and to end in the right position.
Progressing at a completely different tempo
The four principles have not only enabled the researchers to develop computer simulations based on earlier scientific discoveries. To ensure that the simulation is correct, they have also tested whether the simulation reflects reality by subsequently examining both successful and unsuccessful scenarios in mouse embryos.
“We aimed to create a model system that could avoid having to carry out many time- and resource-consuming experiments in the future. By using the system, we can now also start to change the parameters and determine what happens if we change factors in the various stages. We can now instead run simulations first and then examine the interesting scenarios.”
Brickman mentions one example of a research group outside Denmark that published an article in 2017 in which they tested the effect of a specific parameter on over 100 mouse embryos. He and his colleagues have used this study as a litmus test for their new system – with great success.
We can clearly now move forward at a completely different tempo, and we can start to ask questions that we could not previously ask because both time and money prevented us from determining the answers. Then we can test in the laboratory whether reality reflects the results generated by the model.
Tobacco study led to breakthrough
The next step in the research is crucial: transferring the researchers’ computer model from mice to people. Success would have important implications for understanding and applying how human stem cells are created and develop. Nevertheless, Joshua Brickman is very reluctant to predict the most important application of this research because determining which new technologies the basic research may create is very difficult.
“Who would have thought that the early work of Roy Stevens in the 1950s initially funded by the tobacco industry, in an attempt to prove that cigarettes don’t cause cancer, would lead to the discovery of embryonic stem cells over 20 years later.”
Stevens observed that he could identify funny tumours containing a disorganized mixture of all sorts of tissues found in the adult, including teeth, hair, muscle, nervous tissue, and fat. These were unique to a funny strain of mice known as 129 and he found that they were supported by a “stem cell”, and could be formed from early embryo from this 129 mouse strain.
“Today, as a result of Roy Stevens’ work trying to breed the 129 mouse strain for their capacity to make these funny tumours, we have embryonic stem cells and understand some of the associations between stem cells and certain cancers.”
The simple principles of life
Although stem cells are derived from the early developmental stages described in the new model, they are also a potential source of material to cure degenerative diseases such as Parkinson’s and diabetes or to generate the cells that could used to replace damaged organs,
Brickman views the new model’s potential in a much broader context, implying that this sort of approach could be used to address basic questions in evolution or for the understanding of the way in which human organs organize themselves. Through understanding these principles new technologies could be born.
Our new research primarily confirms that the principles of life are usually always simple rather than complex. We could easily have added all sorts of details and exceptions to our model system. Nevertheless, four simple principles turned out to be sufficient to explain the first 5 days of life, and we can also use this theory when we try to understand the further development of humans.
“Four simple rules that are sufficient to generate the mammalian blastocyst” has been published in PLoS Biology in collaboration between DanStem and the Niels Bohr Institute at the University of Copenhagen as part of the Danish National Research Foundation’s Centre for Stem Cell Decision Making. Joshua Brickman is affiliated with the Novo Nordisk Foundation Center for Stem Cell Biology at the University of Copenhagen, which has received grants totalling nearly DKK 700 million from the Novo Nordisk Foundation in 2010–2017.