The repeated light and dark pattern that you can see on the side of this embryo is caused by the presence of somites.
There's a little problem in biology that's so obvious that most biologists don't consider it a problem. Humans and mice (and most other mammals) make almost the same collection of things that they develop from a fertilized egg. And that with almost identical genes. But mice do anything in 21 days; It takes people more than ten times longer to do this.
You could try to attribute this to the different numbers of cells, but as you move through the mammalian variety, none of these really match. It gets even more confusing when you try to account for things like birds and reptiles, which use the same genes to make many of the same things. The math just doesn't work. How do developing organisms manage to consistently balance the number of cells, development time and a static network of genes?
Biologists are only just beginning to find out, and two articles published this week mark some important advances in this area.
Get someones nerve
One of the two UK-based research groups looked at the creation of motor neurons that connect the spinal cord to muscles and allow us to move. Motor neurons take less than a day to produce in zebrafish, around four days in mice, and two weeks in humans – the timing is quite different. However, the process is all controlled by an identical set of genes in these species so it is not obviously a genetic difference.
To find out what was going on, they used a system where stem cells are directed to create motor neurons. They found that the cells also obeyed a kind of internal clock outside the developing embryo: mouse stem cells took two to three days to form motor neurons, while human stem cells took about a week.
Why is that? Perhaps, according to the researchers, human cells do not receive as much of the signal that prompts cells to develop as motor neurons do. The team made additional stem cells and exposed them to a chemical that mimicked this signal. That didn't change anything. It is possible that key genes in motor neuron development were regulated differently in human cells. So they took the human version of one of these genes and put it in mouse cells. It behaved just like the mouse gene. This indicates that gene regulation is not a factor, as it only follows the cell in which it is currently located.
Therefore, the researchers began to study the gene activity in detail. Starting from the DNA, genes are transcribed into RNAs, which are then translated into proteins. And each of these products – the RNA and the protein – have an average lifespan before they are broken down. Since RNA and protein production didn't appear to be the controlling factor, the team checked whether the RNA and proteins lived longer in human cells. They added a label to them and then stopped production so the team could follow the gradual loss of the label as the protein or RNA disintegrated.
This showed that the RNA for important motor neuron genes was present in equivalent amounts in mouse and human cells. However, the protein in mouse cells persisted less than half the time in human cells. While they didn't check certain proteins, it is possible that this is responsible for the faster development in mice. Another factor was cell division. When cells divide, each daughter cell receives half the proteins that their parents had. The researchers found that mouse cells divide faster than human cells, which reduces protein levels even further than the lower stability.
The researchers confirm that they did not check whether proteins that are specifically involved in motor neuron development are more or less stable, or whether this difference exists in other tissues or at other times. Fortunately, another research group, mostly based in Japan, was studying a different tissue at the same time.
On the flank
The researchers looked at structures called somites that form on both sides of the developing spinal cord. These produce things like the ribs and vertebrae, along with many muscles. The ribs and vertebrae are repeated structures, and some have a similar repeated structure in the early embryo, with dozens of them forming in a typical mammal. They form from head to tail, and their formation goes like clockwork: a set number of hours after the previous somite form, a new one condenses from the loose cell cover on the side of the developing spinal cord.
Given the subject we are discussing, it should come as no surprise to you that the clocks run at different times in different species: about 30 minutes for zebrafish, 90 minutes for chickens, two to three hours for mice, and four to six hours for humans. This again begs the question of why the timing of the clock can differ so greatly when all of these species have very similar collections of genes.
Like the other researchers, this group used stem cells from mice and humans and induced them to form somites. Again, the stem cells behaved similarly to the intact embryonic tissues: it took 120 minutes for mouse stem cells to start producing certain genes and 320 minutes for human stem cells.
If the cells were to signal each other to control timing, it would only work if the cells were in close physical proximity. So the authors distributed them in a very sparse culture dish so that only a few cells would have neighbors nearby. Nevertheless, the onset of gene activity specific to this retained the characteristic timing of the two different species.
Like the other group, this research team took the human version of a key somite gene and inserted it into mouse stem cells. This slowed the mouse clock, but only by about 20 minutes – it was still running much faster than in human cells. When the stem cells were used to make actual mice, the clock was slow again, but the human gene was good enough to make a healthy adult mouse.
So the researchers started looking at how the gene was used to make a protein. And a lot of little things seemed to add up. It took about half an hour longer for a gene specific in this way to pass from the first activation to the production of a protein in human cells. There were also delays in the processing of RNAs (called splicing) that were required before they could be translated into proteins. And like the other study, it took longer for the protein to break down in human cells.
All of this would only suggest that human cells have a generally slower metabolism that weakens all of these developmental processes. However, the authors checked the stability of six other thus-specific proteins and found that only half of them lived longer in human cells than in mice. There is clearly something more complex than a general slowdown.
A partial answer
Really, this complexity should come as no surprise. Because while the general outlines of vertebrate evolution may be the same in most species, there are many significant differences – like the development of a relatively large brain in humans or an elongated tail in mice. Given these differences, it is probably unrealistic to think that a single, neat system can handle all of the time changes required to accomplish these things.
The fact that there seem to be a lot of things that contribute to the overall slowdown in humans – slower cell divisions, slower protein destruction, longer RNA processing times – could allow for a greater degree of flexibility. Different tissues can use different subsets of the set of potential clocks to time their development. Unfortunately, this would mean researchers have to pull apart a large collection of smaller effects, and they will learn different things by looking at different tissues.
Most of biology, however, is built on such incremental advances that ultimately manage to create an overall picture.
Science, 2020. DOI: 10.1126 / science.aba7667, 10.1126 / science.aba7668 (About DOIs).