About human and mouse brains
Image source: https://link.springer.com/article/10.3758/s13414-019-01760-1
We recently prepared the manuscript for bioarxiv and submitted to the journal. It took almost 3 years for the whole team to make this work. Here is a little summary of what we found with the intro. So, I decided to practice my sci comm skills.
According to Carl Sagan "the brain is a very big place in a very small space". It applies for almost any brain we could find in Nature, but especially it is true for the human brain. It contains about 100 billion cells, which is the number comparable to the number of stars in our Milky Way galaxy. Huge amount of cells, a number with 11 zeros.
Moreover, all these cells are not the same. There is in fact the whole "Zoo" of these cells in the nervous system. Different cells are a bit like mammals - all of them share the same features, like 4 limbs, one heart, one brain etc. But on the top of that there is a great diversity of mammals. A Grey Whale might not look like a chihuahua, but they are in fact part of the same mammals family. The same applies for the neurons of the nervous system, some of them are so tiny as 10 micrometer in diameter granule cells in the Cerebellum, while the others like Betz cells in the motor cortex could have an apical dendrite as long as 1000 - 2000 micrometers.
Overall, during the course of evolution, the brain as well as the other parts of the body has substantially changed. For example, such area of the human brain as neocortex has expanded enormously compared almost any other mammals relative to the size of the whole brain. Cortex itself could be larger in an elephant or a grey whale, but not proportionally larger. This part of the brain is usually associated with very complex functions, like reasoning, perception and many others. I would say that even we call the class Mammals, where feeding the offspring with milk, but maybe more important feature of all mammals is that they are having the neocortex. It is called "neo-" because it was novel thing that mammals had. This part of the brain is crucial for complex behavior specific for mammals.
The mammalian cortex has an enormous diversity of cells types. It has very quickly expanded during the course of evolution and contains huge amount of cells that are very different from each other. But how could one classify cells? Should it be based on how the cells look like, how they generate electric currents or something else. It turns out that we could put cells into different boxes based on their gene expression.
In this cases, cells that have more similar genes in them, would have more similar properties in terms of shape and their response to the input. In our work we compared the properties of cells from layer 2 and 3 of human and mouse neocortex. These animals are separated by approximately 80 millions years of evolution from the same ancestor. During this time many things have changed in how the brain is organized. It turned out the layer 2-3 has substantially expanded in the relative size. Humans have much more cell types compared to mouse in this region. In our work we characterized the diversity of these cells types and compared them between mouse and human. We found that human cells have larger variation in their cellular properties compared to mouse. This might imply that human cells need to be more diverse to perform more complex operations, which human brain is capable of. But this is the speculation of course, how exactly the structure of the neocortex is related to its function is an open question. Moreover we found certain cell types that might be more affected in the Alzheimer disease, which might get a better idea how to affect these cells using cell-type specific gene therapy in the future. But that is another projection on our side, whether it will be useful or not the time will tell.
Overall, it took an effort of 146 people in 13 different Institutions across the globe to make these results. It was really tremendous efforts of collaboration between scientists from different parts of the world. This is how how big science is done today. And I am happy that I provided my contribution to the whole project together with many others. From now on, we will see where it will be actually published.