Complex organisms are made up of trillions and trillions of cells, and few of these cells appear and act identically to each other. In fact, the small structures that make up animal tissues are specialized. They come in all shapes and sizes, contain all kinds of structures, and perform all kinds of functions.
This variety of cells did not appear overnight. A series of four new studies published in Science used the genetic expressions of different cell types to better demonstrate their development over time, specifically in the brains of reptiles and amphibians.
Scientists have long understood that different types of cells exist throughout the body differentiated through different gene expressions. But only recent research has begun to reveal the full extent of this diversity. In the last few years, e.g. studies have shown that hundreds of cell types exist even in small areas of the brain of an adult mouse, one of the most common model organisms in all of scientific research.
But despite this progress in recognizing the extent of cell diversity, the process by which this diversity develops remains difficult to define. By studying genetic expression in these tiny structures, scientists have developed a better understanding of the evolutionary processes behind cellular diversification in reptiles and amphibians, a pair of unusual scientific models.
In the first of four studies, a team analyzed the genetic expressions of different types of cells in a bearded dragonthe brain using a method called comparative single-cell transcriptomics. They then used their analysis to create a map, called a cell-type atlas, of the different types of cells in the brain of the lizard, which is common to Australia and covered in clusters of spiny scales.
The team compared the cell type atlas of the bearded dragon brain to that of the mouse brain and found that the cell types in broad areas of the brain corresponded to each other. They classified these cells as “conserved,” meaning their expression remains the same over time and between species as a result of natural selection.
However, by more closely comparing the cell type atlases of the bearded dragon and the mouse, the team found several different cell types between the two animals in more specific areas of the brain. This coexistence of conserved and diverse cell types, the scientists say, shows that cells in these areas are plastic, able to change and evolve over time.
According to the scientists, the three additional studies only confirmed the first findings. Again using single-cell transcriptomics, the teams compiled cell-type maps of the telencephalon region of the amphibian brain, particularly that of axolotl, a striped water salamander from Mexico. They then used these maps to isolate cells unique to amphibians and axolotls, paying particular attention to cells involved in brain regeneration after injury. The results once again revealed the ability of brain cells to develop.
“These studies highlight the potential to apply the powerful transcriptomic methods typically reserved for mice to non-standard models,” concluded Lehigh University researchers Dylan Faltain-Gonzalez and Justus Kebshul in related perspectiveAccording to press release. “Each of the papers creates massive single-cell and often multimodal datasets and extracts publicly available data, demonstrating the importance of data sharing and the power of accumulating single-cell data from many species for evolutionary comparisons.”