The axolotl (Ambystoma mexicanum) is an aquatic salamander renowned for its ability to regenerate its spinal cord, heart and limbs. These amphibians also readily make new neurons throughout their lives. In 1964, researchers observed that adult axolotls could regenerate parts of their brains, even if a large section was completely removed. But one study found that axolotl brain regeneration has a limited ability to rebuild original tissue structure.
Different cell types have different functions. They are able to specialize in certain roles because they each express different genes. Understanding what types of cells are in the brain and what they do helps clarify the overall picture of how the brain works. It also allows researchers to make comparisons across evolution and try to find biological trends across species.
One way to understand which cells are expressing which genes is by using a technique called single-cell RNA sequencing (scRNA-seq). This tool allows researchers to count the number of active genes within each cell of a particular sample. This provides a "snapshot" of the activities each cell was doing when it was collected.
This tool has been instrumental in understanding the types of cells that exist in the brains of animals. Scientists have used scRNA-seq in fish, reptiles, mice and even humans. But one major piece of the brain evolution puzzle has been missing: amphibians.
Our team decided to focus on the telencephalon of the axolotl. In humans, the telencephalon is the largest division of the brain and contains a region called the neocortex, which plays a key role in animal behavior and cognition. Throughout recent evolution, the neocortex has massively grown in size compared with other brain regions. Similarly, the types of cells that make up the telencephalon overall have highly diversified and grown in complexity over time, making this region an intriguing area to study.
We used scRNA-seq to identify the different types of cells that make up the axolotl telencephalon, including different types of neurons and progenitor cells, or cells that can divide into more of themselves or turn into other cell types. We identified what genes are active when progenitor cells become neurons, and found that many pass through an intermediate cell type called neuroblasts—previously unknown to exist in axolotls—before becoming mature neurons.
We then put axolotl regeneration to the test by removing one section of their telencephalon. Using a specialized method of scRNA-seq, we were able to capture and sequence all the new cells at different stages of regeneration, from one to 12 weeks after injury. Ultimately, we found that all cell types that were removed had been completely restored.
We observed that brain regeneration happens in three main phases. The first phase starts with a rapid increase in the number of progenitor cells, and a small fraction of these cells activate a wound-healing process. In phase two, progenitor cells begin to differentiate into neuroblasts. Finally, in phase three, the neuroblasts differentiate into the same types of neurons that were originally lost.
Astonishingly, we also observed that the severed neuronal connections between the removed area and other areas of the brain had been reconnected. This rewiring indicates that the regenerated area had also regained its original function.