A key molecular switch can make ape brain organoids grow more like human organoids, and vice versa.
During the early stages of brain development, neurons are made by stem cells called neural progenitors. These progenitor cells initially have a cylindrical shape that makes it easy for them to split into identical daughter cells with the same shape.
The more times the neural progenitor cells multiply at this stage, the more neurons there will be later.
As the cells mature and slow their multiplication, they elongate, forming a shape like a stretched ice-cream cone.
Previously, research in mice had shown that their neural progenitor cells mature into a conical shape and slow their multiplication within hours.
Now, brain organoids have allowed researchers to uncover how this development happens in humans, gorillas and chimpanzees.
They found that in gorillas and chimpanzees this transition takes a long time, occurring over approximately five days.
Human progenitors were even more delayed in this transition, taking around seven days. The human progenitor cells maintained their cylinder-like shape for longer than other apes and during this time they split more frequently, producing more cells.
This difference in the speed of transition from neural progenitors to neurons means that the human cells have more time to multiply. This could be largely responsible for the approximately three-fold greater number of neurons in human brains compared with gorilla or chimpanzee brains.