By Siddhant Pusdekar / The Transmitter
Purkinje cells—the cerebellar cortex’s primary output neuron—underwent a stepwise shape transformation over the course of human evolution, according to a new preprint.
The cells show varying degrees of dendrite complexity across 11 species of primates, the study reveals. In humans, the cells open like a book, allowing for a bigger dendrite surface and more synaptic input, says study investigator Christian Hansel, professor of neurobiology at the University of Chicago. The findings flag Purkinje cells as a key node for evolutionary change, he adds: “I’m not aware of any other type of neuron that changes that extremely.”
Despite the range of behaviors the cerebellum mediates across vertebrates, it was, until recently, considered “not to have changed much during evolution,” says Robert Barton, professor of evolutionary anthropology at Durham University, who was not involved in the preprint.
The new study adds microstructural detail to a growing body of evidence that “there are all sorts of fascinating adaptations, cerebellar adaptations, across the tree of life,” Barton says.
In the new work, posted on bioRxiv in June, Hansel and his collaborators used confocal microscopy to compare the branching patterns of primary dendritic arbors emerging from more than 1,700 cerebellar Purkinje cells across 11 primate species, as well as mice, focusing on a cerebellar region involved in social cognition, emotion and language in humans. They acquired cerebellar tissue samples from a zoo, a brain bank and collaborators’ labs.
In spider monkeys and marmosets, only about 4 percent of Purkinje cells have two or more primary dendrites emerging from the soma. This fraction jumps to about 30 percent in nonhuman apes and 55 percent in humans, the researchers found.
Further analyses of various dendritic characteristics, including branch numbers, lengths and angles of primary dendrites, show that “change in one feature doesn’t necessarily predict change in another,” says Silas Busch, who worked on the study as a graduate student in Hansel’s lab and is currently a postdoctoral fellow at Rockefeller University. “This means that each cell morphology, constructed from combinations of dendritic features, is even more unique than the categories reveal.”
Across the species studied, humans have the highest average distance between dendritic branches, and primary dendrites split closer to the soma than in other primates, the researchers found. In humans, primary dendrites are so widely separated that the cells appear horizontal.