There are differences in the neuronal architecture of primates and non-primates.
A multinational research team has now been able to advance their understanding of species-specific variations in cortical neuron architecture thanks to high-resolution microscopy.
Researchers from the Developmental Neurobiology research group at the Ruhr-Universität Bochum, led by Professor Petra Wahle, have shown that primates and non-primates differ in an important aspect of their architecture: the origin of the axon, the process responsible for the transmission of electrical signals known as action potentials. The results were recently published in the journal eLife.
Axons can originate from dendrites
Until now it was considered textbook knowledge that the axon always, with few exceptions, arises from the cell body of a neuron.
However, it can also come from dendrites, which serve to collect and integrate the incoming synaptic signals. This phenomenon is called “axon-bearing dendrites”.
Different mammalian species and high-resolution microscopy reveal the variable axonal origin
“A unique aspect of the project is that the team worked with archived tissue and slide preparations, including materials that have been used to teach students for years,” explains Petra Wahle.
A variety of animals, including rodents (mouse, rat), ungulates (pig), carnivores (cat, ferret), and macaques and humans of the zoological order primates were also examined. The scientists concluded that there is a species difference between non-primates and primates through the use of five different staining techniques and evaluation of more than 34,000 neurons.
There are noticeably fewer axon-bearing dendrites on excitatory pyramidal neurons in the outer layers II and III of the primate cerebral cortex than on excitatory pyramidal neurons in non-primates. In addition, for inhibitory interneurons, substantial variations in the percentage of axon-carrying dendrite cells between the feline and human species were discovered.
No quantitative differences were observed when comparing macaque cortical areas with primary sensory and higher brain functions. High-resolution microscopy was of particular interest, as Petra Wahle describes: “This allowed the axonal origin to be accurately tracked at the micrometer level, which is sometimes not so easy with conventional light microscopy.”
Evolutionary advantage still puzzling
Little is known about the function of axon-bearing dendrites. Usually, a neuron integrates excitatory inputs arriving at the dendrites with inhibitory inputs, a process called somatodendritic integration. The neuron then decides whether the input is strong enough and important enough to be transferred via action potentials to other neurons and brain regions.
Axon-bearing dendrites are considered privileged because depolarizing inputs to these dendrites are able to directly evoke action potentials without the involvement of somatic integration and somatic inhibition. Why this species difference evolved, and the potential benefit it may have for neocortical information processing in primates, is as yet unknown.
Reference: “Neocortical pyramidal neurons with axons emerging from dendrites are common in non-primates but rare in monkeys and humans” by Petra Wahle, Eric Sobierajski, Ina Gasterstädt, Nadja Lehmann, Susanna Weber, Joachim HR Lübke, Maren Engelhardt , Claudia Distler and Gundela Meyer, Apr 20, 2022, eLife.
The study was funded by the German Research Foundation.