Scientists have identified a type of stem cell that appears to be responsible for the neurons involved in higher brain function…
The mammalian cerebral cortex is layered like an onion, with neurons in different layers responsible for different levels of cognitive function. Neurons in the inner layers are connected to subcortical targets – such as the thalamus and basal ganglia – that deal with basic sensory and motor signals. Neurons in the outer layers are connected to other parts of the cortex, which in humans play a role in higher-level brain processes such as self-awareness, language and problem-solving.
In the developing brain, stem cells in the heart of the cortex produce neurons in sequence from the inner layer outwards. “Neurons migrate past earlier-born neurons to reach a more superficial position,” explains Ulrich Mueller at The Scripps Research Institute in California. This is then repeated to generate all cortical layers, with a neuron’s birthdate determining its layer – and therefore its function.
“However, it had never been established whether the connection between birthdate and neuronal cell type is casual or causal,” says Mueller. “We went to find out.”
In the prevailing model, different types of neurons are generated in successive waves by a single type of stem cell. However, when Mueller and his colleagues studied the developing brains of mice embryos, they found that neurons in the upper layers of the cortex are produced by a different type of stem cell. This is particularly intriguing since upper layer neurons are especially abundant in humans. “Maybe the invention of this new type of stem cell was important in driving brain evolution,” says Mueller.