Mice given grafts of human brain cells showed enhanced cognitive abilities that could explain what underpins the evolution of human intelligence.
What's surprising about this study is that the cells responsible for making mice that could learn far more rapidly and demonstrate enhanced navigation skills were not nerve cells.
Instead, University of Rochester husband-and-wife team Steven Goldman and Maiken Nedergaard have been focusing on cells called astrocytes, which were discovered around the same time as nerve cells but were rapidly dismissed as mere supporting cells. Indeed, scientists had thought that the sheer physical size and neurone count of the human brain was what gave us our intellectual edge.
But in recent years, the astrocyte (a form of glial cell), which is far more complex in structure and at least twice as large in humans compared with non-primates, has been shown to play key roles in regulating the biochemical environment of the nervous system and even controlling the way that nerve cells exchange information.
This suggests that, far from being relegated to the realms of being a mere supporting cell, the evolution of the complex human astrocyte might actually be fundamental to human thinking.
To probe this possibility, Goldman and Nedergaard isolated from human foetal tissue precursor cells that give rise to astrocytes.
Into newborn immune-deficient mice they injected about 100,000 of these precursors on each side of the brain before following the progress of the animals for up to 20 months.
Studies on the animals' brains showed that the injected cells had turned into astrocytes that were indistinguishable from those found in the human nervous system and had migrated into most areas of the mouse brain.
The grafted cells appeared to be chemically wired-up to adjacent nerve cells but showed superior electrical performance, responding to and conveying electrical stimulation more than three times faster than their normal mouse cell counterparts.
In behavioural tests, the animals showed striking performance enhancements in learning and navigating tasks compared with control animals.
The scientists think that the human astrocytes were secreting various modulating factors that were tuning up the performance of the nerve cells with which they were associating, facilitating the formation of new connections to form neural networks, a process called LTP or long-term potentiation and which underlies learning.
Apart from adding insight to the origin of human intelligence, this study, published in Cell Stem Cell, also has clinical implications. According to Goldman, by making glial cells from patients with degnerative brain diseases, this technique can now be used to probe the role that the glial cells play in the disease progression, something which has previously been overlooked...