Why Humans Can't Smell As Well As Other Animals
Research shows that our human olfactory bulb - a structure in the brain processing sensory input from the nose - differs from other mammals in that no new neurons are formed after birth
Researchers from Karolinska Institutet published their findings in the scientific journal Neuron, based on olfactory neurons age-determination using the carbon-14 dating method. Their work helps explain why our human sense of smell is so much worse than that of other animals.
"I've never been so astonished by a scientific discovery," says lead investigator Jonas Frisén, Tobias Foundation Professor of stem cell research at Karolinska Institutet. "What you would normally expect is for humans to be like other animals, particularly apes, in this respect."
It was long thought that all brain neurons were formed up to the time of birth, after which production stopped. A paradigm shift occurred when scientists found that nerve cells were being continually formed from stem cells in the mammalian brain, changing scientific views on the plasticity of the brain and raising hopes of being able to replace neurons lost during some types of neurological disease.
In the adult mammal, new nerve cells are formed in two regions of the brain: the hippocampus and the olfactory bulb. While the former has an important part to play in memory, the latter is essential to the interpretation of smells. In this study, researchers at Karolinska Institutet and their Austrian and French colleagues made use of the sharp rise in atmospheric carbon-14 caused by Cold War nuclear tests to answer how frequently new neurons are formed in the olfactory bulb.
Carbon-14 is incorporated into DNA, making it possible to gauge the age of cells by measuring their carbon-14 isotope uptake. The team found that olfactory bulb neurons in their adult human subjects had carbon-14 levels matching those in the atmosphere at the time of their birth.
This is a strong indication that there is no significant generation of new neurons in this part of the brain, something that sets humans apart from all other mammals.
"Humans are less dependent on their sense of smell for their survival than many other animals, which may be related to the loss of new cell generation in the olfactory bulb, but this is just speculation," says Professor Frisén.
Professor Frisén and his team now plan to study the extent of neuron generation in the hippocampus, a part of the brain that is important for higher cerebral functions in humans.
The present study was made possible by support from the Tobias Foundation, the Swedish Research Council, the Swedish Foundation for Strategic Research (SSF), the American Brain and Behaviour Research Foundation (NARSAD), the Swedish Brain Fund, the Knut and Alice Wallenberg Foundation, AFA Försäkring, the EU and the Stockholm County Council through its ALF agreement with Karolinska Institutet.
Olaf Bergmann, Jakob Liebl, Samuel Bernard, Kanar Alkass, Maggie S.Y. Yeung, Peter Steier, Walter Kutschera, Lars Johnson, Mikael Landén, Henrik Druid, Kirsty L. Spalding & Jonas Frisén
The age of olfactory bulb neurons in humans
Neuron print issue 24 May 2012
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