Of the five senses, smell is the most mysterious. It’s easy to understand the more tangible senses of sight, touch, taste, and sound, but what exactly goes on when we smell something? Scientists have recently gotten a better understanding of the sense of smell with a breakthrough discovery involving the way our bodies develop and maintain the sense. It seems that we develop our sense of smell, or olfactory system, early in life. Once this system is implemented, it stays for life. This discovery is not only important in understanding how our nose works but can also be used to develop a deeper understanding of neurodevelopment and psychiatric disorders.
The study, published in Science, was conducted at Drexel University and involved working with the odorant receptor MOR28 in mice. The scientists were able to create a version of MOR28 that could be expressed or suppressed at key developmental times. In the typical mammalian olfactory system, neurons expressing a receptor gene like MOR28 will be found randomly sprinkled around the lining of the nose. Their long and wiry axons all connect to two symmetrical pairs of structures called glomeruli within the brain’s olfactory bulb. The glomeruli then send the signals to the rest of the body. According to the press release, the study found results which suggest that neurons in the nose influence each other during early development as they find their way to the glomeruli. This goes against the traditional understanding that the neurons have no contact with each other as they try to reach the glomeruli.
Arguably, the most important discovery of the study was the idea of a limited time period for the formation of the olfactory system. “We conclude that there is a critical period for the formation of rerouted-MOR28 glomeruli that ends at birth or shortly thereafter,” the lead researchers Lulu Tsai and assistant researcher, Gilad Barnea wrote in the press release. Following this realization, the researchers then concluded that they did not need to maintain expression of the engineered MOR28 for the rerouted connections to last into adulthood. Once they are established, they will remain. In a different experiment using the same lab-engineered MOR28, the researchers were able to conclude that adults are unable to create new rerouted glomeruli, although they can restore the existing ones.
Overall, the scientists found that the fundamental wiring of the sense of smell is laid out and implemented very early on, and once it is established, it lasts for life. This discovery could also be used to give insight into understanding neurodevelopmental and psychiatric disorders. This is because it suggests that early development has lifelong consequences. The findings could also help regenerative medicine. “It is clear that there is much for us to learn about the development of neural circuits,” Barnea explained in the press release. Still, this major development in understanding the differences between early development and the adult system is a huge breakthrough in science.
Source: Tsai L, Barnea G. A Critical Period Defined by Axon-Targeting Mechanisms in the Murine Olfactory Bulb. Science. 2014.