March 26, 2013
Origins Of Our Smell Cells Pinpointed By Scientists
redOrbit Staff & Wire Reports - Your Universe Online
Biologists at the California Institute of Technology (Caltech) have identified the origin of olfactory nerve cells, finding that neural-crest stem cells — multipotent, migratory cells unique to vertebrates that give rise to many structures in the body — play a key role in building olfactory sensory neurons in the nose.
When human noses detect a scent, two types of sensory neurons are at work. These neurons are particularly interesting because they are the only ones in our bodies that regenerate throughout adult life. However, precisely where those neurons come from in the first place has long perplexed developmental biologists.
Previous hypotheses have given credit to embryonic cells that develop into skin or the central nervous system, where ear and eye sensory neurons, respectively, are thought to originate. But the Caltech team has shown that this understanding is incorrect.
"Olfactory neurons have long been thought to be solely derived from a thickened portion of the ectoderm; our results directly refute that concept," said Marianne Bronner, Professor of Biology at Caltech and author of a paper published March 19 in the journal eLIFE.
The two main types of sensory neurons in the olfactory system are ciliated neurons, which detect volatile scents, and microvillous neurons, which usually sense pheromones. Both of these types are found in the tissue lining on the inside of the nasal cavity and transmit sensory information to the central nervous system for processing.
In the current study, the researchers showed that during embryonic development, neural-crest stem cells differentiate into the microvillous neurons, which had long been assumed to arise from the same source as the odor-sensing ciliated neurons.
Moreover, they demonstrated that different factors are necessary for the development of these two types of neurons. By eliminating a gene called Sox10, they were able to show that formation of microvillous neurons is blocked whereas ciliated neurons are unaffected.
They made this discovery by studying the development of the olfactory system in zebrafish — a useful model organism for developmental biology studies due to the optical clarity of the free-swimming embryo. Bronner said that understanding the origins of olfactory neurons and the process of neuron formation is important for developing therapeutic applications for conditions like anosmia, or the inability to smell.
"A key question in developmental biology — the extent of neural-crest stem cell contribution to the olfactory system — has been addressed in our paper by multiple lines of experimentation," said Ankur Saxena, a postdoctoral scholar in Bronner's laboratory and lead author of the study.
"Olfactory neurons are unique in their renewal capacity across species, so by learning how they form, we may gain insights into how neurons in general can be induced to differentiate or regenerate. That knowledge, in turn, may provide new avenues for pursuing treatment of neurological disorders or injury in humans," Saxena said in a statement.
The researchers now plan to examine what other genes, in addition to Sox10, may play a role in the process by which neural-crest stem cells differentiate into microvillous neurons. They also plan to look at whether or not neural-crest cells give rise to new microvillous neurons during olfactory regeneration that happens after the embryonic stage of development.