The cellular origin of smell, sex and self-recognition has been discovered in a study of mice.

The new research shows that during embryonic development, a set of cells that line what will become the nose, give rise to three types of neurons that carry signals making these three functions possible.

"These cell types are key for the survival of an organism," said researcher Anthony-Samuel LaMantia, director of the George Washington Institute for Neuroscience. "You can't get more essential than that." LaMantia conducted the research while at the University of North Carolina, Chapel Hill.

Nasal neurons

LaMantia's team isolated the stem cells from mouse embryos about halfway through their 20-day gestation. After 20 hours, cells they took from the sides of the lining of the nasal cavity (called the olfactory epithelium) produced only more of themselves — a key feature of stem cells, he said.

The results also showed the nasal stem cells went on to become the three types of nerve cells: olfactory (responsible for our sense of smell), pheromonal (linked to the ability to sense pheromones) and gonadotropin (linked to reproduction).

Once formed, the neurons associated with reproduction, gonadotropin releasing hormone neurons, leave the nasal tissue and migrate to the brain's hypothalamus, a region of the brain that controls many body functions. These neurons are responsible for the release of two hormones: luteinizing hormone, which triggers ovulation in women and testosterone production for males; and follicle-stimulating hormone, which regulates puberty and reproductive processes in the body.

While the sex neurons don't regenerate, the olfactory and pheromonal neurons (called vomeronasal receptor neurons) are unique in that they can regenerate. Humans have nearly lost our ability to sense the chemical signals, pheromones, researchers speculate.

Autism, schizophrenia and other diseases

The results could have important medical implications, particularly for the so-called olfactory receptor neurons. These nerve cells hide out in the nose, detecting chemical signals, which they communicate to the brain via long stems called axons. 

"Over the course of a lifetime, it turns out in most parts of the nervous system, the neurons you start with when you are born are the neurons you end up with. If you lose them, you don't replace them," he said. "Except for in the nose."

There is evidence of the limited addition of new cells in the hippocampus, but otherwise, the nose is unique, and olfactory neurons' ability to regenerate could aid in the study of diseases, like autism and schizophrenia, where the sense of smell is altered, he said. 

Because these neurons could survive a biopsy, they could potentially be used to study the progression of these diseases. And since some, like autism and schizophrenia, are diagnosed only by observation, olfactory neurons might yield diagnostic tools, he said.

The research was published in May in the journal Development.