The Neural Basis of Olfaction

We are all familiar with the sense of smell, or olfaction, one of our five basic senses. A basic biology lesson would remind us that within our noses are odor receptors that are capable of recognizing unique molecules and registering a scent, a theory posited as long ago as the first century B.C. by the Roman philosopher Lucretius. However, it was not until 1991 that the theory became fully grounded in molecular biology with Linda Buck’s and Richard Axel’s cloning of olfactory receptors.

Since then, molecular research into olfaction has accelerated, with a particular focus on the developmental basis for odor receptor neurons and their higher pathways. Remarkably, much of what has been discovered has upended conventional wisdom on brain development, with implications for therapeutics as well as our fundamental understanding of the brain.

Olfaction: A Neural Perspective

The first components of the olfactory system – odor receptors – reside within the nasal epithelium and may bind molecules that enter the nose. With forty million neurons composing at least 1,200 families of receptors, humans are able to uniquely identify a broad and stochastic range of odors. Yet detection is only the first step, as receptor axons carry signals towards a fan-like structure, the glomerulus, where each receptor family converges upon a specific point.

Collectively, these glomeruli compose the olfactory bulb, a brain structure responsible for transmitting scents from odor receptors in the nose to the processing regions of the brain. Located in the forebrain, this critical structure transfers incoming signals from odor receptors into outgoing signals through major projection neurons – mitral cells – that carry odor information up to the cortex.

In addition to the mitral cells, the olfactory bulb is packed with interneurons and modulatory neurons. These critical organizing factors ensure that odor signals are carried to the proper cortical region where chemical messengers impart the sense of smell. However, while this brief description suggests a static scheme of messages and messengers, new discoveries have revealed an incredibly dynamic system.

Two olfactory sensory neurons expressing the marker GFP, visualized with blue dots representing sensory neurons that express different odor receptors. Photo courtesy of Charles Greer.

Plasticity and Regeneration

An expert in central nervous system development and regenerative neural events, Yale Professor of Neurosurgery and Neurobiology Charles Greer has pioneered research into sensory systems, with a particular focus on the olfactory system. Greer emphasized the precocious and inherently complex nature of olfaction, citing that the first vestiges of the olfactory system, the olfactory placode, are evident as early as embryonic day nine in the mouse.

Through his work, Greer has uncovered an extraordinary plasticity in the olfactory system. While most of the other sensory systems – including the visual, auditory, and somatosensory systems – have a fixed and limited number of receptors, the receptors of the olfactory system undergo constant renewal. Odor receptors, the only portion of the central nervous system directly exposed to the environment, are particularly prone to damage from toxins or even dust, requiring renewal within six to ten weeks of exposure. As Greer often enjoys telling his students, “By the end of the semester, you will have replaced all of your olfactory sensory neurons.” But neurons themselves lack the ability to undergo mitosis and replicate themselves, suggesting that the critical process of renewal exists elsewhere.

Current investigations have revealed a population of olfactory stem cells that are capable of giving rise to new sensory neurons that can differentiate and guide their axons to the appropriate site on the olfactory bulb. Nevertheless, their recent discovery has left many questions still unanswered. There is no clear approximation for the size of the stem cell population, although Greer proposes an estimate of a base population of 100,000 stem cells. Furthermore, whether the stem cells are truly pluripotent or specific to particular receptor families remains unknown.

Highly detailed imaging studies suggest that the proliferative region of the brain that gives rise to the neurons – the sub-ventricular zone – retains its developmental capabilities, producing 10,000 to 30,000 new neurons every day. These new neurons migrate into the olfactory bulb and become integrated into the synaptic circuitry of the olfactory system. The mechanisms and factors that guide these developing neurons are only now being uncovered.

Dr. Charles Greer, Diego Rodriguez-Gil and Mary Whitman reviewing research results. Photo courtesy of Yale Department of Neurosurgery.

Developing Protomaps

In order to comprehend how the daughter neurons from olfactory stem cells are able to migrate to a precise location on the olfactory bulb or in the nasal epithelium, the development of the olfactory system must be understood. A leading theory Greer supports is the existence of developmental protomaps. These protomaps function as design schemes in which cellular fate is determined at the time of cellular division. Protomaps have been implicated in the development of the neocortex, especially in primates, where specific topographical birth sites lead to a particular neuronal fate in a designated site in the neocortex.

While debate remains on whether or not there exists an odor receptor or olfactory bulb map, research conducted within the Greer lab suggests that there are indeed protomaps for both odor sensory and odor processing regions and that these maps develop in a parallel but independent manner. Fumiaki Imamura, a postdoctoral associate with the lab, demonstrated one critical aspect by proving that both a spatial and temporal protomap determines the fate of mitral cells, which carry information from the olfactory bulb to the cortex.

Although the particular mechanisms that control this fate determination are not yet fully known, there have been several stunning discoveries. Chief among them is a unique migratory mechanism for mitral cellular axons. In contrast to a typical radial migration, mitral cells migrate radially up from the ventricular zone toward the intermediate zone of the brain and then migrate tangentially. This unusual movement has not yet been explained but may be a result of movement along the axons of earlier cells. Preliminary evidence suggests that the axon guiding molecules SLIT and ROBO may be involved, and the Greer lab plans to examine variations in transcription factors between uniformly and asymmetrically distributed cell populations.

Mitral cells that receive the synapses from the olfactory sensory neuronal axons. Photo courtesy of Charles Greer.

Helper Cells and Molecules

Despite the fact that many mysteries remain, parallel discoveries into the roles of helper cells and critical molecules have shed light on the developing and regenerating olfactory system. One particular group of glial cells, ensheathing cells, is found in high concentrations in the olfactory nerve. Engulfing non-myelinated C fibers, the smallest in the nervous system at 0.2 microns, may play a key role in guiding axons to the correct location. Because the C fibers provide gap junctions between the ensheathing cells and growing axons, it is believed that these cells may be involved in cellular communication and thereby in axonal organization.

Tenascin-C is one critical molecule in the development of the olfactory system. Earlier imaging studies demonstrated an interesting phenomenon: when axons first arrive at the olfactory bulb, they do not immediately form a synapse, but instead sit “quietly” just within range of the bulb. It appears that tenascin-C acts as an important barrier molecule, preventing synaptic formation. One hypothesis states that tenascin-C only breaks down upon the arrival of enough axons; otherwise, binding is prevented and a glomerulus fails to form.

A second hypothesis involves cadherin, a cell-cell adhesion or axon-axon domain molecule. Cadherin has adhesive properties, which may bring together axons based on differential distribution. This molecule may hold the key as to how axons from the same receptor family are able to converge upon a particular glomerulus.

Though these findings have begun to reveal the molecular and cellular basis for olfactory development and regeneration, they represent only a few of dozens or even hundreds of vital factors that bring together the complex olfactory system.

An example of a midline whole mount in which both the olfactory epithelium and olfactory bulb can be seen, showing the convergence of the axons into a single glomerulus in the olfactory bulb. Photo courtesy of Charles Greer.

Next Steps

While Greer dislikes the idea of a “model system,” he points out that olfactory mechanisms are “broadly applicable to other parts of the nervous system.” Understanding fundamental properties of the olfactory system may have tremendous impact for both therapeutics and other systems.

A collaboration with Yale Professor of Neurology and Neurobiology Jeffery Kocsis revealed that removing ensheathing cells from the olfactory nerve and injecting them into an injured spinal cord drastically increased healing as well as the number of axons that grew beyond the lesion to innervate their proper target. The therapeutic potential of such a discovery needs little elaboration, as it opens the possibility of healing previously permanent neural damage.

Discoveries may be serendipitous as well, as an unexpected collaboration with C.N.H. Long Professor and Chair of Cellular and Molecular Physiology Michael Caplan demonstrates. A shared set of 7-transmembrane G-coupled receptors in the cilia of sensory neurons and in the ciliated cells of the kidney revealed a link between polycystic kidney disease and loss of odor detection. This shared mechanism enabled an unpredicted collaboration, as each condition can be studied in the framework of either system.
There still remains much to be discovered before we can begin to claim a true understanding of the olfactory system. Yet the challenge ahead hardly discourages Greer. When asked what really keeps him going, he lightheartedly exclaimed, “Smell is fun!”

About the Author
SUNNY KUMAR is a sophomore Molecular, Cellular, and Developmental Biology major in Saybrook College. He is a Yale Global Health Fellow specializing in infectious diseases and conducts research in Professor Zhong’s lab investigating the genetic basis and mechanism of neurogenesis and asymmetric stem cell division.

The author would like to sincerely thank Professor Charles Greer for his time and excellent research.

Further Reading
Greer CA, Whitman MC. “Adult Neurogenesis and the Olfactory System.” Prog Neurobio. 2010 Oct 1.
Imamura F, Ayoub AE, Rakic P, Greer CA. “Timing of Neurogenesis is a Determinant of Olfactory Circuitry.” Nat Neuroscience. 2011 Mar 14;(3):331-7.
Rela L, Bordey A, Greer CA. “Olfactory ensheathing cell membrane properties are shaped by connectivity.” Glia. 2010 Apr 15;58(6):665-78.
Treloar HB, Ray A, Dinglasan LA, Schachner M, Greer CA. “Tenascin-C is an inhibitory boundary molecule in the developing olfactory bulb.” J Neurosci. 2009 Jul 29;29(30):9405-16.