Olfactory subsystems in mammals: morphology, genetic and evolution

The last few years have seen huge progress in the knowledge of the sense of smell that lead the scientific community to reconsider critical issues affecting the whole sensory system, and even established ideas considered fixed scientific beliefs a few years ago need to be revisited. At the same time, olfactory structures like the ganglion of Grüneberg and the septal organ, which seemed not so long ago forgotten, are now being studied in great detail using different strategies.
This renewed interest for such structures is indeed one of the factors that stand for the definition of Olfactory Subsystems, which –as the name indicate– are nothing else but divisions of the sense of smell as a whole. These divisions imply that the area receiving odorous chemical stimulation –the olfactory sensory epithelium of the nasal cavity– is divided into zones or structures.
Enough evidence supports the existence of such subdivisions or subsystems from a strictly morphological point of view and increasing number of convincing findings indicate a functional separation as well. Sensory neurons from the olfactory epithelium of the nasal cavity express different molecular markers, including receptors, G proteins, ion channels and other signal transduction molecules. Chemosignals are recognized by various types of receptors: odorant receptors, vomeronasal and formyl peptide receptors, trace amine associated receptors, guanylyl cyclases, and transient receptors potential ion channels. However, the still persisting lack of a clear connection between the morphological and molecular subdivisions, prevent us to fully understand the functional roles of each subsystem. It has not been determined whether particular types of receptor-neurons share a single location, that could be in the ganglion of Grüneberg, in the septal organ, in the vomeronasal organ (in any of the two sensorial layers), or in the main olfactory epithelium (in any of its presumptive subdivisions), or on the contrary, whether they are present in multiple locations in several of the mentioned subdivisions. Whichever of those alternatives is found to be true, it would certainly be of great interest to know how the above mentioned receptor-neurons are organized.
Specific receptors in the periphery of the olfactory system transduce the olfactory information activating defined neural circuits in the olfactory bulb and higher processing centers in the brain. It is not known whether the different subsystems have specialized as well as redundant functions in conveying chemosensory information. Gain deeper understanding on the organization of olfactory circuits is critical to provide important insights of the identity and function of the olfactory subsystems.
The conceptual definition of the olfactory subsystems will most probably be supported by the specialists as the relationship between morphology and molecular biology, two fields that should never have followed independent paths, is constantly growing.
In addition to this, the number of functional chemosensory receptor genes and pseudogenes varies enormously among the genome of the different animal species, representing an ideal structural platform to study evolution.
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