Main olfactory epithelium

Pelengaris S.A., Abbott D.H., Barrett J. and Moore H. (1992). Induction of estrous and ovulation in the female Grey Short-tailed Opposum Monodelphis domestica involves the main olfactory epithelium. In Chemical Signals in Vertebrates 6 (Doty R.L. and Muller-Schwarze D., eds.). Plenum, New York, pp. 253-258. 

Two olfactory systems have evolved in terrestrial vertebrates which differ in both their peripheral anatomy and central projections. The main olfactory system is usually conceived as a general analyzer that detects and differentiates among complex chemosignals of the environment (Firestein 2001). Odors are detected by olfactory sensory neurons located in the main olfactory epithelium (MOE) these neurons project to glomeruli in the main olfactory bulb (MOB). The mitral and tufted neurons abutting these MOB glomeruli then transmit olfactory signals to various... 

Keller, M., Douhard, Q., Baum, M.J. and Bakker, J. (2006a) Destruction of the main olfactory epithelium reduces female sexual behavior and olfactory investigation in female mice. Chem. 

As with many macrosmatic mammals, rodents have two separate chemosensory systems, the main olfactory system (MOS) and accessory olfactory system (AOS), which respond to social odors. Importantly, these sensory systems differ not only in their peripheral morphology and central projections, but also in the types of chemosignals that they process (Meredith 1991). Sensory neurons of the MOS, which are located in the main olfactory epithelium and project to the main olfactory bulbs, process volatile chemicals and can detect odors at a distance. In contrast, sensory neurons of the AOS, which are located in the vomeronasal organs (VNO) and project to the accessory olfactory bulbs, primarily process large, non-volatile chemicals and require contact for stimulation (Meredith 1991). 

Fig. 35.1 Simplified diagram of chemosensory circuit in amygdala. Vomeronasal input via accessory olfactory bulb (VNO/ AOB) is analyzed in anterior and posterior medial amygdala (MeA, MeP). MeP appears to be inhibited by intercalated nucleus (ICNc) for heterospecific and artificial stimuli. MOE/ MOB Main olfactory epithelium/Main olfactory bulb. ACN Anterior Cortical Nucleus. PC Piriform Cortex. BLA Basolateral amygdala. ICNr rostral part of medial intercalated nucleus. ICNc caudal part of ICN. MPOA Medial Preoptic Area. VMH Ventro-medial hypothalamus...

The VNO is lined with bipolar receptor cells (Fig. 5.11). The receptor cells possess microvilli, in contrast to the cilia on the receptor cells of the main olfactory epithelium. The VNO of male and female mice have sensory cells with receptors that respond to male urine, and others that respond to female... 

In vertebrates, most of the olfactory neurons of the nasal epithelium protrude from the apical dendritic knob a variety of cilia into the protective mucus layer, thus enlarging the sensory surface area of the cells. A subpopulation of sensory neurons in the main olfactory epithelium and all of the neurons in the vomeronasal organ (VNO) are structurally different their apical region of the dendrite extends in an array of microvilli. 

Odorants are detected in a specific region of the nose, called the main olfactory epithelium, that lies at the top of the nasal cavity (Figure 32.2). Approximately 1 million sensory neurons line the surface of this region. Cilia containing the odorant-binding protein receptors project from these neurons into the mucous lining of the nasal cavity. 

Mammalian pheromones released into their environment can readily reach their target tissue, either the main olfactory epithelium (MOE) or the VNO. Both target tissues are lined with an olfactory neuroepithelium that contains membrane-bound receptor proteins, which comprise the largest known family of G-protein-coupled [262] receptors in mammals. The number of mammalian olfactory receptors [263-265] found has been astonishing, but not unreasonable. The MOE and VNO have some common features, but also significant difierences in neuron types, primary structures of receptor proteins and signal transduction [266]. 

Trinh K, Storm DR (2003) Vomeronasal organ detects odorants in absence of signaling through main olfactory epithelium. Nat Neurosci 6(5) 519—525 Tsuboi A, Miyazaki T, Imai T, Sakano H (2006) Olfactory sensory neurons expressing class I odorant receptors converge their axons on an antero-dorsal domain of the olfactory bulb in the mouse. Eur J Neurosci 23(6) 1436-1444... 

Fig. 3 Vomeronasal system. Schematic representation of a rodent nasal cavity and brain (lateral view). Accessory olfactory bulb (AOB) mitral cells project to vomeronasal and extended amygdala. Inset The VNO is a bilateral tubular structure located at the base of the nasal septum. VSNs that express the same V1R or V2R converge on a small number of glomeruli in the AOB. Sensory neurons located in the apical layer of the epithelium project to the anterior part of the AOB, whereas those present in the basal layer project to the posterior part. MOE main olfactory epithelium, MOB main olfactory bulb, BSTMPM posteromedial bed nucleus of the stria terminalis, MEA medial amygdaloid nucleus, BACfF bed nucleus of the accessory olfactory tract, PMCO posteromedial cortical amygdaloid area...

Wang Z, Balet Sindreu C, Li V, Nudelman A, Chan GC, Storm DR (2006) Pheromone detection in male mice depends on signaling through the type 3 adenylyl cyclase in the main olfactory epithelium. J Neurosci 26 7375-7379... 

Wang Z, Nudelman A, Storm DR (2007) Are pheromones detected through the main olfactory epithelium Mol Neurobiol 35 317-323... 

The pheromone receptors play an important role in modulating behavior in rodents whether they are involved in human behavior is a matter of debate. Pheromone receptors belong to class C. They are specifically expressed in the vemeronasal organ in rodents, which is a specific structure separate from, but in proximity to, the main olfactory epithelium. While there are more than 100 active receptors in the mouse, only 11 have been identified in humans, and their ligands are unknown. 

Unfortunately, no other study to our knowledge has directly compared the effects of mechanism of odor delivery to the VN epithelium and main olfactory epithelium on the response of accessory and main olfactory bulb neurons. We can therefore make no direct comparisons between our data and that of others. 

---