Vomeronasal organ epithelia

The origin of the nervous tissue which comprises the sensory epithelium of the vomeronasal organ is in the anterior neural crest, from which the anterior neurogenic placode appears at the rostral tip (Fig. 4.1). 

Altner H. and Muller W. (1968). Electrophysiological and electron microscopical investigation of the sensory epithelium in the vomeronasal organ in lizards (Lacerta). Z Vergl Physiol 60, 151-155. 

Gaafar H.A., Tantawy A., Hamza M. and Shaaban M. (1998). The effect of ammonia on olfactory epithelium and vomeronasal organ neuroepithelium of rabbits a histological and histochemical study. J Otorhinolaryngol 60, 88-89. 

Johnson E.W., Eller P. and Jafek B.W. (1993). An immunoelectron microscopic comparison of olfactory marker protein localization in the supranuclear regions of the rat olfactory epithelium and vomeronasal organ neuroepithelium. Acta Oto-Laryngol 113, 766-771. 

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Segovia S., Paniagua R., Nistal M. and Guillamon A.. (1984). Effects of post-pubertal gonadectomy on the neurosensorial epithelium of the vomeronasal organ in the rat. Dev Brain Res 316, 289-291. 

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These cues are important in rearing, territorial, courtship and, in particular, sexual behaviors. The vomeronasal organ (VNO) is separate from the main epithelium in mammals, comprising a thin epithelial tissue within a bony capsule in the lateral wall of the nasal cavity. It is probably vestigial in humans. The VNO epithelium contains at least two populations of microvillar chemosensory neurons one is in the more apical aspects of the epithelium, while the other lies in the more basal region. These two populations of vomeronasal neurons (VNs) are defined by the differential expression of several genes. For example, the apical VNs express the G-protein subunit Ga, while the basal neurons express Ga0. Apical and basal VNs also... 

Monti-Bloch, L. and Grosser, B. I. (1991) Effect of putative pheromones on the electrical activity of the human vomeronasal organ and olfactory epithelium. J. Steroid Biochem. 39, 573-582. 

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). 

The female elephant pheromone (Z)-7-dodecenyl acetate occurs bound to urinary proteins. When it is taken up by a male elephant, the acetate is bound to proteins of the trunk mucus. This might facilitate transport of the pheromone to the vicinity of the sensory epithelium of the vomeronasal organ (Rasmussen and Schulte, 1998). 

Although the vomeronasal system is specialized to detect stimuli in a liquid environment, it probably is not functional in utero, at least in mice. Fluorescent microspheres were not taken up by the vomeronasal organ as the access canal is not open yet in utero. In rats, by contrast, the canal is open before birth and the microspheres can be taken up. The olfactory epithelium of the main olfactory system plays a greater role prenatally, as evidenced by the uptake of radiolabeled 2-deoxyglucose (Coppola and Coltrane 1994). Fetal mice respond to amyl acetate and isovaleric acid delivered into the nasal cavity through a tiny cannula (Coppola, 2001). In both rats and mice, the main olfactory system, and not the vomeronasal system, appears to mediate prenatal olfaction (Coppola, 2001). 

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. 

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. 20. Olfactory epithelium projections to the MOB. Photomicrographs of sagittal sections through the olfactory bulb In sections stained for Nissl (A) or with WGA HRP after injection of the tracer in the olfactory epithelium (B). Note that most of the olfactory bulb is comprised by the main olfactory system while a small portion of the dorsocaudal bulb is occupied by the accessory olfactory bulb in the rat. Note also in B that the WGA HRP did not transport to the glomeruli of AOB since the tracer did not gain access to the vomeronasal organ that is embedded in the nasal septum. Bar in B, 1 mm.

Figure 1. Cross section through the snout of an axolotl, decalcified and stained with cresylecht violet. The midline of the snout is shown on the left, lateral is to the right, and the dorsal surface of the snout is toward the top. This section is taken through the point at which the vomeronasal organ connects with the nasal cavity. The vomeronasal organ is lined with vomeronasal sensory epithelium (vom), and the medial portions of the nasal cavity are lined with olfactory epithelium (olf).

Figure 5. Electro-olfactogram (EOG) responses recorded from the olfactory and vomeronasal epithelia in axolotls. Odorants consisted of 100 pi of 1 mM L-methionine (met), of water containing whole-body odorants from sexually-mature adult females (female), or of water containing whole-body odorants from size-matched, sexually-mature adult males (male). (A) Responses recorded from the olfactory epithelium of an adult male (B) responses recorded from the vomeronasal organ of the same individual. (C) Responses recorded from the olfactory epithelium of an adult female (D) responses recorded from the vomeronasal organ of the same female. Note that in all cases, responses elicited by odorants from opposite-sex individuals are larger than those elicited by odorants from same-sex individuals. Adapted from Park et al., 2004.

Each vomeronasal organ was embedded whole in paraffin, cut into 10 micron coronal sections, and mounted on slides. Every thirtieth section was Nissl stained with 1% cresyl violet (Figure IB), and the area of the vomeronasal sensory epithelium determined using the same computer program described above. Damaged sections were replaced by adjacent sections. VNO epithelial areas for each section were integrated to determine the volume of the vomeronasal sensory epithelium for each animal. 

Figure 1. A. Sagittal section of the anterior telencephalon stained with antibody to olfactory marker protein. Ant=anterior AOB, AOB=accessory olfactory bulb, FCx=frontal cortex, MOB=main olfactory bulb, Post=posterior AOB. Rostral is to the left. B. Vomeronasal organ stained by the NissI method. Bv=blood vessel, Lu=lumen, S=septum arrows point to sensory epithelium.

Figure 7. Mean volume SEM of sensory epithelium of the vomeronasal organ of male and female opossums. indicates that differences are significant (p=.0016).

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