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|Frontal section of nasal cavities of a human embryo 28 mm. long (Vomeronasal organ of Jacobson labeled at right)|
|Gray's||subject #223 996|
During embryological development, it forms from the nasal (olfactory) placode, at the anterior edge of the neural plate (cranial nerve zero). It is a chemoreceptor organ which is completely separated from the nasal cavity the majority of the time, being enclosed in a separate bony or cartilaginous capsule which opens into the base of the nasal cavity. It is a tubular crescent shape and split into two pairs, separated by the nasal septum. It is the first processing stage of the accessory olfactory system, after which chemical stimuli go to the accessory olfactory bulb, then to targets in the amygdala and hypothalamus.
The vomeronasal organ is mainly used to detect pheromones, chemical messengers that carry information between individuals of the same species, hence is sometimes referred to as the "sixth sense." The VNO has two separate types of neuronal receptors, V1R and V2R, which are seven-transmembrane receptors that are coupled to G proteins. The receptors are distinct from each other and form the large family of receptors in the main olfactory system. Evidence shows that the VNO responds to nonvolatile cues which stimulate the receptor neurons. Information is then transferred to the accessory olfactory bulb as well as other centres of the brain such as the anterior part of the hypothalamus.
Its presence in many animals has been widely studied and the importance of the vomeronasal system to the role of reproduction and social behavior (through influence on anterior hypothalamus) has been shown in many studies. Its presence and functionality in humans is widely controversial, though most studies agree the organ regresses during fetal development.
The VNOis found at the base of the nasal cavity. It is split into two, being divided by the nasal septum, with both sides possessing an elongated c-shaped, or crescent, lumen. It is encompassed inside a bony or cartilaginous capsule which opens into the base of the nasal cavity. The vomeronasal receptor neurons possess axons which travel from the VNO to the accessory olfactory bulb (AOB) or, as its also known, the vomeronasal bulb. These sensory receptors are located on the medial concave surface of the crescent lumen. The lateral, convex surface of the lumen is covered with non sensory ciliated cells, where the basal cells are also found. At the dorsal and ventral aspect of the lumen are vomeronasal glands, which fill the vomeronasal lumen with fluid. Sitting next to the lumen are blood vessels that dilate or constrict to pump the lumen.
In mammals, the sensory neurons of the vomeronasal organ detect specific chemical compounds contained within scents that are often, but not always, large non-volatile molecules. This is sometimes viewed as the 6th sense. Notably by way of the vomeronasal organ, some scents act as chemical-communication signals (pheromones) from other individuals of the same species. Unlike the main olfactory bulb that sends neuronal signals to the olfactory cortex, the VNO sends neuronal signals to the accessory olfactory bulb and then to the amygdala and hypothalamus, which may explain how scents influence aggressive and mating behavior. In essence, the pheromonal information sensed by the VNO is transferred to the accessory olfactory bulb and to the hypothalamus. The hypothalamus functions as the body's thermostat, as well as the body's neuroendocrine system, which also controls aspects of reproductive physiology and behavior. For example, in many vertebrates, nerve signals from the brain pass sensory information to the hypothalamus about seasonal changes and the availability of a mate. In turn, the hypothalamus regulates the release of reproductive hormones required for breeding. . However, it is key to note that the vomeronasal organ detects other compounds in addition to pheromones and that some pheromones are detected by the main olfactory system. In the past VNO's were considered to be atrophied or vestigial in adults, like the vermiform appendix. However, experimental studies showed potentially functional VNO near the base of nasal septum in adults. In the experiment, applying chemicals derived from adult human skin to the VNO showed changes in the autonomic nervous system (ANS), and in the periodicity of the follicle stimulating hormone (FSH) and Luteinizing (LH) from the pituitary gland, indicating that potentially functional VNO-hypothalamic-pituitary-gonadal axis exists. .
Sensory epithelium and receptorsEdit
The VNO is a tubular crescent shape and split into two pairs, separated by the nasal septum. The crescent lumen is lined with receptor neurons on the medial concave side and is filled with fluid from the VN glands. There VN neurons are isolated from the nasal cavity and therefore isolated from the air stream that passes during normal respiration. This means that a stimulus requires arousal of the vascular pump which is lateral to the lumen. The medial, concave area of the lumen is lined with a pseudo stratified epithelium that has three main cell types: receptor cells, supporting cells, and basal cells. The supporting cells are located superficially on the membrane while the basal cells are found on the basement membrane near the non sensory epithelium. The vomeronasal sensory cells form in the olfactory placode along with other sensory olfaction neurons. They are located in a sensory epithelium and are separated from olfactory epithelium, lining an elongated cavity (lumen) inside the bone capsule which encloses the organ. A thin duct, which opens onto the floor of the nasal cavity inside the nostril, is the only way of access for stimulus chemicals . The vomeronasal sensory neurons communicate with the hypothalamus to change neuroendocrine function. These sensory receptors are often referred to as pheromone receptors since vomeronasal receptors have been tied to detecting pheromones.
The receptor cells are G-protein-coupled receptors which detect the pheromones, which are frequently referred to as pheromone receptors. The receptor neurons possess apical microvilli whose axons merge together to form VN nerves which move from the paired olfactory bulbs to the main olfactory bulb, entering the posterior dorsal aspect through the AOB. There have been two different G-protein-coupled receptors identified in the VNO, each found in distinct regions. These are V1 and V2. V1 and V2 are seven transmembrane receptors which are not closely related to the main olfactory receptors.
- V1 receptors, V1Rs, are linked to the G protein, Gαi2. They are located on the apical compartment of the VNO and a relatively short NH2 terminal and have a great sequence diversity in their transmembrane domains.
- V2 receptors, V2Rs, are linked to the G-protein, Gαo. These have long extracellular NH2 terminals which are thought to be the binding domain for pheromonal molecules, and are located on the basal compartment of the VNO. V2R genes can be grouped in to four separate families, termed A - D, and D. Family C V2Rs are quite distinct from the other families and they are expressed in all basal neurons of the VNO.
The vomeronasal organ’s sensory neurons act on a different signaling pathway than that of the main olfactory system’s sensory neurons. Upon stimulation activated by pheromones, IP3 production has been shown to increase in VNO membranes in many animals, while adenylyl cyclase and cyclic adenosine monophosphate (cAMP), the major signaling transduction molecules of the main olfactory system, remain unaltered. This trend has been shown in many animals, such as the hamster, the pig, the rat, and the garter snake upon introduction of vaginal or seminal secretions into the environment.
V1Rs and V2Rs are suggested to be activated by distinct ligands or pheromones. The evidence that Gi and Go proteins are activated upon stimulation via different pheromones supports this.
- Gi proteins are activated upon stimulation with lipophilic volatile oderents.
- Go proteins on the other hand is activated by nonvolatile proteins.
Vomeronasal sensory neurons are extremely sensitive and fire action potentials at currents as low as 1 pA. Many patch-clamp recordings have confirmed the sensitivity of the vomeronasal neurons. This sensitivity is tied to the fact that the resting potential of the vomeronasal neurons are relatively close to that of the firing threshold of these neurons. Vomeronasal sensory neurons also show remarkably slow adaptation and the firing rate increases with increasing current up to 10 pA. The main olfactory sensory neurons fire single burst action potentials and show a much quicker adaptation rate. Activating neurons that have V1 receptors, V1Rs, cause field potentials that have weak, fluctuating responses that are seen the anterior of the accessory olfactory bulb, AOB. Activation of neurons that contain V2 receptors, V2Rs, however, promote distinct oscillations in the posterior of the AOB.
- Salamanders perform a nose tapping behavior to supposedly activate their VNO. [How to reference and link to summary or text]
- Snakes use this organ to sense prey, sticking their tongue out to gather scents and touching it to the opening of the organ when the tongue is retracted.
- The organ is also well developed in some primates such as Nycticebus tardigradus or Cebus capucinus.
- Elephants transfer chemosensory stimuli to the vomeronasal opening in the roof of their mouths using the prehensile structure, sometimes called a "finger", at the tips of their trunks. [How to reference and link to summary or text]
- Painted Turtles use this organ to use their sense of smell underwater.
In some other mammals, the entire organ contracts or pumps in order to draw in the scents.[How to reference and link to summary or text]
Some mammals, particularly felids and ungulates, use a distinctive facial movement called the flehmen response to direct inhaled compounds to this organ. The animal will lift its head after finding the odorant, wrinkle its nose while lifting its lips, and cease to breathe momentarily. Flehmen behavior is associated with “anatomical specialization”, and animals that present flehmen behavior have incisive papilla and ducts, which connect the oral cavity to the VNO, that are found behind their teeth.
- House cats often may be seen making this grimace when examining a scent that interests them.
Kudjakova et al. performed exploratory behavioral studies on non purebred rats by extirpating the VNO. The study showed that the exploratory behavior of the rats with extirpated VNO’s were significantly different from both control groups of rats. These results suggest that removal of the VNO removed the experimental rats from important social information. This is seen in the reduced exploratory activity in the experimental animal and the lower number of species-specific reactions.
Another study conducted by Beauchamp et al. investigated the role of the VNO in male guinea pigs social behavior. Half of the guinea pigs vomeronasal systems were removed, while the other half were put under fake surgeries with their vomeronasal systems left intact. The findings suggested that the VNO in the male domestic guinea pig is necessary for the maintenance of normal responsiveness to sex odors. However, “in its absence, other sensory systems are capable of maintaining normal sexual behavior under conditions of laboratory testing.”
These behavioral studies show the importance of the vomeronasal system in animals’ social networks and everyday activities. The importance of the vomeronasal system to the role of reproduction and social networking has been shown in many studies.
Anatomical studies demonstrate[How to reference and link to summary or text] that in humans the vomeronasal organ regresses during fetal development, as is the case with some other mammals, including apes, cetaceans, and some bats. In fact, the human embryonic VNO possesses bipolar cells and luteinizing hormone releasing hormone (LHRH) producing cells, both that are characteristics of developing vomeronasal systems in other animals. It is debatable, and somewhat controversial, whether or not there is a presence of the vomeronasal system in adult humans.
Many studies have been performed to find if there is an actual presence of a VNO in adult humans. Trotier et al. estimated that around 92% of their subjects that had no septal surgery had at least one intact VNO. Kajer and Hansen, on the other hand, stated that VNO structure disappeared at later stages in development. Won (2000) found evidence of a VNO in 13 of his 22 cadavers (59.1%) and in 22 of his 78 living patients (28.2%).  Given these findings, some scientists have argued that there is a VNO in adult humans.  However there is no reported evidence that humans have active sensory neurons like those in working vomeronasal systems of other animals. Furthermore, there is no evidence to date that suggests there are nerve and axon connections between any existing sensory receptor cells that may be in the adult human VNO and the brain. Likewise, there is no evidence for any accessory olfactory bulb in the adult humans, and the key genes involved in VNO function in other mammals have pseudogeneized in humans. Therefore while the presence of a structure in adult humans is debated, a review of the scientific literature by Tristram Wyatt concluded, "most in the field... are sceptical about the likelihood of a functional VNO in adult humans on current evidence." 
- Accessory olfactory system
- Olfactory system
- Flehmen response
- Vomeronasal cartilage
- Cranial nerve zero
- ↑ Jacobson, L. (1813). Anatomisk Beskrivelse over et nyt Organ i Huusdyrenes Næse. Veterinær=Selskapets Skrifter [in Danish] 2,209–246.
- ↑ Kimball, J.W. Pheromones. Kimball's Biology Pages. Sep 2008. 
- ↑ Berliner,D.L.,Monti-Bloch,L.,Jennings-White,C.&Diaz-Sanchez,V.J.Steroid Biochem.Mol.Biol.58,259-265(1996)
- ↑ http://www.neuro.fsu.edu/~mmered/vomer/
- ↑ Kudjakova TI, Sarycheva NY, Kamensky AA. Characteristics of Exploratory Behavior and the Level of Uneasiness of White Nonpurebred Rats after Extirpation of the Vomeronasal Organ (VNO). Doklady Biological Sciences. 2007 July; 208-211.
- ↑ Beauchamp GK, Martin IG, Wysocki CJ, Wellington JL (1982). Chemoinvestigatory and sexual behavior of male guinea pigs following vomeronasal organ removal. Physiol. Behav. 29 (2): 329–36.
- ↑ Won Johnny. The Vomeronasal Organ: An objective anatomic analysis of its prevalence. Ear, Nose & Throat Journal. 2000 Aug. 
- ↑ Johnson A, Josephson R, Hawke M.; "Clinical and histological evidence for the presence of the vomeronasal (Jacobson's) organ in adult humans;" J Otolaryngol. 1985 Apr;14(2):71-9; PMID: 4068105
- ↑ Wyatt, Tristram D. (2003). Pheromones and Animal Behaviour: Communication by Smell and Taste. Cambridge: Cambridge University Press. ISBN 0-521-48526-6. p295
- Døving KB, Trotier D (1998). Structure and function of the vomeronasal organ. J. Exp. Biol. 201 (Pt 21): 2913–25.
- Silvotti L, Moiani A, Gatti R, Tirindelli R (2007). Combinatorial co-expression of pheromone receptors, V2Rs. J. Neurochem. 103 (5): 1753–63.
- Keverne EB (1999). The vomeronasal organ. Science 286 (5440): 716–20.
- Meredith M (2001). Human vomeronasal organ function: a critical review of best and worst cases. Chem. Senses 26 (4): 433–45.
- Evans CS (2006). Accessory chemosignaling mechanisms in primates. Am. J. Primatol. 68 (6): 525–44.
- Wekesa KS, Anholt RR (1997). Pheromone regulated production of inositol-(1, 4, 5)-trisphosphate in the mammalian vomeronasal organ. Endocrinology 138 (8): 3497–504.
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