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An organ thought to supplement the olfactory system in receiving pheromonic communication. The sensory part of the organ is in two long, thin sacs, situated on either side of the nasal septum at its base. [TFD][VHOG]

uberon

organ of Jacobsen, VNO

(...) the vomeronasal organ is known only in some tetrapods. It is absent in most turtles, crocodiles, birds, some bats, and aquatic mammals. In amphibians, it is in a recessed area off the main nasal cavity. (...) In mammals possesing this organ, it is an isolated area of olfactory membrane within the nasal cavity that is usually connected to the mouth via the nasopalatine duct (reference 1); The opinions concerning the presence and functioning of the vomeronasal organ in humans are controversial. The vomeronasal cavities appear early in human foetuses. (...) Historical examination of the nasal septum revealed the presence of vomeronasal cavities in approximately 70% of adults. In contrast to the situation in other mammals, the organ is not supported by a rigid tube of bone or cartilage (reference 2); (...) the best evidence for the homology of the human VNO to that of other primates (and of mammals in general) is ontogenetic in nature, based on a common embryonic origin from a thickening (vomeronasal primordium) on the medial aspect of each olfactory pit (reference 3); (...) suggesting that lungfish possess a region homologous to the accessory olfactory bulb of tetrapods. Based on these results, it seems appropriate to refer to the recess epithelium as a primordium of the vomeronasal organ (reference 4). [debated][VHOG]

UBERON:0002255

Generally formed only in tetrapods; lungfish have rudimentary VN organs; true VN organs are not normally found in recent fishes, birds, aquatic reptiles, aquatic mammals (Bertmar 1980). Humans: 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 was controversial, though most studies agree the organ regresses during fetal development. Many genes essential for VNO function in animals (such as TRPC2) are non-functional in humans (Liman ER. Use it or lose it: molecular evolution of sensory signaling in primates. Pflugers Arch. 2006;453(2):125-31.)