Vomeronasal organ

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Frontal section of nasal cavities of a human embryo 28 mm. long. (Vomeronasal organ of Jacobson labeled at right.)

The vomeronasal organ (VNO) or Jacobson's organ is an auxiliary olfactory sense organ in some tetrapods. In adults, it is located in the vomer bone, between the nose and the mouth. It develops from the nasal (olfactory) placode, at the anterior edge of the neural plate.

Its function is somewhat mysterious. The sensory neurons within the vomeronasal organ detect distinct chemical compounds, often but not always large non-volatile molecules. The vomeronasal organ is used in the detection of pheromones in some animals such as mice, although some pheromones are detected by the main olfactory epithelium, and the vomeronasal organ detects other compounds in addition to pheromones. Cats sense nepetalactone from catnip using the vomeronasal organ.

Snakes use it to sense smell prey, sticking their tongue out and touching it to the opening to the organ. Elephants transfer chemosensory stimuli to the vomeronasal opening in the roof of their mouths using the "finger" at the tips of their trunks. Some mammals use a distinctive facial movement called flehmen to direct compounds to this organ, while in some other mammals the entire organ contracts or pumps to draw in compounds.

Anatomical studies demonstrate that in humans the vomeronasal organ regresses during fetal development, as is the case with some other mammals, including other apes, cetaceans, and some bats. There is no evidence of a neural connection between the organ and the brain in adult humans. Nevertheless, a small pit can be found in the nasal septum of some people, and some researchers have argued that this pit represents a functional vomeronasal organ. Thus, its possible presence in humans remains controversial.