The vomeronasal organ (VNO) is a small, tube-shaped chemical sensor located inside the nose that detects pheromones and other social chemical signals in most land-dwelling vertebrates. It sits on both sides of the nasal septum, the wall dividing the left and right nasal passages, and operates as a second, parallel smell system distinct from the main one you use to detect everyday odors like food or flowers. In humans, the structure is present but almost certainly nonfunctional, a leftover from our evolutionary past.
Where It Sits and What It Looks Like
The VNO is enclosed in a capsule made of bone or cartilage at the base of the nasal septum. A narrow duct connects its internal cavity to the nasal passage, allowing chemical signals dissolved in mucus to reach the sensory tissue inside. The organ’s lumen, its hollow interior, is filled with secretions from dedicated glands that help capture and transport chemical stimuli to the sensory surface.
The sensory lining of the VNO is a crescent-shaped tissue roughly 180 micrometers thick in mice. It contains three cell types: sensory neurons that detect chemicals, supporting cells that provide structural scaffolding, and basal cells that replenish the tissue over time. The sensory neurons extend tiny finger-like projections called microvilli into the lumen, where they make contact with incoming chemical signals. On the opposite wall of the organ, a nonsensory lining resembles the tissue found elsewhere in the nasal cavity.
Surrounding the nonsensory side is a network of large blood vessels and sinuses. These blood vessels act as a pump: when they expand and contract under nervous system control, they create a suction effect that draws chemical-laden mucus into the organ. This pumping mechanism is critical because the VNO, unlike the main olfactory tissue, doesn’t sit in the direct path of airflow.
How It Differs From Regular Smell
Animals with a functional VNO essentially have two separate smell systems. The main olfactory system handles environmental odors, the kind of information that helps an animal find food, avoid predators, or navigate its surroundings. The vomeronasal system is specialized for social chemistry: pheromones from the same species and chemical signals from other species that carry biologically relevant information.
The hardware is different at every level. In the main olfactory lining, each sensory neuron grows multiple hair-like cilia and expresses one of roughly 1,000 different odor receptor genes. These neurons use a signaling cascade based on a molecule called cAMP to send electrical signals to the brain. Vomeronasal neurons, by contrast, extend microvilli rather than cilia and express a completely different set of receptors. They use around 200 receptors from one family (V1R) in the upper layer of the sensory tissue and about 80 from another family (V2R) in the deeper layer, each linked to a different signaling protein.
The brain wiring is separate too. Main olfactory neurons send their signals to the main olfactory bulb at the front of the brain. Vomeronasal neurons send theirs to a distinct structure called the accessory olfactory bulb, which then feeds information into brain regions that influence hormones and reproductive behavior.
How Pheromone Detection Works
When a pheromone molecule binds to a receptor on a vomeronasal neuron’s microvilli, it kicks off a chain of molecular events. The receptor activates a signaling protein, which triggers an enzyme that breaks a membrane molecule into two smaller signals. One of those signals opens a specific ion channel called TRPC2, which sits right at the microvilli where pheromones make contact. This channel lets positively charged ions flood into the cell, generating an electrical signal.
That initial burst of calcium entering through the channel then opens a second set of channels that allow chloride ions to flow. Together, these currents amplify the electrical response. Potassium channels also contribute to shaping the final signal. The net result is that the neuron fires more rapidly, sending pheromone information along its axon toward the accessory olfactory bulb.
The two layers of the VNO’s sensory tissue handle different types of information. Neurons in the upper layer, expressing V1R receptors, project their signals to the front portion of the accessory olfactory bulb. Neurons in the deeper layer, expressing V2R receptors, project to the rear portion. This spatial organization means the brain can process different categories of social chemical signals through partially separate circuits.
What It Does in Animals
In species with a working VNO, the organ plays a central role in reproductive and social behavior. Pheromone signals detected by the VNO travel through the accessory olfactory bulb to brain areas that regulate the hormonal system. This pathway can trigger mating behavior, aggression toward rivals, recognition of offspring, and changes in reproductive cycles.
Many mammals display a distinctive behavior called the flehmen response to push pheromones into the VNO. Horses, cats, goats, and other species curl their upper lip and hold their head high, which presses air and dissolved chemicals toward the vomeronasal duct. You may have seen a cat make this face after sniffing another animal’s urine: that lip curl is the flehmen response in action.
The VNO also plays a surprising role during embryonic development that has nothing to do with smell. Neurons that produce gonadotropin-releasing hormone (GnRH), a key signal for puberty and fertility, are actually born in the tissue of the developing olfactory region near the VNO. These neurons then migrate along the vomeronasal nerve, crossing into the brain at a structure called the cribriform plate and traveling into the hypothalamus. Once there, they orchestrate the hormonal signals that drive reproduction throughout life. When this migration fails, the result is a condition that causes absent puberty and infertility.
Evolutionary Origins
The vomeronasal system is found in land-dwelling vertebrates, from amphibians and reptiles to most mammals. The main olfactory system, by comparison, appears across all vertebrate groups including fish. For a long time, researchers thought the VNO was strictly a feature of life on land, but genetic studies have found that the key genes specific to the vomeronasal system are present and active in certain fish, expressed in specific regions of the fish olfactory lining. This suggests that a precursor to the VNO existed before fish and land animals diverged, making its origins far older than the organ’s visible anatomy might suggest.
Among mammals, the VNO varies enormously. Rodents have a large, well-developed organ with hundreds of receptor genes. Many primates, including humans and Old World monkeys, show signs of dramatic evolutionary loss, with most of those receptor genes degraded into nonfunctional versions.
The Human VNO: Present but Not Working
Anatomists can find a vomeronasal structure in the nasal septum of most human adults, but the scientific consensus is that it is vestigial, meaning it persists as a structural remnant without performing its original job. Three independent lines of evidence support this conclusion.
First, the human VNO lacks neurons and nerve fibers. Without sensory cells, there is no way to detect chemical signals or transmit them to the brain. Second, humans do not have an accessory olfactory bulb, the brain structure that would receive and process signals from a working VNO. Without this relay station, even if the organ could detect something, the information would have nowhere to go. Third, the genes that would make the system work have been disabled by mutations over evolutionary time. The TRPC2 gene, which codes for the ion channel essential to pheromone signal transduction in the VNO, is a pseudogene in humans: it still sits in our DNA but can no longer produce a functional protein. Most of the V1R receptor genes are similarly broken.
This does not mean humans are immune to social chemical signals. There is evidence that some pheromone-like effects in humans, such as the influence of body odor on attraction, are handled by the main olfactory system rather than the VNO. The machinery for detecting social chemistry wasn’t entirely lost; it was likely consolidated into the one olfactory system that remained intact.