The vomeronasal organ, also known as Jacobson’s organ, is a specialized chemosensory structure found in many animal species. It functions as an auxiliary sense organ, distinct from the primary olfactory system that detects typical smells. This organ processes non-volatile chemical compounds, often related to social or reproductive cues.
Anatomy and Location
The vomeronasal organ consists of a pair of small, blind-ended tubular or sac-like structures. They are situated within the nasal cavity, embedded in the soft tissue of the nasal septum, adjacent to the vomer bone. The vomer bone is a thin, plowshare-shaped bone that forms part of the nasal septum, sitting between the nose and the mouth. This placement means the organ is located just above the roof of the mouth in many species.
The VNO is an elongated, C-shaped, or crescent-shaped lumen. It is enclosed within a bony or cartilaginous capsule in many animals. In some species, like cats and dogs, it connects to the oral cavity via ducts behind the incisor teeth, while in others, like horses, it connects directly to the nasal passages. The lumen contains a sensory epithelium with specialized receptor neurons, along with supporting and basal cells.
Function in the Animal Kingdom
In many non-human animals, the vomeronasal organ detects non-volatile chemical cues, particularly pheromones. These chemical signals are exchanged between individuals of the same species, influencing a wide range of behaviors. For instance, the VNO triggers mating rituals in rodents by detecting pheromones that signal reproductive readiness. It also assists in identifying social status within a group and can influence aggressive behaviors.
Snakes, for example, rely on their vomeronasal organ to track prey. They flick their forked tongues to collect scent molecules, then touch them to the organ’s openings upon retraction. This allows them to “taste” the air for chemical trails. Maternal recognition of offspring in some species also involves VNO processing of specific chemical signals. The organ’s sensory neurons are highly sensitive, capable of specialized detection.
A distinctive behavior associated with the vomeronasal organ in many mammals is the Flehmen response. This involves an animal curling back its upper lip, often exposing its front teeth, and inhaling with its nostrils usually closed for several seconds. This action draws non-volatile chemical compounds, such as pheromones found in urine or genital secretions, into the vomeronasal organ for analysis. Species like cats, horses, and various ungulates display this “lip-curl” or “grimace” to gather chemical messages about other individuals, including their reproductive status.
The Human Vomeronasal System
The presence and functionality of a vomeronasal organ in humans has been a subject of extensive scientific discussion. During human fetal development, a VNO-like structure is commonly observed, resembling the organ found in other species. This embryonic structure contains bipolar cells. However, as human development progresses, this structure undergoes regression or becomes significantly reduced.
In adult humans, the vomeronasal organ is inconsistently present as a small pit or blind-ended tube in the antero-inferior part of the nasal septum. While some macroscopic studies report its presence in a significant percentage of adults, microanatomical studies provide a more nuanced view. There is a lack of evidence for active sensory neurons or nerve connections from this structure to the brain’s accessory olfactory bulb in adult humans. The accessory olfactory bulb, which processes VNO signals in many animals, is also absent in humans.
Genetic studies indicate that the genes responsible for encoding the sensory receptor proteins found in functional vomeronasal systems of other mammals have mutated into pseudogenes in humans. These pseudogenes are non-functional, meaning they cannot produce the active proteins required for chemical detection. The absence of both functional sensory cells and the specific neural pathway to the brain leads most researchers to conclude that any remnant VNO structure in adult humans is non-functional or vestigial for chemosensation.
Chemical Communication Without a VNO
Given the scientific consensus that the human vomeronasal organ is non-functional, humans rely on their main olfactory system, or standard sense of smell, for detecting chemical signals in their environment. This primary system, located in the main olfactory epithelium of the nasal cavity, is developed and capable of perceiving a vast array of volatile chemical compounds. Our main olfactory receptors detect odors that influence our perception of the world, from food aromas to environmental cues.
Humans engage in chemical communication, where certain body odors or compounds can subtly influence mood, physiological states, or social perceptions. These chemical signals, sometimes controversially referred to as human pheromones, are typically detected by the primary olfactory receptors rather than a dedicated vomeronasal pathway. For example, studies suggest that human body odors can convey information about health or emotional states. Some research indicates potential influences on hormonal cycles, such as menstrual synchrony, though the exact mechanisms and the “pheromonal” nature of these effects remain debated and require further rigorous investigation.
The main olfactory system transmits signals to the main olfactory bulb, which then projects to the olfactory cortex and other brain regions, including those involved in emotion and memory. This pathway allows for learned associations with odors and contributes to our complex social interactions. Therefore, even without a functional vomeronasal organ, humans possess a sophisticated system for chemical sensing that plays a role in our behavior and interactions, albeit through a different sensory pathway than that seen in many other animals.