Most animals have a very strong sense of smell, as it is important for finding food and potential mates as well as determining the presence of nearby predators. Thus, nonhuman mammals and reptiles have two systems: the main olfactory system to detect volatile odors (chemical stimuli) and an accessory olfactory system to detect fluid-phase stimuli as well as volatile odors. Fluid-phase stimuli are commonly known as pheromones, and it is important to note that some pheromones can be detected via the main olfactory system. The accessory olfactory system has a special organ to sense the fluid-phase stimuli. This structure is a bone that separates the left and right nasal cavities (between the nose and the mouth) and is called the vomeronasal organ. This organ specifically lies between the vomer and nasal bones and was first critically described by Dutch surgeon Ludwig Lewin Jacobson (1783—1843), thus the organ is often referred to as Jacobson’s or Jacobsonian’s organ. Mammals curl back their upper lip (flehmen) to stimulate the vomeronasal organ, while snakes stick out their tongue, simultaneously touching the vomeronasal organ, to detect prey.
There has been quite a bit of controversy between scientists about whether or not humans have a truly functioning vomeronasal organ. Popular culture and popular science literature speculate that there is a functioning human vomeronasal organ that is stimulated by pheromones, but basic scientists have argued against that notion, particularly since the definition of pheromone includes: “chemical substances released by one member of a species as communication with another member, to their mutual benefit” (Meredith, 2001).
Anatomy and Physiology
The vomeronasal organ is the peripheral sensory organ of the accessory olfactory system. In animals—nonhuman mammals, amphibians, and reptiles—it is a paired organ found at the roof of the mouth. In 90 percent of humans, however, it is found unilaterally at the base of the nasal septum (Meredith, 2001). The vomeronasal system is involved in both chemical and pheromone communications. It consists of an epithelium with microvillar vomeronasal sensory neurons and a lamina propria that lies below the epithelium. The vomeronasal sensory neurons have axons that exit the epithelium through the lamina propria as an axon bundle, which in turn enters the brain for perception. In mice and hamsters, studies have shown that testosterone and luteinizing hormone levels increase when males are exposed to female mice or hamster chemosensory stimuli. This process occurs through the vomeronasal system. Similarly, female prairie voles will enter estrus (or their uterus will enlarge) when their vomeronasal organ is stimulated by the scent of a nearby male. Such responses have not been observed in humans.
In human embryos, the vomeronasal organ is very similar to those of other species such that it contains bipolar cells and generates luteinizing hormone-releasing hormone (LHRH)—producing cells. However, as the embryo matures (by 30 weeks of age), the vomeronasal organ becomes more simplified compared to other species. In adults, the vomeronasal organ is found in the nasal septum; however, it does not have large blood vessels to supply the cells or any supporting cartilage. Furthermore, following nasal surgery where the septum is removed, studies have shown that the human vomeronasal organ consists of a blind-ending tube that is lined with a pseudostratified epithelium with submucosal glands. However, no clear axon bundles have been observed penetrating the lamina propria in the same way it is seen in vomeronasal epithelia of other species. As this is very different from other species, it appears that this blind-ending tube is the remnant of the vomeronasal organ in adult humans.
In animals with a well-developed vomeronasal organ, the sensory neurons’ axons bundle together and synapse in the accessory olfactory bulb’s primary neurons, called mitral cells. Here, the signal is then passed on to the amygdala and the hypothalamus for perception. It has a direct involvement with sex hormone activities that include the start of mating behaviors as well as influencing aggressiveness.
Jennifer L. Hellier
See also: Olfactory Mucosa; Olfactory System; Pheromones; Senses of Animals
Dénes, Lorand, Zsuzsanna Pap, Annamaria Szántó, Istvan Gergely, & Tudor Sorin Pop. (2015). Human vomeronasal epithelium development: An immunohistochemical overview. Acta microbiologica et immunologica Hungarica, 62(2), 167—181. http://dx.doi.org/10.1556/030.62.2015.2.7
Meredith, Michael. (2001). Human vomeronasal organ function: A critical review of best and worst cases. Chemical Senses, 26(4), 433—445. Retrieved from http://chemse.oxfordjournals.org/content/26/4/433.full
Shigeru Takami, Maiko Yukimatsu, George Matsumura, Sawa Horie, & Fumiaki Nishiyama. (2016). Morphological analysis for neuron-like cells in the vomeronasal organ of human fetuses at the middle of gestation. Anatomical Record (Hoboken), 299(1), 88—97. http://dx.doi.org/10.1002/ar.23290.