The vomeronasal organ or Jacobson 's organ is a chemoreceptor organ that is part of the olfactory system of many animals and is located in a kind of cartilaginous capsule, separated from the main olfactory epithelium.
This organ, which is part of the accessory olfactory system, is found in most amphibians, reptiles, and non-primates, but is absent in birds, adult catarrhine monkeys, and apes.
Jacobson's organ in reptiles (Source: Dario Aralezo via Wikimedia Commons)
The accessory olfactory system is responsible for sensory perception of chemical compounds related to social and reproductive behavior in many vertebrate animals; therefore, there are different families of receptors associated with this system.
The vomeronasal organ, as the main organ of the accessory olfactory system or the vomeronasal system, perceives and processes stimuli. It contains a sensory epithelium that is not directly exposed to the air, so it needs a “pumping” mechanism to fill it with mucus where the molecules responsible for the smell are embedded.
This organ was discovered by the Danish anatomist Ludvig Jacobson in 1811 and some authors describe it as a cluster of sensory cells present in the nasal chamber that have the ability to detect odoriferous particles from different sources.
Histology
Jacobson's organ has a tubular appearance. It is divided by the nasal septum (which is part of the cartilaginous bony skeleton that separates the two nostrils and gives the nose its shape) and each side of it has a crescent-shaped lumen.
This organ is enclosed by a cartilaginous capsule that connects to the nasal cavity through a duct at the base of it. In some carnivorous and ungulate animals, it connects to the oral cavity through what is known as the nasopalatine duct.
Its crescent-shaped lumen is lined with receptor neurons and is filled with fluid produced by the vomeronasal glands. On the lateral sides of the lumen there are a large number of blood vessels and paranasal sinuses that are connected to neurons of the autonomic nervous system.
The nerve fibers of the autonomic nervous system are responsible for inducing vasodilation and vasoconstriction, allowing the "pumping" of chemical-laden mucus into the lumen.
These nerve fibers run, along with a group of gonadotropin-releasing hormone (GnRH) secreting cells, from the nasal placode to the brain, to and from where they send specific sensory signals.
In the lumen of the vomeronasal organ, there is a pseudostratified epithelium, where at least three types of regenerative cells are distinguished, responsible for many of the transmission functions of said organ.
Features
The vomeronasal organ, as mentioned, has important functions in the processing of the chemical messages encoded in the odoriferous molecules, especially those related to sexual activity and aggressive and territorial behaviors.
Diagram of the neurobiological circuit of the sexual behavior of a female mammal and the participation of the vomeronasal organ (Source: Yohan Castel via Wikimedia Commons)
Many animals use this organ to improve their perception of the environment that surrounds them and increase their capabilities during the hunt.
In dogs
The vomeronasal organ in dogs consists of two elongated fluid-filled "sac" structures that open into the mouth and nose. It is located on the palate (the "roof" of the mouth), behind the upper incisors.
The olfactory receptors in the nasal cavity are different from those associated with Jacobson's organ in dogs. In the first case, the nerve cells of the olfactory epithelium have dendrites that have cilia-filled endings that are lined with mucus.
The sensory neurons of the vomeronasal organ, on the other hand, do not have cilia, but rather have a cell surface filled with microvilli.
As in other animals, nerve fibers connected to the vomeronasal organ send nerve impulses related to sexual and social behavior, especially related to pheromones, to the hypothalamus.
In cats
In cats, the vomeronasal organ is located in the lower part of the nasal cavity and is a bilateral formation in close association with the maxillary and incisor bones. The lateral region of the organ is inserted into the nasal mucosa.
The function of the Jacobson's organ in felines is very similar to that of other animals, which is why it is related to social and reproductive behavior and also to territoriality and feeding.
In humans
The presence of the vomeronasal organ in humans was first hinted at by the German anatomist Ruysch, who observed it near the nasal septum of an infant.
However, there are those who consider that the discovery of this organ in humans is due to Kölliker, who, in 1877, made a deeper description of it.
The vomeronasal organ in humans has a tubular canal-like structure, with a "blind" opening into the nasal cavity that is easily observable in histological sections of the nasal septum. It is prominent in the fetal state, but has an almost vestigial appearance in adults.
Unlike other animals, the Jacobson's organ in humans is not associated with nerve fibers or sensory neurons and its size and shape can vary considerably from one person to another.
Features
The functions of this organ in humans are still the subject of deep discussion. Apparently, the genes that code for some proteins involved in the signal transduction processes in the vomeronasal organ of other species have, in humans, mutations that yield non-functional products.
In addition, there are no accessory olfactory bulbs that are responsible for the reception of any type of information from the receptor cells of the Jacobson's organ, so it is considered that it does not fulfill sensory functions.
Some researchers, despite all the above, have found certain evidence that suggests that this organ in humans fulfills, rather, endocrine functions, which has not been reported in any other animal.
In other living beings
In most animals that have a well-developed Jacobson's organ, such as reptiles, for example, it is found inside the oral cavity and the tongue is responsible for introducing the odoriferous particles from the outside to the mouth, thus facilitating perception.
Whatever the mechanism of "entry" of the odoriferous molecules into the Jacobson's organ, once they come into contact with their specific receptors (located on the surface of the sensory cells), these trigger the brain signaling pathway.
Snakes are a good example of an animal species with a well-developed vomeronasal organ, as they use it to locate themselves spatially when they follow the trail of prey.
Crocodiles and some aquatic animals (mammals, reptiles, and some fish) do not have this type of chemoreceptor organ.
References
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- D'Aniello, B., G, S., Scandurra, A., & Pinelli, C. (2017). Identification of accessory olfactory system and medial amygdala in the zebrafish. Frontiers in Neuroanatomy, 70, 1–3.
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- Keverne, E. (1999). The Vomeronasal Organ. Science, 286, 716-720.
- Miller, LR, & Gutzke, WHN (1999). The role of the vomeronasal organ of crotalines (Reptilia: Serpentes: Viperidae) in predator detection. Animal Behavior, 58, 53–57.
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