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The dog vomeronasal organ: A review

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The dog vomeronasal organ: A review

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The vomeronasal organ (VNO) or Jacobson's organ is an auxiliary olfactory sense organ in many vertebrates. This organ is involved in the Flehmen response and it is necessary for the detection of pheromones, chemical signals released to members of the same species for social and sexual communication. In this review the Authors report on the VNO in dog pointing out the morphogenesis, the structure and the neural pathway toward dog accessory olfactory bulb (AOB). By recent immunohistochemical studies it is highlighted that dog VNO neurons express only VR1 genes in their genome, unlike the rodents where VR1 and VR2 genes are expressed. Besides, the organization of AOB is lower than that of the rodents. This feature might be the result of VNO regressive process of a macrosmatic mammal, like dog.
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The dog vomeronasal organ: a review
Alessandra Coli1,*, Maria Rita Stornelli1, Elisabetta Giannessi1
1Dipartimento di Scienze Veterinarie, Università di Pisa - Italy
Abstract: The vomeronasal organ (VNO) or Jacobson’s organ is an auxiliary olfactory sense organ in many verte-
brates. This organ is involved in the Flehmen response and it is necessary for the detection of pheromones, chemical sig-
nals released to members of the same species for social and sexual communication.
In this review the Authors report on the VNO in dog pointing out the morphogenesis, the structure and the neural
pathway toward dog accessory olfactory bulb (AOB). By recent immunohistochemical studies it is highlighted that dog
VNO neurons express only VR1 genes in their genome, unlike the rodents where VR1 and VR2 genes are expressed. Be-
sides, the organization of AOB is lower than that of the rodents. This feature might be the result of VNO regressive
process of a macrosmatic mammal, like dog.
Key Words: dog; morphofunctional study; vomeronasal organ.
* Corresponding Author: alessandra.coli@unipi.it
Introduction
The vomeronasal organ (VNO) consists of a pair of blind, tubular, mucomembranous ducts of
olfactory epithelium lying on each side of the rostral base of the nasal septum. It is surrounded by a
thin plate of cartilage and opens rostrally into the incisive duct of the same side that connects the
nasal and oral cavity.
VNO is considered a second olfactory organ involved in parental, social and sexual communica-
tion as a chemosensory organ which has the chemoreceptors necessary for the detection of
pheromones, chemical signals
released into the environment
to members of the same
species and of flavor of food.
The VNO is also related to
‘Flehmen reaction’, a peculiar
retraction of upper lip (Ylmaz
et al., 2007).
VNO neurons are closely
related to the hypothalamus
and participate in its activa-
tion via the accessory olfacto-
ry bulb (AOB) and the amyg-
dala (Keverne, 1999).
Dog Behavior, 1-2016, pp. 24-31
doi 10.4454/db.v2i1.27
Received, 01/25/2016
Accepted, 03/09/2016
Fig. 1. Dog nasal septum (lateral view), sensory (yellow-brown) and
respiratory (red-orange) mucosa, VNO localization (panel). From Bar-
rios et al., 2014b.
04Stornelli 24_colore_Layout 1 12/04/16 10:17 Pagina 24
VNO in the animal kingdom
VNO was first described by Jacobson (1813) in mammals nose: he assessed that the VNO was a
sensory and secretory organ.
25
Fig. 2. Jacobson’s drawing of the deer head (Cervus sp.). From Døving & Trotier, 1998.
Fig. 2 is a reproduction of Jacobson’s drawing of the head of a deer (Cervus sp.), showing the lo-
calization of VNO with its nerves running to the olfactory bulb. Jacobson also pointed out that the
VNO diameter became greater cranio-caudally and that many glands were localized in the dorsal
and ventral VNO mucosa, with small secretory ducts ending in the lumen of the organ. Jacobson
described the VNO in many domesticated animals (cat, cow, dog, goat, horse, pig and sheep), and
not domesticate, (tiger, camel, buffalo, deer and seal) but not in birds (Jones & Roper, 1997) al-
though it is present in the embryo at a very early stage of development. The presence of VNO was
confirmed in most mammals (Von Mihalkovics, 1899; Pearlman, 1934) and it is well developed in
some primates (Nycticebus tardigradus and the Cebus capucinus) but reduced or absent in
Macaca mulatta (Jordan, 1972; Stark, 1975). Retzius (1894) demonstrated the chemosensory func-
tion of the organ in reptiles (snake embryo) for the similarity in morphology between the olfactory
neurons and VNO neurons running to the accessory olfactory bulb. Von Mihalkovics (1899) evi-
denced that the sensory ciliated cells were on the medial concave surface of the VNO cavity while
the lateral convex surface is lined with no sensory cells. He also described a cavernous tissue, or
eminentia fungiformis”, under the mucosa.
VNO development: morphogenesis
With regard to the VNO development (Garrosa et al., 1998), the VNO originates from the me-
dial wall of the olfactory pit (VNO placode) and consists of a cellular bud growing dorsally, cau-
dally and medially that forms the VNO groove. During morphogenesis the VNO groove closes
forming a blind-ended tube laterally the nasal septum, which opens into the oral cavity by the
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incisive duct in dog. In cross section the VNO shows a shape similar to a kidney and the epitheli-
um on the concave side, in a medial position, become wider and neuroblasts, supporting and un-
differentiated cells develop. The epithelium on the convex side (‘‘non-sensory’’ epithelium), is in
a lateral position and become thinner. During late morphogenesis, VNO acquires adult shape
and at birth, medial and lateral epithelia becomes similar to the adult. Histochemical and ultra-
structural features suggest that full performance of the VNO does not occur at birth but in pre-
pubertal ages.
VNO histology
The structure of the dog VNO has been reported in several studies (Barone et al., 1966; Klein,
1881; Ramser, 1935), where VNO is described as reduced, containing no true olfactory receptors
(Barone et al., 1966) and histologically and ultrastructurally similar to that of the cat, guinea-pig,
mouse and rabbit (Ciges et al., 1977).
26
Fig. 3. Dog VNO with incisive duct. From Evans: Miller’s Anatomy of the dog, Saunders, Elsevier, 1993.
Adams & Wiekamp (1984) prove that dog VNO is highly developed. They study the VNO in
mature dogs with the optical, transmission electron and scanning electron microscopes. The au-
thors report a crescent-shaped lumen adjacent to the nasal septum and enclosed partially by a J-
shaped vomeronasal cartilage, lacking in the dorsal lateral wall. In cross section dog VNO lumen
shows a lateral convexity along most of its length and a concavity medially in anterior portions and
ventrally in caudal portions.
Both convex and concave surfaces of the epithelium show a well vascularized lamina propria.
Small nerves deep to the medial epithelium are unmyelinated while those deep to the lateral epithe-
lium usually contains some myelinated fibers. Dorsal and lateral to the VNO glands present ducts
opened into the lumen through the epithelial commissures and the lateral epithelium, less fre-
quently through the medial epithelium. The lumen is filled with fluid from the VNO glands.
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As reported by Adams & Wiekamp (1984), the epithelium of the lateral convex mucosa, named
Non-Sensorial Epithelium (NSE) (Barrios et al., 2014b) is pseudostratified contained basal, non-cil-
iated columnar, ciliated columnar and goblet cells. Ciliated cells had a like-dome apex with numer-
ous cilia and long basal bodies. The non-ciliated cells have a surface border of microvilli and are the
most represented. The cilia irregularly distributed might favor the mixing of fluid in VNO lumen
so that the contact between molecules and receptor cells is enhanced.
As reported by Keverne (1999), the epithelium of the medial concave mucosa, named Sensorial
Epithelium (SE) (Barrios et al., 2014b) is pseudostratified containing basal or stem cells along the
basal membrane, supporting cells lying in superficial layer of the epithelium, and receptor cells.
This epithelium extends far caudally, unlike rat (Vaccarezza et al., 1981) and hamster (Taniguchi &
Mochizuki, 1982). The receptor neurons show apical microvilli, unlike the presence of only one cil-
ium on the surface of other mammalian VNO receptors (Ciges et al., 1977; Vaccarezza et al., 1981;
Salazar et al., 1984).
27
Fig. 4. Dog VNO (transverse section,
caudal portion). Lateral epithelium (le)
with few ciliated columnar cells (cc)
and non-ciliated columnar cells (nc).
Medial epithelium (me). From Adams &
Wiekamp, 1984.
Fig. 5. Dog VNO (transverse section); left is lateral, up is dor-
sal. Lumen (asterisk); nerves (1); vessels (2); gland (3); connec-
tive tissue (4); cartilage (whitearrow). E/E. Scale bar: 500μm.
From Barrios et al., 2014.
04Stornelli 24_colore_Layout 1 12/04/16 10:17 Pagina 27
Sano and Okano (1995) divide dog VNO in a vestibular, rostral, body and caudal portion, with
SE only in the medial wall of the body portion, which is comprised between the level of the canine
teeth and the edge of the third premolar teeth.
In mammals, by in situ hybridization, two families of G protein-linked receptors (VR1 and VR2)
are expressed in a distinct region of the VNO; each neuron expresses only one receptor (Dulac &
Axel, 1995). Barrios et al. (2014) point out that in the dog VNO only VR1 is expressed.
The neural patways: from the VNO to the brain
The neural pathways from the VNO mucosa to the brain are distinct from those of normal olfac-
tory mucosa (Scalia & Winans, 1975). This pathway projects to an accessory olfactory bulb (AOB)
(Salazar et al., 1992) by two or three nerves branches merging before the ethmoid bone. The nerves
penetrate the lamina cribosa by a single orifice to run along the media part of the olfactory bulb.
Dog AOB position and morphology remained vague for a long time (Cajal, 1902; Miodonoski,
1968). Salazar et al. (1992 and 1994) show that dog AOB has a lower organization compared to that
of the rodents; it presents a thick glomerular layer and thinner layer of mitral/tufted granular and
scattered glial cells. For this reason the dog AOB has a very simple structure with reduced lamina-
tion; in particular plexiform and mitral/tufted layers are indistinguishable. Nakajima et al. (1998)
by an immuno-histochemical study demonstrate the distribution of neurons in dog AOB: they
point out that the granular cells are present but in small number in respect to other mammals. Be-
cause the granular cells are inhibitory interneurons in the olfactory pathway, some differences of
the AOB function between dog and other animals are conceivable.
The mammalian olfactory system shows differences among mammals; in particular in rodents
are detected four different nasal areas: main olfactory epithelium (MOE), septal organ (SO),
Grüneberg’s ganglion (GG) and vomeronasal organ (VNO). Barrios et al. (2014b) point out that
dog olfactory system lacks of the SO and the GG, besides the absence of VR2 genes in the canine
genome. All these features might be the result of a regressive process of the dog olfactory system
(Salazar et al., 1984), despite that dog is a considered a macrosmatic mammal for its particular
sense of smell. The larger size of the canine nasal cavity might allow a better physical discrimina-
tion among odors than in the mouse (Schoenfeld & Cleland, 2005).
Another possible explanation of the dog VNO regression might be related to its domestication
dating back to 15.000 years ago because a smaller range of olfactory stimuli than undomesticated ani-
mals like rodents are required (Leonard et al., 2002; Savolainen et al., 2002). Also Salazar et al. (1984)
assumes that the more domesticate the specie the smaller the functional importance of the VNO.
Confirming this hypothesis Shi & Zhang (2008) and Niimura (2012) report that dog has 811 func-
tional olfactory receptor genes instead the mouse that has 1035 functional olfactory receptor genes.
The Authors agree with Barrios et al. (2014b) that a study of wolf VNO would significant to de-
termine whether the dog VNO might be a result of domestication.
Unresolved issues and future directions
Actually there are many evidences that the main olfactory system is involved also in pheromonal
communication. Indeed, previous works considered the “dual olfactory hypothesis”, sustaining that
the main and accessory olfactory bulb project in non overlapping areas of the brain in the basal te-
lencephalon. According to this hypothesis the olfactory and vomeronasal systems were organized
as anatomical pathways serving different functions. Recently (Martinez-Marcos, 2009; Tirindelli et
al., 2009), reported that, by genetic studies, the olfactory and vomeronasal epithelia are able to
process both olfactory cues as pheromones. A hypothesis predicts that main olfactory and
28
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vomeronasal pathways converge overlapping in the rostral basal telencephalon defining a new cate-
gory of mixed chemosensory cortex. Other hypothesis states that the two pathways of the accessory
olfactory bulb projects primary to amygdala as target and then tertiary pathways directed to ventral
striatum converge to a same sensory cortex. These hypotheses indicate that anatomical and func-
tional distinction between the olfactory and vomeronasal systems should be rewieved and they
could justify that, despite the reduction of the VNO development resulting in domestication, dog
should be considered a macrosmatic species.
The VNO function can also be evaluated by the studying behavioral alterations in spontaneous
or experimentally induced VNO lesions. Wysocki et al. (1991) in rodents, after VNO removing,
conclude that vomeronasal organ is determinant for modulating the behavior in reproductive func-
tion by the perception of pheromones. Booth et al. (2000) evidence the role of the sheep VNO in
lacking of recognition of neonatal offspring by cauterization of the nasoincisive duct which makes
nonfunctional this organ. Asproni et al. (2015) study the effects of inflammatory lesions of cat
VNO and point out the association between inflammatory lesions with intraspecific aggressive be-
havior. This type of investigation could also be applied to the dogs to determine the actual involve-
ment of the VNO in intraspecific communication and in the adaptive mechanism to new environ-
ments, as a recent study suggests (Osella et al., 2015).
Acknowledgments: we are grateful to Dr. Carla Lenzi for her contribution to bibliography re-
search.
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L’organo vomeronasale nel cane: una review
Alessandra Coli1*, Maria Rita Stornelli1, Elisabetta Giannessi1
1Dipartimento di Scienze Veterinarie, Università di Pisa - Italia
Sintesi
L’organo vomeronasale (VNO) consiste di un paio di dotti tubulari, a fondo cieco, mucomembranosi, rivestiti di epi-
telio olfattivo che sono collocati, uno per lato, nella porzione rostrale della base del setto nasale. Il VNO è circondato da
un sottile strato di cartilagine e si apre rostralmente nella cavità nasale o in quella buccale.
Il VNO è considerato un secondo organo olfattivo, coinvolto nella comunicazione in ambito parentale, sociale e ses-
suale, possedendo chemiorecettori necessari per il rilevamento dei feromoni.
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Il VNO fu descritto per la prima volta da Jacobson nel 1813 nel naso dei mammiferi e fu da lui ritenuto un organo con
attività sensoriale e secretoria.
La presenza del VNO fu confermata nella maggior parte dei mammiferi da studi posteriori ed è ora noto come sia ben
sviluppato in alcune specie di primati non umani-
Qualche decennio dopo le osservazioni di Jacobson, un altro anatomista, Retzius, lo descrisse nei rettili. Nel 1899 Von
Mihalkovics evidenziò che l’epitelio ciliato era posizionato sulla superficie concava mediale dell’organo mentre la superfi-
cie laterale era rivestita di cellule senza funzioni chemiorecettoriali, al di sotto delle quali è presente un tessuto cavernoso
detto “eminentia fungiformis”.
La struttura del VNO del cane è riportato in diversi studi dove è descritto come un organo di dimensioni ridotte, non
contenente veri recettori olfattivi e istologicamente simile a quello del gatto, cavia, topo e coniglio.
L’epitelio che riveste la porzione laterale, denominato Epitelio Non Sensoriale (NSE) è pseudistratificato e contiene
cellule basali, colonnari non ciliate, colonnari ciliate e globose. Le cellule ciliate hanno un apice a cupola con numerose ci-
lia e corpi basali. Le cellule non ciliate sono le più numerose e hanno una superficie cosparsa di microvilli. Le cilia, distri-
buite in modo irregolare, potrebbero favorire la miscelazione del fluido nel lumen del VNO in modo che il contatto tra le
molecole e le cellule recettoriali sia migliorato.
L’epitelio che riveste la porzione mediale, denominato Epitelio Sensoriale, è pseudo stratificato e contiene cellule basa-
li che sostengono le cellule più superficiali e quelle recettoriali. Questo tipo di epitelio si estende molto causalmente, a dif-
ferenza di quanto avviene nel ratto e nel criceto.
Sano e Okano suddividono il VNO del cane in quattro porzioni: vestibolare, rostrale, corpo e caudale. L’epitelio sen-
soriale sarebbe presente nella porzione del corpo che è compresa tra il canino e l’apice del terzo premolare.
La via neurale dal VNO al cervello è distinta da quelle delle mucosa olfattiva normale. Questa via proietta al bulbo ol-
fattivo accessorio attraverso due o tre branche nervose che emergono prima dell’osso etmoide.
Il sistema olfattivo presenta differenze notevoli tra i diversi mammiferi; in particolare nei roditori sono individuabili
quattro differenti aree nasali: l’epitelio olfattivo principale, l’organo settale, il ganglio di Grüneberg e il VNO. Il sistema
olfattivo del cane è privo dell’organo settale e del ganglio di Grüneberg. Tutti questi aspetti, oltre alla mancanza del gene
VR2 nel genoma canino, potrebbero essere il risultato di un processo di regressione del sistema olfattivo del cane, nono-
stante questo animale sia considerato un mammifero macrosmatico per il suo particolare olfatto. Tuttavia la maggior di-
mensione della cavità nasale del cane potrebbe comunque permettere una migliore discriminazione fisica degli odori ri-
spetto al topo.
Un’altra possibile spiegazione della regressione del VNO nel cane potrebbe essere la domesticazione a cui questo ani-
male è stato sottoposto da almeno 15 mila anni.
Recentemente alcuni studi hanno valutato il ruolo del VNO nella comunicazione animale, osservando le alterazioni
del comportamento quando tale organo sia lesionato, in modo spontaneo o sperimentale.
Nei roditori il VNO risulta indispensabile nel modulare il comportamento riproduttivo, permettendo la percezione
dei feromoni sessuali. Negli ovini, il riconoscimento dell’agnello da parte della madre non avviene se si rende non funzio-
nante il VNO attraverso la cauterizzazione del dotto naso-incisivo. Nel gatto, processi infiammatori del VNO sembrano
correlati a fenomeni di aggressività.
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... The perception of smell in most mammals is directly influenced by the structure of the olfactory organ itself. It is made up of two main parts, the main olfactory system and that responsible for the detection of semiochemicals, the vomeronasal organ (VNO), also called the Jacobson organ, which each have separate access routes for fragrances [1][2][3] and communicate with other centers within the central nervous system [4]. In consequence, signals originating from the main olfactory system and the VNO are distinct [5]. ...
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