This blog was designed as an assignment for the BIOL 3500 course at Memorial University of Newfoundland.
 |
| Fig 1 |
It’s no secret that dogs have an
excellent sense of smell! It's what makes them a great asset to police forces,
hunters, search and rescue teams and now-a-days even scientists researching
cancer and bacteria (as you will see in this blog).
But have you ever wondered why they
have this amazing sense? What makes them different from other animals and even
humans? This blog aims to answer these questions and present the science behind
the common knowledge (with some fun facts thrown in for good measure).
 |
| Fig 2 |
Air takes a different journey through the dog nose than it would traveling through the human nose and many of these differences account for the
dog’s superior sense of smell.
To fully understand the anatomy of the dog nose, one must realize that
inhaling air is not just important for identifying odors, but also important in
respiration to obtain oxygen from the environment and release carbon dioxide back out. Thus, the anatomy of the
dog nose is specialized to maximize both of these functions. (Lawson et al. 2012)
The dog nose has two main passages that are separated by
the lamina transversa. One of these leads to the lungs for gas exchange
while the other goes to the olfactory area. This is the first difference we see from humans who have only one passage for both functions with the olfactory epithelia found on the roof
of the nose. (Craven et al. 2009)
Firstly, the air surrounding the nose enters the nasal vestibule and gets mixed up to ensure that the olfactory epithelia gets a “representative
sample” containing all the different chemicals in the air. The air splits here
and travels down each passageway.
 |
| Fig 3: The flow of air through the dog nose. (Lawson et al. 2012) |
The respiratory pathway is the least complicated of the two. The
air leaves the nasal vestibule, goes through the maxilloturbinate to the
nasopharynx then the lower respiratory tract. (Lawson et al. 2012.) As it travels this route
the air gets warmed/cooled, moistened and filtered. (Craven et al. 2007.
The more complicated pathway is the olfactory pathway. The
air entering this pathway leaves the nasal vestibule and travels through the
dorsal meatus which leads to the olfactory region. Travelling through this
airway, the air bypasses the maxilloturbinates and instead goes to the
olfactory recess, turns 180° and goes
forward through the ethmoturbinates and then on to the nasopharynx (Lawson et al. 2012). Since the ethmoturbinates have a scroll-like appearance,
they provide a large surface area for odors to be absorbed (Craven 2009) and
the air spends more time in this region than any of the other turbinates (Lawson et al. 2012).
(Craven et al. 2007)
There are 4 main types of epithelium in the nose!
1) Simple squamous found in the nasal vestibule
2) Respiratory pseudo-stratified
columnar epithelium in the maxilloturbinate and frontal sinuses
3) Olfactory pseudo- stratified
columnar epithelium (contains olfactory cells) in the ethmoturbinates
4) Transitional epithelium in
the region of the posterior nasal vestibule to the anterior maxilloturbinate
* If you notice, the nasal vestibule has simple squamous
epithelium and the maxilloturbinates, which it leads into, is covered in
respiratory pseudo-stratified columnar epithelium. The transitional epithelium thus
allows the switch between the two.
 |
| Fig 4: Olfactory and respiratory epithelium (Lawson et al. 2012) |
As well, most of the
interior of the nose is covered in a mucous secretion which serves many
purposes such as removing odorous molecules, aiding in heat transfer and
keeping the cilia alive, just to name a few.
(Craven et al. 2007)
It is hard to distinguish where the respiratory epithelium ends and the olfactory epithelium begins as there seems to be some olfactory cells mixed in amongst the non-sensory cells of the respiratory epithelium.
Two of the main functions of the respiratory epithelium - filtration and
warming/cooling - are accomplished by a couple of structural features. Firstly,
the lamina propria (connective tissue) found underneath the epithelial layer
contains an abundance of blood vessels which can dilate and constrict to aid in
warming and cooling respectively. Secondly, filtration is aided by the motile
cilia projecting from the surface. One note worth mentioning is that the
respiratory epithelium does not participate in olfaction.
 |
| Fig 5: Olfactory and Respiratory Epithelium (Lawson et al. 2012) |
Olfactory epithelium differs from respiratory epithelium in a couple of
ways. Although it also has a layer of lamina propria underneath its epithelial
layer, unlike that of the respiratory epithelium it is not vascularized. It
also has cilia but they also differ. These cilia are non-motile and
actually extend from the dendrites of the olfactory receptor cells.
At the surface of the olfactory epithelium lies many olfactory receptor
neurons (ORN's) to which odors bind and are recognized. However, these are specific in
that only certain odors bind to certain ORN’s. Dividing them into two main
categories makes it easier to predict what odors bind to which receptor. Type 1
ORN’S bind hydrophilic molecules- those that are soluble in the mucous membrane
of the nasal cavity while Type 2 bind hydrophobic particles. (Lawson et al.
2012).
As well, not only do dogs have more
genes dedicated to encoding for olfactory receptors than humans (1,300 versus
the 650-900 for humans) (Quignon et al. 2005) but a larger percentage of them
are true genes (Quignon et al. 2003). For canines, 18% of these are pseudogenes
compared to the 63% for humans!
What about sniffing?
 |
| Fig 6 |
I’m sure you’ve all
seen a dog sniffing when trying to locate something, when it encounters
something new or it’s favorite food, etc. But does sniffing really enhance
olfaction? Yes! A dog will move its nose independently to sniff, dilate its
nostrils, breathe more frequent and have higher air flow rates than during
normal respiration. Seems obvious this would increase olfaction, right? But
something else quite interesting happens when a dog sniffs which increases
sensitivity. (Craven et al. 2009). The mechanism of sniffing actually opens a
direct passage in the nose to the dorsal metus which leads straight to the
olfactory region. Here, the odours concentrate and interact with the receptors
on the olfactory epithelium (Craven 2007).
Just how much better smellers are dogs than humans?
(PBS Article)
*Their nose contains up to 300 million olfactory receptors in their nose compared to our ~6 million
*They can detect odor
concentrations at 1-2 parts per trillion (10 000- 100 000 times that of humans)
*The area of their
brain devoted to smells is about 40 times greater than ours when looked at in
proportion
Dogs possess a
vomeronasal organ (Jacobson’s organ)- located on the bottom of the nasal passage (above
the roof of the mouth, behind the upper incisors) devoted to identifying
pheromones. Nerves lead from here to the brain.
Detecting Bacteria
The canine’s sense of smell is so
astonishing that they can be trained to detect bacteria! In one study, a 2 year
old beagle was trained to sit or lie down when it detected Clostridium difficle in both stool samples and patients. The dogs
sensitivity and specificity was both 100% in the stool samples and it correctly
identified 25 of the 30 cases of patients with the bacteria and 365 out of 370
controls (Bomers et al. 2011).
Detecting Cancer
Dogs are now being
trained to detect cancers in humans! In one study a Labrador Retriever was
trained to detect colorectal cancer in exhaled breath and stool samples. In the
breath samples there was a 0.91 sensitivity and 0.99 specificity. In the stool
samples, there was sensitivity of 0.97 and specificity of 0.99. This supports
the claim that there actually are certain specific compounds present in the
body of those with cancer (Sonoda et al. 2011).
Bloodhounds!
 |
| Fig 8 |
The bloodhound has
long flappy ears that sweep the odors up towards its nose making it a more
efficient tracker!
And to end, here's a picture of my dog, Milo ...
And to end, here's a picture of my dog, Milo ...
Bomers, M. K., van Agtmael, M. A., Luik, H., van Veen, M. C., Vandenbroucke-Grauls, C. M., & Smulders, Y. M. (2011). Using a dog's superior olfactory sensitivity to identify Clostridium difficile in stools and patients: proof of principle study. BMJ (Clinical research ed.), 345, e7396-e7396.
Craven, B. A., Neuberger, T., Paterson, E. G., Webb, A. G., Josephson, E. M., Morrison, E. E., & Settles, G. S. (2007). Reconstruction and morphometric analysis of the nasal airway of the dog (Canis familiaris) and implications regarding olfactory airflow. The Anatomical Record, 290(11), 1325-1340.
Craven, B. A., Paterson, E. G., & Settles, G. S. (2009). The fluid dynamics of canine olfaction: unique nasal airflow patterns as an explanation of macrosmia. Journal of The Royal Society Interface, 7(47), 933-943.
Sonoda, H., Kohnoe, S., Yamazato, T., Satoh, Y., Morizono, G., Shikata, K., ... & Maehara, Y. (2011). Colorectal cancer screening with odour material by canine scent detection. Gut, 60(6), 814-819.
Quignon, P., Giraud, M., Rimbault, M., Lavigne, P., Tacher, S., Morin, E., ... & Galibert, F. (2005). The dog and rat olfactory receptor repertoires. Genome biology, 6(10), R83.
Quignon, P., Kirkness, E., Cadieu, E., Touleimat, N., Guyon, R., Renier, C., ... & Galibert, F. (2003). Comparison of the canine and human olfactory receptor gene repertoires. Genome biology, 4(12), R80-R80.
http://www.pbs.org/wgbh/nova/nature/dogs-sense-of-smell.html
