2012 Group Project 3
Taste or more appropriately gustation, is a fundamental survival tool in animals as it directs the consumption of essential nutrients. The five tastes that exist within the human gustatory system: salty, sweet, sour, bitter and umami, all signify basic physiological requirements. Salty tastes denote the presence of Na+, an important ion involved in the transportation and retention of water across cell membranes. Sweetness is the recognition of carbohydrates, essential for maintaining optimal brain function and providing the basis for energy production in muscle tissue via ATP hydrolysis. Similarly umami codes for the presence of L-amino acids, especially L-glutamate which is an integral component of protein synthesis.
These different modalities not only function to maintain the proper intake of nutrients within the body but also, by way of aversion, prohibit the consumption of undesired or poisonous materials. The bitter modality for example is generally regarded as displeasing to the human palate. It represents a high acid content which may be a result of foods which have become rotten or are inherently poisonous to the body.
Research has historically limited itself to investigating the mechanisms of these five taste qualities though it must be noted many other modalities may exist. Of particular conjecture is the existence of fatty acid recognition. In the past the detection of fat in food has been attributed to somatosensory inputs  from the tongue, that is, the oily feel and texture of fat rather than its actual ‘taste’ allows the brain to encode for its presence. Further research is therefore required to elicit the precise mechanisms of fat detection.
The five taste qualities are not all detected by the same type of receptor cell located within the taste bud. Like rods and cones in the eye which detect different wavelengths of light, there are specific types of receptors for different tastes. For example, sweet, umami and bitter are recognised by Type II G-protein coupled receptors, whereas sour is related to Type III presynaptic cells. The cell type involved in salty taste transduction is unknown however it is known that sodium ions can enter the receptor cell membrane via ion channel permeation.
Type II receptors
When a bitter, sweet or umami ligand binds to a type II G-coupled receptor, a cascade of chemical reactions causes the release of Ca2+ which in turn mediates exocystosis of ATP. The function of this ATP is threefold:
- Stimulates gustatory nerve afferents which project to gustatory nuclei in the spinal cord
- Excites Type III presynaptic nerve fibres to release serotonin.
- Has a positive feedback effect on type II receptor cells, increasing the level of ATP production.
The role of serotonin is believed to be in the form of lateral inhibition i.e. when a bitter quality is recognised, adjacent receptors for sweetness are deactivated and thus the two tastes may be clearly differentiated. It also has a negative feedback effect on receptor cells, inhibiting umami, bitter and sweet taste transduction.
The idea of a tongue map has disseminated through society for many years. This concept purports that different areas of the tongue are specialised to detect either sweet, salty, sour and bitter tastes. Recent research however has completely nullified such claims and suggests instead that different forms of taste are recognised all over the tongue as well as via the palate. Thus the term taste map has come to take on a new meaning and that is, the precise areas of taste modalities processing in areas of cortex and its subsequent neural inputs.
First order neurons - From the receptors located in the taste buds, gustatory nerve afferents project to the ipsilateral rostral 1/3 of the nucleus tractus solitarius (NTS), located in the medulla. This rostral 1/3 is commonly referred to as the gustatory nucleus.
The gustatory nucleus receives input from cranial nerves VII (Facial n.), IX (Hypglossal n.), and X (Vagus n.) via special visceral afferent (SVA) nerve fibres. A summary of their functions is as follows:
- Facial nerve n. - carries taste information from the anterior 2/3 of the tongue.
- Hypoglossal n. - carries taste information from the posterior 1/3 of the tongue.
- Vagus n. - carries taste information from the palate
Second order neurons - From the NTS second order neurons carry taste information to the Ventral posteromedial (VPM) nucleus in the thalamus.
Copious scientific conjecture surrounds how each taste modality is transmitted to the brain. The ‘labelled-line’ hypothesis suggests that there are taste specific neurons which exclusively carry that taste modality to cortical areas. In analogous terms it can be viewed like the pipelines leading toward a house. Each pipeline carries its own utility, there is one for gas, another for water and finally for electricity with each terminating in slightly different areas of the house. In the same way, on the tongues there are different receptors for each taste which concurrently have individual nerve tracts leading to the primary gustatory cortex.
Contrapuntally there is electrophysiological evidence that single nerve afferents carry multiple modalities. These studies show one nerve afferent may have both a strong and a weak activation in response to a multiple taste stimuli.
The location of the taste perception centres has been observed via functional magnetic resonance imaging (fMRI) studies. The primary taste cortex has been identified as being located in the anterior insula/frontal operculum (I/fO), with the secondary taste cortex in the caudolateral orbitofrontal cortex. Within the insula there are further subdivisions related to each taste modality. Two photon calcium imaging research has outlined certain ’hot-spots’ of activation which are clearly delineated in relation to each taste. For example, the bitter modality is represented on the insula cortex approximately 1mm posterior to the middle cerebral artery whereas the sweet modality is represented 2.5mm rostrodorsal to the bitter field with no apparent overlap. These findings lend weight to the idea that there is only one receptor for each taste quality.
It is interesting to note the relationship between taste and the reward centres of the brain. There is evidence of neural input to the ventral tegmental area (the origin of the mesolimbic dopaminergic reward pathway) and nucleus accumbens (involved in the conversion of motivation into physical action). Neural connections such as this provide the foundations in explaining concepts such as flavour learning and preference. It may also give some insight into chronic problems such as food addiction and craving. As of yet however the gustatory neural network is not well understood and more research is required to elucidate the relationship between pleasure and taste sensation
Timeline of Developmental Processes of the Gustatory System
The surface of the developing tongue is covered by nearly flat epitheliuem (Figure 1) . The first gustatory papillae of the tongue appears in the caudal midline near the foramen caecum  . On the dorsal midline the first circumvallate papilla develops (Figure 2) 
|Weeks 6 to 7||
Nerve fibres approach the basal lamina of lingual epithelium . At this stage the lingual epithelium consists of two to three cell layers and there is not yet any sign of cell specializations indicating early taste bud formation.
A series of epithelial swellings in the anterior part and midline of the tongue indicate early forming fungiform papillae (figure 2) .
The lingual epithelium shows first signs of taste bud development as nerve fibres penetrate epithelial basal lamina  and form synapses with taste bud progenitor cells. The synapses reach a maximum around the 12th to 13th week . However, at this time these cells are still poorly differentiated, elongated epithelial cells  . The synapses demonstrate the neuronal connection between the taste bud primordium and the central nervous system . Ciliated cells also appear around the 8th week, however the significance of these cells which are scattered randomly across the lingual surface remains unclear .
“By this stage the top surface of circumvallate papillae usually contains a taste pit partly filled with microvilli of presumed underlying taste bud cells” (Figure 10) 
At this stage, the lingual epithelium compromises of about four cell layers, and the first shallow grooves above the taste bud primordium are developed .
Untypically differentiated apical cellular processes extend onto the surface 
Taste bud cells are more clearly differentiated into epithelial cell types II and III .
By this stage the taste bud primordial are all located on the top of dermal papillae (Fig 6a) . There is also maximum synapses between cells and afferent nerve fibres, which intermingle with each other to form a plexus-like structure .
|Weeks 14 -15||
The shape of the taste buds primordial begins to resemble those of adult taste buds . By the 14th week the taste pores develop as the taste pits are filled by microvilli , indicating the possibility that the taste buds begin their gustatory function , however they still lack an electron-dense mucous material (Fig.10) . The tastebuds only achieve a fully developed function in week 15 of gestation with the development of type I cells to produce the mucous material in the taste pit .
History of Discoveries
|350BC||Aristotle writes about the basic tastes, sweet and bitter, which can be modified, he says, by salty and acidic.|
|1901||D. Hanig publishes a paper containing data of taste sensitivity in different regions of the tongue. The data are later misinterpreted, giving rise to the myth of the ‘tongue map’|
|1908||Fifth basic taste discovered: savouriness, described as umami, which is conferred by glutamate.|
|1931||Bitter taste sensitivity found to vary among humans (1)|
|1931-32||Genetecists confirm findings about sensitivity to bitter tasting PTC and discover non-tasting is a recessive genetic trait (2,11)|
|1939||Geneticists show that chimpansees like humans, vary in their ability to perceive the bitterness of PTC|
|1992||Discovery of gustucin, a teste cell-specific G-protein, in the taste buds. Gustucin is later shown to mark bitter, umami and sweet cells (13)|
|1996||Witt M and Reutter K of the Source Department of Anatomy, Technical University Dresden, Germany, carried out a transmission electron microscopy study to investigate the embryonic and fetal development of Human taste buds. Their results suggest an "at least dual function of embryonic/fetal taste buds", including non-gustatory, paracrine functions prior to the 14th week and gustatory after the 14th week. PMID 8955790|
|2000||First taste sonsors, the T2R receptors, discovered (3)|
|2001||The sweet receptor is discovered (5): a combination of TaR2 and T1R3.|
|2002||Amino acid detector, T1R1 and T1R3 identified (6) = Umami|
|2005||Sweet taste receptor found (15) in the GI tract|
|2006||Cells for sour taste discovered, identified by PKD2L1 (4-7)|
|2007||Harlow DE, Barlow LA of the Department of Cell and Developmental Biology, Rocky Mountain Taste and Smell Center, University of Colorado Denver Health Sciences Center provide evidence of the embryonic origin of gustatory cranial sensory neurons PMID 17826760|
|2009||The Car4 receptor, which senses the carbon dioxide in fizzy drinks is found on sour cells (8)
|2010||ENaC identified as the sodium-salt taste receptor (9)|
Adult Tongue and Taste Buds – Structure and Function
The tongue is located on the floor of the oral cavity, It is a muscular structure with sensory units crowning. The tongue is divided into an anterior two thirds and a posterior one third. These regions are divided by a V-shaped groove at the back of the tongue (sulcus terminalis). The anterior two thirds of the tongue is covered by stratified squamous epithelium, It contains a roughened surface and has projections called papillae that vary in shape and number.
'The Image Below is a very simplistic Diagram of the surface of the Tongue showing the locations of the different papillae and other important features
There are 4 types of papillae on the tongue. The most numerous papillae are the filiform papillae, which function to provide a surface that aids in holding food on the tongue during chewing but do not contain taste buds. The larger, less numerous fungiform papillae which contain taste buds, as do foliate papillae. Circumvallate papillae form a wide V at the sulcus terminalis also containing taste bus. There are no papillae or taste buds located on the posterior third of the tongue, having mucosal folds and the lingual tonsils instead.
The tongue is for tasting, swallowing, and speech. The functional unit of the taste bud is a taste cell, there are between 50 and 100 taste cells in each taste bud these taste cells represent all 5 different tastes. Historically is was believed that different areas of the tongue were responsible for different taste sensations although this has since been disregarded. The papillae contain taste buds which are connected to the oral cavity via a taste pore, the function of the taste bud is to transmit a chemical signal from the oral cavity to a taste cell, this chemical signal is the converted into an electrical impulse and delivered to the brain via nerve fibres for interpretation.
Knocking out P2X Receptors
Gustatory abnormalities has not been widely researched and what has been researched has been through animal testing, most commonly we have found on mice.
In Huang, 2008 this team used the release of the neurotransmitter, ATP (adenosine triphosphate) as a quantitative measurement of gustatory sensation and taste. This was done by using a comparison of wild type (WT) and double knockout (DKO) mice. P2X receptors, P2X2 and P2X3 were knocked out in the DKO mice. The premise of this article was that knocking out P2X receptors reduces transmitter secretion of ATP in taste buds, therefore they cannot taste.
It should be noted that the taste buds in DKO are functional, but are not stimulated by the administration of tastants.
The transmission of release of ATP is secretion through gap junction hemichannels (pannexin 1 gap junction).
When both P2X2 and P2X3 are knocked out, no taste is elicited. However they found that if either P2X2 OR P2X3 was knocked out there was a taste response. So the inference made from this is that if one of the two receptors from the P2X family was knocked out there still can have taste response. The WT mice showed significant stimulation by tastants whereas DKO had little to no stimulation of ATP release.
FGF signalling and genes
Sprouty, or Srpy, genes have been related to regulating the development of circumvillate papillae (CVP). The CVP are large dome shaped papillae, which form a 'V' just in front of the terminal sulcus.
By knocking out Spry genes using mice which had Spry1 and Spry2 knocked out showed that the number of CVP doubled. However, when Fibroblast Growth Factor gene ("Fgf10") was absent, the number of CVP was significantly reduced, if not completely absent. The correlation between Spry1/2 and Fgf10 is that Spry1/2 antagonises "Fgf10" to limit the size of the CVP progenitor placode. Exclusive expression of Fgf10 in the mesenchyme is necessary for the formation of CVP.
Taste buds develop from the mesenchyme but require local signalling to properly differentiate. Some signalling factors for proper development of taste buds besides FGF are: Sonic Hedgehog (SHH), Bone Morphogenetic Proteins (BMPs), Epidermal Growth Factor (EGF).
As mentioned above, this article shows that the anterior and posterior developments of the tongue are derived from embryonic tissues, where the anterior tongue is derived front the ectoderm and the posterior tongue is derived from the endoderm. Secondly, FGF is required to regulate the growth of taste buds, while Spry genes limit the number of CVP. Fgf10& Spry 1&2work antagonistically through receptor tyrosine kinase (RTK)signalling.
However, the deletion of Spry2led to the increase of CVPs it should be noted that they found that a significant decrease in number of fungiform papillae. Contrastingly, the absence of Fgf10, while leading to the absence of CVPs lead to an increase in number and size of fungiform papillae. Therefore these genes have different effects on the anterior and posterior developing tongue and taste buds.
We have an evolving understanding of Embryonic Taste development, through the use of state-of-the-art technology and research techniques we are able to make brilliant discoveries that continue to connect the dots of this amazing natural process of human development. The majority of research in taste development is involving mice, these mammals show similar embryonic pathways to humans and research is performed in ethical and humane methods.
the below image shows histological stains of a mice tongue showing dysplasia"
Synergistic relationships of Six Genes
In an animal study using mice by Suzuki Y, Ikeda K, Kawakami K.(2011) Firstly nominating "Six Genes" as a major component in gustatory development, stating that deficiencies in certain Six genes (specifically Six1 & Six4) leads to poor development. Their research also highlights evidence of cooperative relationships between Six genes for normal advance. This experiment involved breeding mice containing exclusively Six1 and Six4 genes and examining the expression of these genes in papillae under high powered microscope observation. Understanding the role of certain genes along with the intrinsic relationships they hold is crucial for the ability to identify possible causes and correction of any abnormalities 
Neural Crest responsibilities
Another Animal Study involving mice explores a new idea of Neural crest (NC) contribution in taste development, specifically the development of papillae and taste buds. Liu HX, Komatsu Y, Mishina Y, Mistretta CM. (2012) suggest that Neural crest cells travel to the location of the tongue in early embryonic stages, gain epithelium phenotypes, multiply and then differentiate to eventually form taste papillae. The experiment involved the comparison of 2 different types of Cre line mice, which both express Cre gene in neural crest protocol, the different distribution patterns where observed in specific regions that NC is responsible for.
Augmentation of Endoderm
In an animal study conducted by Rothova M, Thompson H, Lickert H, Tucker AS.(2012) exploring the historically debated issue of endoderm contribution to tongue development showed promising evidence that position of taste buds are patterned by the border of ectoderm and endoderm derivative epithelium. This study was accomplished via microscopic examination of previously stained specimens. Concluding endoderm has direct influence on gustatory develoment 
Changes in Taste Cells over time
Research by Ozdener H, Spielman AI, Rawson NE.(2012) developing a culture which allows taste cells to survive for up to 12 months, empowers researchers to study the processes of proliferation, differentiation and function. This experiment will provide a precidnet for future study of taste cells, as these cells are able to operate and grow normally. 
A tamoxifen treatment which supresses Sonic hedgehog (Shh) secretion in mice proving to reduce the number of cells visable within papillae, in contrast mice not treated with tamoxifen showed a mark increase of cells within papillae, these cells are assumed to become taste cells in later development. Specific mice were bred to trace the destination of taste placode cells, the study concluded that Shh expressing placodes are taste bud proginators which in turn become taste cells within taste buds although do not have any prescuros relationship with pappilae. The results were obtain by examination of mice embryos using Bright-field or multichannel fluorescent images through the use of an "Axiocam CCD camera and Axioplan fluorescence microscope with Axiovision software" a study by Harlow, Yang, Williams, Barlow (2011)
WNT family exhibit various roles
The WNT gene family has a function of signally proteins for various reason such as development, In 2010 Liu, Staubach Grosse, Walton, Saims, Gumucio, Mistretta explored recent findings on the role of WNT's in tongue and papillae development, Concluding that WNT/β-catenin is essential for fungiform papillae differing to WNT5a which proved to be principle in tongue development. Intrinsic chemical mediators were considered by manipulating a tissue culture and then throughaly examined by scanning photomicrograph. 
The below image shows and example of tongue abnormalities, this is called "double tongue" each side has independent movement 
ectoderm- outer germ layer of embryo
endoderm - inner germ layer of embryo
epithelium - basic animal tissue that is composed of tightly packed cells, usually covering the outer portion of organs
exocystosis - movement of contents out of cell
ganglia - the accumulation of a nerve cell body
growth factor - a substance that stimulates the growth of cells
gustatory - anything that relates to the taste sense
hydrolysis - breakdown of a chemical when it reacts with water
mesoderm - middle germ layer of embryo
mesenchyme - multipotential cells
neural crest - a part of the ectoderm found on both sides of the neural tube
neuron - most simplistic unit of the nervous system
neurotransmitter - chemicals which allow the passing of signals from neuron to neuron via connecting part (synapse)
papillae - small rough surface projection
six genes - a family of genes
Sonic hedgehog (Shh) - signalling protein invloved in normal development
sulcus terminalis - describes a physical transition point separating the anterior 2/3 of the tongue from the posterior 1/3
tamoxifen - hormonal altering drug
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