The origin of Taste Buds
It is speculated that taste buds evolved as a means of preventing the consumption of poisonous or rotten food (4).
Studies show that all sensory receptor cells originate from embryonic neurogenic ectoderm, however, the origin of taste bud progenitor cells is actually from epithelial cells which have been innervated by nerves at the basal end of the bud (6).
Where are Taste Buds Found?
Taste buds are located on the anterior surface of the tongue, the soft palate tissue at the posterior roof of the oral cavity (1), the esophagus and the epiglottis (3).
The tongue has four types of papillae all within the anterior epithelium, three of which house taste buds:
- Filform papillae - cover the majority of the anterior surface, highly keratinized and lack taste buds
- Fungiform papillae - fewer in number, lightly keratinized and scattered taste buds
- Foliate papillae - lateral ridges with taste buds, not well developed in adults
- Vallate papillae - largest but fewest in number, form an inverted V separating the anterior and posterior surface of the tongue; contains more than half of the taste buds of the human oral cavity. The buds form within a groove surrounding each papillae into which serous salivary glands empty, continuously moving dissolved particles over the buds.
Structure and Function of Cell Types
Although located within different papillae and vary in location from tongue to esophagus, taste buds have the same structure and are comprised of three cell types.
- Gustatory
- Supportive
- Stem
The bud is a cluster of 50-75 cells located within stratified epithelium and is connected to the surface via a taste pore (1).
Gustatory cells are are the major cell type of the bud with microvilli at the apical surface entering the taste pore where contact is made with dissolved particles from the mixing of food and saliva (1). These chemoreceptor cells, of which there are approximately 100 per bud, bind and convert external stimuli into action potentials (2, 3); the basal portion forming synapses with afferent axons of the facial nerves and glossopharyngeal nerves which carry the signals to the brain (5).
The second cell type is the Supportive cell found amonst the gustatory cells, both of which are elongate. Apart from the supportive role as indicated by the name, the function of these cells is not known (1).
The Stem cells lie beneath the other two cell types and differentiate into one or the other; regenerating gustatory cells every 7-10 days (1).
Sense of Taste
There are five main categories of taste that are preceived by humans (1)
- Salty - genereated by metal ions
- Sour - generated by hydrogen ions of acids
- Sweet - generated by sugars and similar organic compounds
- Bitter - generated by alkaloids and certain toxins
- Umami (Savoury) - generated by certain amino acids, for example glutamate
Each tastant has a function. Whether we are consciously aware of it or not, our taste buds pick up on what is necessary for survival versus those food items that are actually harmful to us.
The salt and sour tastants lend themselves to maintaining homeostasis of salt and acid concentrations in the body. The bitter tastant allows identification of poisonous foods, such as those compounds found in poisonous plants. The sweet tastant is linked to foods that are high in calories whereas the umami tastant appears to be linked to foods that are high in protein (5).
Perhaps the "cravings" one experiences are our bodies way of telling our conscious selves what we are lacking.....like chocolate!
Pathology
Head trauma involving the anterior fossa has been shown to result in long term or permanent loss of taste receptor function (7).Seizures which involve the temporal lobe, such as those typical of epilepsy, can result in short term sensory loss of taste (7).
Age may also play a role in the loss of taste sensation but research suggests that by understanding the pathways, receptor mechanisms and response patterns of taste, not to mention the over lap of the olfactory system with taste perception, it may be possible to engineer molecules that will stimulate the chemoreceptors of the taste buds (5).
Three known disorders for the sense of taste (8):
- Hypogeusia - a diminished taste sensation to the five basic categories.
- Ageusia - loss of all taste sensation (very rare).
- Dysgeusia - the continual presence of a foul, salty, rancid or metalic taste sensation that does not correspond to an ingested substance.
Taste perception disorders can result from:
- Head trauma (as mentioned above)
- Respiratory and/or middle ear infections
- Various chemicals, exposure to environmental factors ie: insecticides; ingested perscriptions
- Radiation treatment
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References:
1. Mescher, Anthony L. 2010. Junqueira's Basic Histology, Twelfth Edition. The McGraw-Hill Companies, Inc.
2. http://en.wikipedia.org/wiki/Sensory_receptors
3.http://en.wikipedia.org/wiki/Taste_bud
4. http://answers.yahoo.com/question/index?qid=20060717154001AAMA3hI
5. Bradbury, Jane. 2004. Taste Perception: Cracking the Code. PLoS Biology, volume 2, issue 3, p 0295-0297.
6. Stone, Leslie M., Thomas E. Finger, Patrick P. L. Tam, Seong-Seng Tan. 1995. Taste receptor cells arise from local epithelium, not neurogenic ectoderm. Proc. Natl. Acad. Sci. USA. Vol. 92, pp. 1916-1920
7 British Medical Journal. 1971. Taste and Smell. British Medical Journal. pp. 508-509
8. http://www.nidcd.nih.gov/health/smelltaste/taste.html#3
