Taste Bud. Its Histology for under graduate dental students

Taste Bud: The Histology

Tongue

The tongue is a mass of striated muscle covered by mucosa, which manipulates ingested material during mastication and swallowing.

 The muscle fibers are oriented in all directions, allowing a high level of mobility. 

Connective tissue between the small fascicles of muscle is penetrated by the lamina propria, which makes the mucous membrane strongly adherent to the muscular core.

The lower surface of the tongue is smooth, with typical lining mucosa.

The dorsal surface is irregular, having hundreds of small protruding papillae of various types on its anterior two-thirds and the massed lingual tonsils on the posterior third, or root of the tongue (Figure (a)).

The papillary and tonsillar areas of the lingual surface are separated by a V-shaped groove called the sulcus terminalis.

The lingual papillae are elevations of the mucous membrane that assume various forms and functions. There are four types (Figure (A)):

1. Filiform papillae (Figure (A)) are very numerous, have an elongated conical shape, and are heavily keratinized, which gives their surface a gray or whitish appearance. They provide a rough surface that facilitates movement of food during chewing.
2. Fungiform papillae (Figure (A)) are much less numerous, lightly keratinized, and interspersed among the filiform papillae. They are mushroom-shaped with well-vascularized and innervated cores of lamina propria.
3. Foliate papillae consist of several parallel ridges on each side of the tongue, anterior to the sulcus terminalis, but are rudimentary in humans, especially older individuals.
4. Vallate (or circumvallate) papillae (Figure (A)) are the largest papillae, with diameters of 1 to 3 mm. Eight to twelve vallate papillae are normally aligned just in front of the terminal sulcus. Ducts of several small, serous salivary (von Ebner) glands empty into the deep, moatlike groove surrounding each vallate papilla. This provides a continuous flow of fluid over the taste buds that are abundant on the sides of these papillae, washing away food particles so that the taste buds can receive and process new gustatory stimuli. Secretions from these and other minor salivary glands associated with taste buds contain a lipase that prevents the formation of a hydrophobic film on these structures that would hinder gustation.

(A)Dorsal surface of tongue

Taste buds are ovoid structures within the stratified epithelium on the tongue’s surface, which sample the general chemical composition of ingested material 

(Figures (A) and (B)). 

Approximately 250 taste buds are present on the lateral surface of each vallate papilla, with many others present on fungiform and foliate (but not the keratinized filiform) papillae.

 They are not restricted to papillae and are also widely scattered elsewhere on the dorsal and lateral surfaces of the tongue, where they are also continuously flushed by numerous minor salivary glands.

A taste bud has 50 to 100 cells, about half of which are elongated gustatory (taste) cells, which turn over with a 7- to 10-day life span.

 Other cells present are slender (b) Micrograph shows a single very large vallate papilla with two distinctive features: many taste buds (TB) around the sides and several small salivary glands (Gl) emptying into the cleft or moat formed by the elevated mucosa surrounding the papilla.

 These glands continuously flush the cleft, renewing the fluid in contact with the taste buds.

 The base of each bud rests on the basal lamina and is entered by afferent sensory axons that form synapses with the gustatory cells.

 At the apical ends of the gustatory cells, microvilli project toward a 2-μm-wide opening in the structure called the taste pore.

 Molecules (tastants) dissolved in saliva contact the microvilli through the pore and interact with cell surface taste receptors (Figure (B)).

Taste buds detect at least five broad categories of  tastants: 

1.  Sodium ions (salty);
2.  Hydrogen ions from acids (sour);
3.  Sugars and related compounds (sweet)
4.  Alkaloids and certain toxins (bitter); and 
5.  Amino acids such as glutamate and aspartate (umami; Jap. umami, savory).

 Salt and sour tastes are produced by ion channels and the other three taste categories are mediated by G-protein–coupled receptors.

 Receptor binding produces depolarization of the gustatory cells, stimulating the sensory nerve fibers that send information to the brain for processing.

 Conscious perception of tastes in food requires olfactory and other sensations in addition to taste bud activity.

(B) Histology of taste bud

Taste and its pathway

Taste: Taste allows person to select food according to its desire.

Taste Buds: these are mainly present on the dorsal surface of the tongue but few 

present on palate, pharynx, glottis.

Size: 50-70 milli microns

Number: 10000

30-80 cells are present in each taste bud.

Particular type of substances activates particular sense receptor known as taste modalities i.e sweet, salty, sour, bitter.

Taste areas in brain:

1.   Insular Cortex

2.   Somatosensory cortex

Taste reflex:

Taste reflex is concerned with the saliva secretion. Large number of nerve impulses from nucleus tactussolitarius are transmitted directly into the superior and inferior salivary nucleus, from there signals are transmitted to the salivary glands to help the control of secretion of saliva during ingestion of food. This is taste reflex.

Cell types of taste buds:

1.   Type-I cell/light cell – secrete pit substances

2.   Type-II cells/Dark cell – contains numerous vesicles

3.   Type-III cell/ Intermediate cell- synapse with numerous nerve fibers

4.   Type-IV cell/basal/precursor cell – production of new cells

There are two types of cells involved in generation of action potential:

·        Transducing

·        Gustducin

Factors affecting taste:

1.   Temperature of oral cavity

2.   Hormonal

3.   Genetic

4.   Age

5.   Anaesthetic drugs (Local)

6.   Use of tobacco, pan etc

Taste pathway:

Taste pathway is concerned with transmission of taste sensation to the brain from tongue as explained by the figure below:

(C) Diagrammatic representation of taste pathway