Organ of taste

Localization and histotopography

The organ of taste develops in embryogenesis from the stratified epithelium of the lingual papillae. In the adult organism, taste buds are located predominantly on the lateral walls of foliate, fungiform, and vallate papillae.

In histological preparations, taste buds are clearly visible: they appear as light, round or oval structures standing out against the background of the darker stratified squamous epithelium. The buds contact the narrow clefts (spaces) between the papillae. The connection of the bud with the external environment is carried out through the opening — the taste pore, where dissolved molecules of tastants enter from the oral cavity.

Cellular composition of the taste bud

A single taste bud comprises 30 to 60 cells. Histologically and functionally, they are divided into four main types:

1. Receptor (sensory) epitheliocytes (type I). They possess a dark spindle-shaped nucleus. The nuclei of these cells form the upper row in the structure of the bud.
2. Supporting epitheliocytes (type II). They have a round nucleus located in the middle row and often a vacuolated cytoplasm. Under electron microscopy, they appear as "dark" cells. They extend the entire height of the bud — from the basement membrane to the apical pore — and bear microvilli at their apex. They do not perform a sensory function, but actively synthesize the adsorbing substance.
3. Basal epitheliocytes (type III). Their nuclei and the cells themselves are localized at the very base of the bud and do not reach its surface. These are cambial (stem) elements ensuring the constant renewal of the population (on average every 10 days). They are capable of differentiating into both sensory and supporting epitheliocytes.
4. Perigemmal (peripheral) cells. They are located along the outer contour (periphery) of the taste bud.

Cytophysiology and chemotransduction

The apical surface of receptor and supporting cells facing the taste pore is covered with microvilli. Between the microvilli lies the adsorbent — a substance of complex composition necessary for concentrating taste molecules from saliva.

Specific protein receptors are embedded in the membrane of the sensory cell microvilli. The organization of reception is strictly differentiated: in the anterior part of the tongue, receptor cells contain predominantly sweet-receptor protein, while in the posterior part, they contain bitter-receptor protein.

The process of signal generation (chemotransduction) is indirect and involves a cascade of biochemical reactions:

• Primary contact: taste molecules bind to membrane receptors of the microvilli.
• Enzymatic activation: the activity of membrane enzymes changes.
• Second messengers: the concentration of intracellular mediators (e.g., cAMP) changes in the cytoplasm.
• Ionic shift: mediators affect the ion channels of the plasmalemma, altering their permeability.
• Potential change: a shift in the cell membrane potential occurs.
• Synaptic transmission: excitation is generated at the basal surface of the sensory cell, which is transmitted via synaptic contacts to afferent nerve endings.

Neural pathways of gustatory sensitivity

Innervation of taste buds is provided by dendrites of sensory neurons running within several cranial nerves (nervi craniales).

• Anterior part of the tongue: impulses are collected by dendrites passing initially in the branches of the trigeminal nerve (n. trigeminus, cranial nerve V), and then joining the facial nerve (n. facialis, cranial nerve VII). The cell bodies of these sensory neurons are located in the geniculate ganglion (ganglion geniculi) within the temporal bone.
• Posterior regions of the tongue: gustatory fibers run predominantly within the glossopharyngeal (n. glossopharyngeus, cranial nerve IX) and vagus (n. vagus, cranial nerve X) nerves.
• Central pathway: the axons of all participating sensory neurons project to the brainstem, where their terminal point is the nucleus of the solitary tract (nucleus tractus solitarii).

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Morphological differences of foliate, fungiform, and vallate lingual papillae.
• Ultrastructural features of "light" and "dark" taste bud cells under electron microscopy.
• The role of second messengers (cAMP) and membrane enzymes in the chemotransduction cascade.
• Detailed anatomy of the gustatory pathways in the brainstem.

Other topics in the section "Sense organs: organs of hearing, equilibrium, and taste"

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