Localization and Histotopography
During embryogenesis, the organ of taste develops from the stratified epithelium of the lingual papillae. In adults, taste buds are located predominantly on the lateral walls of foliate, fungiform, and vallate papillae.
Histologically, taste buds are clearly visible as pale, rounded or oval structures that stand out against the darker surrounding stratified squamous epithelium. The buds contact narrow spaces between the papillae. Connection with the external environment is established via the taste pore, an opening through which dissolved tastants from the oral cavity enter.
Cellular Composition of the Taste Bud
A single taste bud comprises 30 to 60 cells, which are histologically and functionally divided into four main types:
- Receptor (sensory) epithelial cells (Type I). These possess dark, spindle-shaped nuclei forming the upper tier within the bud structure.
- Supporting epithelial cells (Type II). These feature round nuclei located in the middle tier and often have vacuolated cytoplasm. Under electron microscopy, they appear as "dark" cells. They extend the entire height of the bud—from the basal lamina to the apical pore—and bear microvilli at their apex. While they lack sensory function, they actively synthesize an adsorbent substance.
- Basal epithelial cells (Type III). Their nuclei and cell bodies lie at the base of the bud and do not reach the surface. These are stem/progenitor elements that ensure continuous population renewal (averaging every 10 days). They can differentiate into either sensory or supporting epithelial cells.
- Perigemmaチュラル (peripheral) cells. 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. Interspersed among the microvilli is an adsorbent substance of complex composition, necessary for concentrating tastants from saliva.
Specific protein receptors are embedded in the microvillous membrane of sensory cells. Taste reception is strictly segregated topographically: receptor cells in the anterior part of the tongue predominantly express sweet-receptive proteins, whereas those in the posterior part express bitter-receptive proteins.
The signal generation process (chemotransduction) is indirect and involves a cascade of biochemical reactions:
- Primary contact: Tastant molecules bind to membrane receptors on the microvilli.
- Enzymatic activation: Membrane enzyme activity is altered.
- Second messengers: Intracellular mediator concentrations (e.g., cAMP) change in the cytoplasm.
- Ionic shift: Mediators act on plasma membrane ion channels, 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 and transmitted via synaptic contacts to afferent nerve endings.
Neural Pathways of Gustatory Sensitivity
Taste buds are innervated by the dendrites of sensory neurons running within several cranial nerves (nervi craniales).
- Anterior part of the tongue: Impulses are collected by dendrites that initially course through branches of the trigeminal nerve (n. trigeminus, CN V) and subsequently join the facial nerve (n. facialis, CN VII). The cell bodies of these sensory neurons reside in the geniculate ganglion (ganglion geniculi) within the temporal bone.
- Posterior region of the tongue: Gustatory fibers run primarily within the glossopharyngeal (n. glossopharyngeus, CN IX) and vagus (n. vagus, CN X) nerves.
- Central pathway: Axons of all participating sensory neurons project to the brainstem, terminating in the nucleus of the solitary tract (nucleus tractus solitarii).