Taste Buds Explained

The molecular origin of taste buds depends on Eya1, Sox2, and Shh, among other genetic factors. Similarly, taste sensory neuron development depends on a set of genes required to form three epibranchial placodes, the geniculate, petrosal, and nodose. Placodal neuron differentiation depends on Neurog2 in mammals. Additional genes are required for these ganglia to develop the peripheral axons that innervate the three regions of the tongue containing taste buds, fungiform (geniculate, VII), foliate (petrosal, IX), and circumvallate (nodose, X). Taste buds sense at least five distinct taste modalities: sweet, bitter, umami, sour, and salt, and possibly more (e.g., fat). G-protein-coupled receptors on Type II cells are required for sweet, umami, and bitter transduction and Type III transduce sour. The cellular location of salt transduction remains unclear. Two additional cell types are common in the taste bud: the Type I cell, which functions as glia, and a differentiating Type IV cell type. Axons of the three peripheral ganglia reach the hindbrain to provide the solitary tract. The nucleus of the solitary tract (nST) has partially distinct and partially overlapping sensory input. Output of the nST stays ipsilateral to innervate the thalamus before it innervates the cortex, adjacent to the rhinal fissure that will in humans to form the insula cortex.