It's a Taste Bud.

Shown below is a lower power section of the tongue, arrows indicate the taste buds. This particular section of tongue is called the foliate papillae, and the taste buds are embedded within the nonkeratinized stratified squamous epithelium.



Taste buds are barrel-shaped, and contain 30-80 spindle shaped cells. Their apical ends terminate just below the epithelial surface (that's the darker red line of cells in the high power view). That's called a taste pit, and the taste pit communicates with the surface through a small opening called a taste pore.

Taste buds are one of the only specialized sensory cells in the oral mucosa. The stimulation of taste seems to be initiated by the material of the taste pits and the microvilli of the cells projecting into those pits. Adsorption of molecules onto the membrane receptors on the surface of the taste bud cells activates a signaling cascade and releases neurotransmitters that then stimulate nerve fibers surrounding the lower half of the taste bud cells.