The vallate (or circumvallate) papilla is the
largest and least numerous type. They're found towards the back
of the tongue, usually in pairs. This very large structure has a core of connective tissue (CT).
The papilla proper is set down below the surface of the tongue in a "moat" that completely surrounds it. Taste buds (TB) are present on the sides of the papilla, and also frequently on the side of the moat. Small lingual salivary glands (SG) empty into the moat to keep the taste buds washed clean and ready for re-use. Like the rest of the tongue, the surface of the papilla is covered with stratified squamous epithelium. Taste buds (which you see in this low power view as light-colored dots along the side) are embedded in this epithelium. Taste buds are also often seen on the sides of the "moat."
In
this higher magnification view, you can see some of the salivary glands associated
with the "moat" and emptying into it.
There are two types of these lingual salivary glands*. They're serous and mucous in nature. The
secretions of these glands are wash the chemoreceptor sites
of the taste buds free of molecules, so that new
sensations can be detected. These aren't the only salivary
glands in the tongue, by the way; there are small ones scattered
throughout the muscle bundles.
*Any salivary gland actually embedded in the substance of the tongue is a "lingual" salivary gland. Don't get this term confused with the "sublingual" gland, which is a distinct anatomic structure.
Here's
a very nice view of the duct (D) draining the products of the salivary
glands into the "moat" (M) around the vallate papilla.
Monkey tongue; H&E stain, 1.5 µm plastic section, 20x, 20x, and 40x
