Histology of Tongue

Tongue organ consists essentially of a lining of stratified squamous epithelium and striated muscle, arranged in a number of layers. The connective tissue attaches the tough mucous membrane of the tongue to the muscular mass.

The epithelium of the tongue is thickest and has the heaviest stratum corneum on the dorsal surface. The tongue bears various papillae, which are named for their characteristic gross morphology.

The papillae are macroscopic projections formed with a central core of connective tissue and a covering layer of stratified squamous epithelium.

The tissue core may give rise to small microscopic papillae (commonly referred to as papillary bodies) over which the epithelium is moulded. According to the shape, the (macroscopic) papillae of the tongue are divided into various types- Filliform, Fungiform, Cirucumvallate (present in all animals) and Foliate (present in horse, donkey, rabbit).

Filliform papillae

• The filliform papillae consist of a connective tissue core derived from the lamina propria and an epithelial covering characterized by a heavy stratum corneum.
• The papillae are usually slender and pointed. In most species the papillary core does not extend above the level of the glossal epithelium.
• The visible projection is made up entirely of epithelium.
• Large conical papillae, whose core projects beyond the surface of the tongue, occur in all domestic mammals except the horse and donkey.
• In ruminants the connective tissue core gives rise to several small secondary papillae, whereas the epithelial coat is raised into a single cornified one. In the horse, donkey and pig, the papillary core is an enlarged similar to papilla, from the top of which a cornified thread projects above the epithelial surface. In carnivores, the connective tissue core extends above the surface epithelium and bear papillae of unequal sizes.

Fungiform papillae

• These are relatively few in number and interspersed among the filiform palpillae.
• They have rounder and broader summits and narrow attached ends.
• The connective tissue core is rich in nerves and characterized by papillary body, have relatively less cornified epithelium containing taste buds.
• Taste buds may or may not be present.
• Taste buds are very few in the Ox and horse. They are numerous in sheep, goat, pig etc.

Circumvallate papillae

• These are very few in number and are arranged in ’V’ shaped row nearer the posterior part of the tongue. They resemble fungiform papillae but are much larger and are surrounded by a cleft (moat) lined with epithelium. They project above the lingual epithelium only slightly or not at all.
• Their connective tissue core bears microscopic papillae and is rich in nerves and lymphocytes. The epithelial surface facing the moat contains many taste buds.
• Deeper to that papillae lie groups of serous glands called Van Ebner’s glands whose excretory ducts open into the moat at various levels.
• These bear numerous taste buds in pigs, and dogs and less so in cats. In the horse and pig, the taste buds are present over the entire papillary wall of the moat and are restricted to the bottom only in carnivores.

Foliate papillae

• These are present only in some animals and each consists of a series of parallel connective tissue leaves, rich in nerves and bearing secondary papillae that project into the covering stratified squamous epithelium.
• Gustatory furrows separate them from each other.
• The epithelium covering the sides of the leaves bears taste buds.
• Deeper to the papillae lie serous glands whose ducts empty into the gustatory furrows.
• absent in ruminants, rudimentary without taste buds in cats but are present in the horse, dog and rabbit.
• Taste buds: Those in the horse are melon-shaped and in Ox and Sheep they are Ovoid. In pigs, they are spindle-shaped and may extend into the tunica propria in the dog. They are more or less spherical in the cat and they are poorly defined.

Lingual glands : Mixed glands are present in the margin of the tongue in the horse, in the root of the tongue in Ox and horse.