VM8054 Veterinary Histology

Exercise 17

Digestive System: Oral Cavity

Author: Dr. Thomas Caceci
Adam and Eve had many advantages, but the principal one was that they escaped teething.

--Mark Twain (1835-1910)
Pudd'nhead Wilson, Chapter IV
Objectives for This Exercise
See Lecture Material For This Topic

The oral cavity is the first portion of the digestive tract, and consists of the mouth and the structures which are enclosed within it. The mouth isn't merely a hole in the face.  It's a complex structure with nutritional, respiratory, and communicative functions in all mammals and most other groups of animals.

Additionally, it serves as the principal means of manipulating (if you'll pardon the expression) the environment for those animals which don't have hands, such as dogs.  A dog carries things in his mouth because he has no other way to do it. (If you have time someday, try to get a dog to bark when he has something in his mouth.  This takes some work, but the noise he makes when trying to eat and communicate simultaneously is worth hearing.)

One can get a clue to the nature of an animal's diet by the type and shape of the teeth, which differ greatly in carnivores and herbivores.

Accessory structures which contribute to the mouth's function include the tongue, the teeth, and the salivary glands. The tongue and the teeth will be dealt with below: click here for a discussion of the structure of salivary glands.


Oral Epithelium

The oral cavity in domestic animals is lined with stratified squamous epithelium.  Depending on the nature of the animal's diet, this may or may not be keratinized.  Those animals which eat a great deal of roughage, such as ruminants and horses, usually have a very heavily keratinized oral cavity, while those which eat softer food show less of it.  There is a considerable amount of variation in this rule, both across species lines and among individuals in a species.

To see the nature of the epithelium of the oral cavity, click here.


Tongue

Slide 1210 is a section of a tongue.  The tongue in mammals is an extremely muscular organ within whose substance there are a number of smaller salivary glands, and any number of motor and sensory nerve fibers.  The bulk of the tongue is skeletal muscle, arranged in three layers, all at right angles to each other.  This provides for an amazing degree of flexibility and is vital to vocalization.  There may be a considerable amount of adipose tissue present as well.  The orientation of the muscle layers gives the tongue a distinctive appearance in LM sections that isn't easily mistaken for anything else.

To see examples of the tongue and its musculature, click here.

Lingual Papillae

Filiform Papillae

On the tongue's surface this particular slide shows numerous filiform papillae.  These are slender conical structures, which project up, forming a velvety covering on the tongue.  The filiform papilla is one of 4 types of lingual papillae, and its function is purely mechanical; it has no sensory structures associated with it, as do the others.

In some species (such as cats and cattle) filiform papillae are extremely large and rough, and the peculiar rasping effect of a cat's tongue is due to these papillae.  It's also what a cat uses to clean her fur.  The stiff papillae make effective bristles for removing debris.

To see examples of filiform papillae, click here.

Vallate Papilla

The remaining types of lingual papillae are sensory in function.  They include:  the fungiform papillae, which are less common than the filiforms; the vallate or circumvallate, which are the largest and most prominent; and the foliate papillae, which are most easily seen in the Order Lagomorpha (rabbits, hares, and conies).

A vallate papilla is present on slide 1210.  This type of papilla is set into a deep pocket in the tongue's surface, and anchored at the bottom by a short broad stalk.  It does not protrude above the general level of the surface.  It's surrounded by a deep "moat" into which some of the lingual salivary glands secrete.  Vallate papillae are the largest type, easily visible with the naked eye in most animals.  They are paired and located near the back of the tongue.

To see a vallate papilla, click here.

Vallate papillae usually show taste buds.  The taste buds are well defined, lightly staining areas which enclose several banana shaped sensory cells.  You should see some on the undersides of the papilla proper, and you may also see some on the tongue side of the "moat."  Taste buds are discrete and well encapsulated by CT, which can be demonstrated with special stains.

The sensory cells of the buds are located below the level of the epithelium, and they communicate with the outside via a so called taste pore.  Through this pore project the microvilli or "taste hairs" on the tops of the sensory cells.

To view taste buds, click here.


Fungiform and Foliate Papillae

You don't have examples of these in your set, but demonstrations are provided.  The fungiform papilla, as the name implies, are mushroom shaped structures.  They rise above the general level of the filiform papillae, and they usually have taste buds. The foliate papilla is best seen in rabbits.  The term means "leaflike" as in the leaves of a book, and these are seen in the demonstration as if you were viewing the top edge of a page of paper.  They are set in the sides of the tongue, and you are looking at them from the dorsal surface.  Foliate papillae carry taste buds.

To see fungiform and foliate papillae, click here.


Teeth

Sound teeth are one of the more important things in life, and an abscessed tooth is likely to make the whole world look black to the person suffering from it.  We featherless bipeds, and most of our pets, have brachydont teeth, the type found in carnivores and the "typical" tooth model.  Ruminants and horses have a different type which won't be considered in this exercise.  You will find a typical brachydont tooth on slide 85. The tooth on this slide is developing in a socket of the jawbone.  This bony alveolus is part of the skeleton, but strictly speaking the tooth isn't.

Primitive connective tissue, or mesenchyme has condensed in the future central region of the tooth, in what will become the pulp cavity.  Inside this region has differentiated a population of specialized cells, the odontoblasts.The odontoblasts make the bulk of the tooth's hard material.  They are visible as tall columnar cells, ranged side by side on the inside surface, closely applied to the wall.  Odontoblasts make dentin, by laying down a matrix material and then calcifying it.  As they do so, they move backwards towards the center of the pulp cavity. This process continues after eruption, and as the animal ages the pulp cavity slowly diminishes in size.

On the outside surface of the tooth, a similar process is occurring; you will easily see a row of nicely columnar cells facing the odontoblasts.  In this region enamel is being made by ameloblasts.  As with the manufacture of dentin, ameloblasts also lay down an uncalcified matrix first, then harden it.  In brachydont teeth, enamel is made only before eruption.  Once the tooth erupts, the ameloblasts die, and if it's damaged or chipped, it can't be renewed.  In some animals, especially in rodents, a portion of the enamel making apparatus remains viable below the gumline, and in these new enamel is made continuously.  For rodents this provides a fresh, sharp, chisel-like edge to the front of the incisors.  That's why they gnaw continuously:  to wear away the excess and keep the edge sharp.  If they are stopped from chewing their teeth will continue to grow until they puncture the roof of the mouth.

To view the layout of forming teeth, and to see the details of enamel and dentin formation, click here.

To see the roots of a forming tooth, click here.


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