ORAL CAVITY
Objectives
You should be able to recognize the following:
| Oral mucosa | Lip | ||||
| oral surface | |||||
| Masticatory mucosa | external surface | ||||
| vermilion border | |||||
| Tongue | minor salivary glands | ||||
| ventral unkeratinized epithelium | |||||
| dorsal keratinized epithelium | Salivary Glands | ||||
| filiform papilla | parotid gland | ||||
| fungiform papilla | submandibular gland | ||||
| foliate papilla | sublingual gland | ||||
| circumvallate papilla | striated duct | ||||
| trench | intercalated duct | ||||
| glands of von Ebner | interlobular ducts | ||||
| taste buds | serous demilune | ||||
| lingual tonsil | |||||
| Hard palate | |||||
| suture | |||||
| glands | |||||
| mucoperiosteum | |||||
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 |
Blue Histology |
Slides
 | D-86 | Lip (H&E) | |
| D-91 | Hard palate (H&E) | |
| D-92 | Tongue, rabbit (Masson stain) | |
| D-93 | Anterior tongue (Masson stain) | |
| D-94 | Tongue (H&E) | |
| D-95 | Tongue (H&E) | |
| D-96 | Posterior tongue (H&E) | |
| D-126 | Parotid gland (H&E) | |
| D-128 | Submaxillary = submandibular gland (H&E) | |
| D-129 | Sublingual gland (H&E) |
Optional
slides
| D-90 | Hard palate (H&E) | |
| D-121 | Pancreas (H&E) | |
| D-127 | Submaxillary gland (PAS stain) | |
| D-130 | Sublingual gland (H&E) | |
| D-170 | Lacrimal gland (H&E) |
Prologue
Oral mucosa
Most of the oral cavity is lined with an "oral mucosa", characterized by non-keratinized stratified squamous epithelium. Mixed serous-mucus glands underlie it in most places.
In areas that receive abrasion the epithelium is keratinized and called "masticatory mucosa. This is the case for the gingiva around the teeth and the hard palate. The dorsum of the tongue also has a keratinized mucosa, albeit a complex one. Anywhere else the epithelium may become converted to a keratinized form if stressed, for example by a dental prosthesis which rubs against it.
Tongue
The tongue is composed of interlacing bundles of striated muscle covered by stratified squamous epithelium. The anterior 2/3 is coated on the dorsum with two types of papillae. Filiform papillae are the most numerous and are evenly distributed. They are slender flame-like keratinized projections of mucosa. A coated tongue results from accumulations of the epithelial cells normally shed from the surface of these papillae. Interspersed in lesser numbers are the fungiform papillae, which are larger, with a bulging, flattened top that contains taste buds. At the border of the posterior 1/3 of the tongue nine to twelve circumvallate papillae are arranged in a V-shape pointing posteriorly. These are shaped like fungiform papillae, but are much larger (2-3 mm) and surrounded by a trench. Taste buds are abundant in their epithelium in the trench. They are pale oval bodies extending the thickness of the epithelium, from the basement membrane up to the tiny pore opening at the surface. The sensitive epithelial cells each contain a short "taste hair" protruding above the surface. At the other end they are attached to tiny end branches of axons of the VIIth and IXth cranial nerves. Taste buds are also scattered individually in the mucosa of the soft palate, epiglottis and pharynx.
The posterior 1/3 of the upper surface lacks papillae and is notable for its prominent aggregations of lymphoid tissue known as the lingual tonsils.
Salivary glands
Large numbers of serous and mucous glands are scattered through the tongue, the inner surface of the lip and the mucosa of the oral cavity. In addition to these small local salivary glands the three large, paired, named glands located outside of the mucosa produce most of the saliva. The parotid, submandibular (= submaxillary) and sublingual glands differ most obviously in the ratio of serous to mucous secretory units. They have a complex set of intralobular ducts that modify the composition of the fluid as it flows through them. The larger excretory ducts of these salivary glands eventually fuse with the stratified squamous epithelium (non-keratinized) which lines the oral cavity, and from which the duct epithelium was embryologically derived.
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Slide descriptions
Slide D-86 Lip (H&E)
The inside of the lip exemplifies the oral lining. Hold this transverse section of the lip up to the light and figure out how it was cut. Distinguish this lingual side (with the thicker epithelial layer) from the external side with a thin epidermal layer. The upper surface in between is the vermilion border. Note where it is on your slide because its boundaries with the skin and oral mucosa are gradual under the microscope. The center of the lip is composed of the orbicularis oris muscle cut in cross section. This is the muscle you exercise when you pucker up for a kiss (with your mother).
Under low magnification examine the cutaneous part, with its hair follicles and glands. Follow the transition of the well keratinized epithelium into the vermilion area of the lip (illustration). The mucosa here has no hair follicles or glands. The dermal papillae are long and extend far up in the epithelium (illustration). They contain a particularly large number of blood vessels which impart the red color to the lips. The thinness of epithelium over them maximizes their effect. Observe that the epithelium over the vermilion border is very lightly keratinized, (illustration).
Continuing around the lip, the degree of keratinization gradually decreases until, on the oral surface, nuclei can be seen in the cells all the way to the surface (illustration). Note the generalization that stratified squamous epithelium usually is substantially thicker when it is nonkeratinized than when it has a dense protective layer of keratin on its surface. This is obvious even by holding your slide up to the light and examining it by eye.
Find the labial glands in the lamina propria ("labial" refers to the lip). They are mixed serous and mucous, but the ones caught on your slides are predominantly the latter. Hunt around for the few serous alveoli here and there in the glands. The serous cells are pyramidal, with a round nucleus in the basal part of the cell where the cytoplasm stains darker due to stores of ribonucleoprotein. Their apical cytoplasm is pink in H & E and contains zymogen granules, precursors of the carbohydrase enzymes, ptyalin (salivary amylase) and maltase. Cell boundaries are indistinct. The mucous cells appear pale blue with the nuclei scrunched flat against the base of the cells. They produce a viscid fluid consisting mainly of mucin. This material is usually extracted during tissue preparation, leaving only a spongy network of residual cytoplasm.
D-93 Anterior tongue (Masson stain)
You have a series of slides showing the anterior and posterior aspects of the tongue; D-92, D-93, 94, 95, and 96. D-93 is a nice one to begin with. It is a section through the entire tip of the tongue of a small monkey and has been stained with Masson stain. The slide shows the unkeratinized epithelium lining the ventral surface (illustration) and the masses of skeletal muscle fibers running in various directions. Its only limitation is that the papillae on the dorsum are not too well preserved. Wait for the other slides to examine these structures in detail.
Observe the very orderly arrangement of the muscle in three perpendicular directions (illustration). This is what makes the tongue so wonderfully dexterous. A giraffe's tongue is even prehensile. The human tongue is almost that nimble. I know a person who can tie a knot in the stem of a cherry with just his tongue. Even the most tongue tied of us can rattle off, "What noise annoys an oyster? Any noise annoys an oyster, but a noisy noise annoys an oyster most." (Even Australians who talk without moving their lips, even they have to wiggle their tongues with lingual muscles just like regular people)
D-92 Tongue, rabbit (Masson)
Slide D-92 was prepared from a rabbit, but cut a bit farther back than D-93. The muscle shows up well as do numerous nerve bundles needed to regulate it. Pause and observe them. Again, this slide is not ideal for examining lingual papillae, which are poorly developed in the rabbit, (illustration). Any one who has been licked by a bunny knows how soft its tongue is. Please raise your hand if you have had this experience so that you can tell the rest of the class what it feels like.
To see the various types of lingual papillae go to slides D-94, D-95 and D-96 .
Filiform papillae are the most numerous type of papillae and are nicely preserved in D-94. To observe them, find areas in which the plane of section is completely perpendicular to the surface of the tongue. Observe that papillae are mucosal structures with a core of lamina propria surrounded by epithelium. Oblique sections will give complex patterns to these structures as shown above for slide D-92 (in the same way that oblique sectioning can make a simple epithelium look stratified, (illustration).
Fungiform papillae are big enough (~ 1/2 mm) to be located by the naked eye. Indeed, you can easily see them on your partner's tongue as tiny red spots. They contain taste buds, although you may not see any on a single section. Finding a fungiform papilla is hit or miss on each of your slides, so look around. Start by holding your slides up to the light and looking at each one for an example. If you are unlucky enough to happen to miss fungiform papillae on all of your section get a new slide from the front of the laboratory.
Circumvallate papillae are worth spending a little extra time on. First locate them on your classmate's tongue. Then find an example on slide D-96 of the human tongue. At higher magnification observe the trench around this papilla, the serous glands (of von Ebner) underneath, and the taste buds in the walls, (illustration). The glands empty into the trench to keep the taste buds washed. In favorable sections you can see that the taste buds extend the entire thickness of the epithelium. You may be able to see the taste pore on one of your taste buds and at least imagine the taste hairs, modified cilia, protruding.
D-95 has a foliate papilla, a type that is common in monkeys, but residual in man. It has some of the characteristics of the circumvallate papilla (trenches and taste buds), When found in humans foliate papillae usually appear as ridges at the sides of the tongue. In this monkey the papillae were more dorsal.
Compare D-96
with D-94 and D-93 as examples of posterior and anterior tongue. Which part of
the tongue has more glands? Which more fat? D-96 may show some of the lymphoid
deposits (lingual tonsils)
that characterize the posterior tongue.
D-91 Hard palate (H&E)
Hold this slide up to the light and orient yourself to this frontal section. Why is this portion of the palate called the "hard" palate ? What kind of epithelium lines the roof of the mouth? What kind of epithelium lines the opposite surface of the hard palate?
Can you identify the kinds of glands under the oral epithelium? Note that they are located laterally. In the midline the masticatory mucosa is closely attached to the periosteum of the palate bone (illustration). This confluence is called a mucoperiosteum, (illustration). If you were going to give an injection in the hard palate you obviously would want to place the needle laterally instead of in the midline.
D-126 Parotid gland
The parotid is a branched acinar (actually tubulo-acinar, or better tubulo-acinar), all-serous gland. Under minimal magnification its lobular divisions are clearly visible. Within the lobules the secretory acini are easily distinguished from the ducts. The many round open spaces are scattered fat cells (check them out if you do not believe me. The occasional one shows its flattened nucleus.) Fat cells happen to be characteristic of the parotid gland. Do not mistake them for mucous acini in this purely serous gland.
Examine the acini under high power. They have round, basally located nuclei and basophilic cytoplasm as you have come to expect for serous-secreting cells. The apical half of the cell is filled with secretory granules. These should be acidophilic as they contain secretory proteins but the granules have been dissolved out in most places leaving just a foamy-looking cytoplasm. This is not unusual for routine histology. Look around for acini which retain their secretory granules and note that, indeed, they are orange.
Begin your consideration of the duct system of this gland with the help of the facing diagram in your syllabus. Ducts are named in two ways; by location and by function. Thus, these ducts of the parotid may be interlobular or intralobular. Physiologically they are of three types: excretory, striated and intercalated. The diagram shows that the first of these types is interlobular, while both striated and intercalated ducts lie within the lobule.
When this is clear to you scan this slide with your 3.5X objective. Note how the connective tissue divides the gland into lobules of various sizes. You may see medium-sized blood vessels in this connective tissue. Find a cross-section of an interlobular duct and observe it with your 10X objective. This is an excretory type of duct, whose function is simply to carry away the secretion. The duct will be surrounded by interlobular connective tissue and lined by an epithelium that can be simple columnar, pseudostratified, or stratified columnar depending on its size. Unfortunately, the epithelium in these large ducts is too poorly preserved on this slide to warrant lots of study.
Instead, move to the purple-staining parenchyma within a lobule. Scan around for cross-sections of acidophilic intralobular ducts lined with a simple cuboidal/columnar epithelium. Switch to your 43X objective and study one of these striated ducts. Note the eosinophilic cells which line them. Their staining characteristic comes from their large numbers of mitochondria for pumping ions into and out of their lumen. Your textbook explains why they are called both "striated ducts" and "secretory" ducts. These ducts have the important physiological function of regulating the ion content of the saliva. They are not just passive pipes to the surface, as the excretory ducts are. Striated ducts are connected to the purple-staining secretory alveoli by even smaller intercalated ducts. To identify them, slowly scan around the lobule, looking between the alveoli for a circle of 3-5 elongated tall squamous / low cuboidal cells, smaller than the alveoli. Be sure to look at your atlas before hunting for them and you may not wish to bother.
Finally, glance at the connective tissue stroma around the epithelial structures. One particular cell type of the intralobular stroma worth mentioning is the plasma cell. It secretes antibodies (IgA) as an important component of saliva. Can you recognize these cells?
D-128 Submaxillary (submandibular) gland (H&E)
Scan this compound, branched tubuloacinar mixed gland with your 3.5X objective. It has a substantial proportion of both mucous and serous cells, (illustration). The mucous-secreting cells are palely stained, with nuclei flattened against the cell bases. They are closely related to the goblet cell. The serous-secreting cells stain vigorously with hematoxylin. Their basophilia suggests vast quantities of ribosomes and, indeed, this cell type makes enzymes for export in a watery secretion. Note that small groups of serous cells often cap the end of mucous acini. These half-moon-like figures are called serous demilunes. Your text has an important picture of the relationships of these cells.
The submandibular gland has both striated and intercalated (2nd example) ducts. As the duct diagram in your syllabus shows most of the intralobular duct system is made up of striated variety (illustration). Short segment of intercalated ducts run into striated ducts. This is readily apparent on your slide. Thus, the large number of obvious striated ducts in sections is one way to distinguish the submandibular gland from the other compound glands of the body.
The interlobular ducts are very nicely preserved here and I have enjoyed looking at their epithelia, seeing some cases of the elusive stratified columnar variety and even epithelia that cannot decide whether to be simple or stratified. I hope that you also have the joy of finding this interesting.
D-129 Sublingual gland (H&E)
Scan this predominantly mucous gland using your 10X objective. It has mainly large light blue mucous alveoli with only a few darker staining serous units (illustration). Look closely to convince yourself that the parotid really is compound, branched tubuloacinar in form and that most of the serous cells are arranged into demilunes at the tips of the secretory alveoli (illustration). With your 3.5X objective, again find large interlobular ducts surrounded by a large amount of connective tissue that radiates to define and surround the lobules. There are not many intralobular ducts in this gland. One reason is that the secretory units are very elongate (as you can observe and the duct diagram shows) and so fewer ducts are needed. The intralobular ducts that are present are mainly excretory; that is, they are just passive pipes for conducting secretions. They are undistinguished looking and surrounded by a fair amount of connective tissue. At 10X verify that the intralobular ducts appear structurally similar to the interlobular ones. You may come across a striated duct if you are lucky. Intercalated ducts are not present in this organ so please don't find one! Another feature to look for are plasma cells. Their antibodies (IgA) are a component of saliva.
Both the parotid and submandibular glands secrete through single large, named duct; Stenson's and Wharton's ducts, respectively.The sublingual is different. It empties through several terminal ducts. Therefore it is a collection (or complex) of glands.
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Optional slides
D-90 Hard palate (H&E)
Repeat your observations on the palate from D-91 on slide.
Hold the
slide up to the light and orient yourself to this frontal section. What two features
tell you that this portion of the roof of the mouth is not the soft palate (answer)?
It is said that woven bone sometimes persists in the hard palate into adulthood.
Can you see any? There are some great views of Sharpey's fibers here. You can
also review the type of glandsin the hard palate and the appearance of a mucoperiosteum.
D-127 Submaxillary gland (PAS stain)
This slide has been stained for carbohydrates to emphasize the mucous cells. Note the purple-red mucous alveoli interspersed with the pale serous alveoli, (illustration). Clearly, this is a mixed mucous serous gland. What percent of the secretory cells would you say are mucus-producing? Is this about the same estimate you would have made for D-128? Can you see pale serous demilune cells capping a red mucous alveolus? Do the ducts stain the way you expect?
D-130 Sublingual gland (H&E)
This slide pretty much duplicates D-129.
It does show, however, that the percentage of serous secretory units varies from
specimen to specimen. This section (illustration)
shows substantially more than D-129. You will be working with spit from sublingual
glands for a long, long time so you might as well check out both slides of this
tissue carefully. Are you surprised that the stratified squamous epithelium
on this slide is not keratinized?
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The lacrimal gland and pancreas are other all-serous compound glands that bear some resemblance to the parotid. You looked at them earlier but if you have time take the opportunity to compare the three again. The duct diagram (in the written syllabus) will be useful.
D-170 Lacrimal gland (H&E)
The lacrimal gland is mainly tubular. Most of the distance
that the secretion flows is through the elongated secretory units themselves instead
of ducts. Consequently, it has relatively few intralobular ducts (and these are
only excretory). This parallels the case of the largely-tubular mucous sublingual
gland. Since the secretory units are long they have to have a substantial sized
lumen towards their proximal ends. Thus, many of the sections have very obvious
lumens in them, making some look like lifesavers, (illustration).
Compare this with the acinar parotid gland, in which you have a dickens of a time
seeing the lumen in most of the acini, (illustration).
D-121 Pancreas (H&E)
Although the pancreas is all serous, it has several features that easily distinguish it from the parotid:
| Islets of endocrine tissue | |
| No fat cells | |
| Only intercalated ducts within the lobule and some are pretty big |
Remember also those centroacinar cells of the pancreas? They are not too easy to find. Why not "give them another go" (as the Australians say) if you have the slide on your microscope.