TOOTH AND GINGIVA

Objectives

You should be able to find the following structures in decalcified or ground sections, as appropriate:
ToothCementum
   crown   acellular
rootcellular
cementocyte
Dentin
dentinal tubuleDentino-enamel junction
peritubular dentinjunction of enamel and cementum
contour lines of Owen
 Alveolar bone
Enamel 
enamel rodsPeriodontal ligament
stria of RetziusSharpey's fibers
gnarled enamel
ameloblastGingiva
 free
Pulp cavityattached
odontoblastenamel space
sulcus
 attachment epithelium
 
 * * * * *

Slides to examine
 D-87Roof of mouth, fetus (H&E)
 D-152 Deciduous tooth (H&E)
D-153Molars, monkey (H&E)
D-154Tooth, adult, posterior (ground)
D-155Tooth, adult,  (ground)  
D-156Tooth, anterior X.S. (ground)  
D-158Maxillary tooth, monkey (H&E)
D-159Mandibular tooth, monkey (H&E)

Prologue

In the short time available for study of the tooth and related structures, only normal morphology will be considered. The fine details and other relationships will be presented later in the Spring quarter course, Oral Histology and Embryology (46lB-46lC). However, this will be your only chance to study histological material with your own microscope and at your own pace.

Teeth, like bone and other calcified tissues can be prepared for histological examination in two ways, by grinding to a thin section and by decalcification. Slides D-154, D-155 and D-156 were prepared by cutting slabs of extracted teeth as thinly as possible with a diamond saw and then grinding them thinner with sandpaper. The sections remain fully mineralized, but no soft structures or cells survive. No two sections will be exactly alike in terms of orientation detail and damage, so be prepared to look at all three slides to find the essential features displayed. Enamel can be visualized only slides prepared by grinding because mature enamel contains almost no organic material, Decalcification completely removes it leaving only an "enamel space" where the enamel was. In contrast, dentin and cementum have enough organic material (especially collagen) to survive decalcification. They can be seen in both ground and decalcified preparations.

It is important to include a general knowledge of tooth structure in this course so that you can relate its parts to other tissues and structures that we have already studied. All parts of the tooth and adjacent structures, except for its enamel and the epithelium of the gingiva are specialized connective tissues. Dentin and cementum are obviously mineralized, dense connective tissues. The periodontal ligament, as the name implies, is a regularly arranged dense connective tissue with some special features of its own. The gingiva is a mucosa (composed of an epithelium with underlying connective tissue) and its attachment to the tooth is of particular importance in dentistry.

Enamel is the hardest structure in the body. Since its origin is ectodermal, it should come as no surprise that its extracellular matrix does not collagen, as is found in mineralized connective tissues. Instead it has special enamelin proteins. Enamel is the secretory product of epithelial cells (ameloblasts) of the tooth germ. In effect each ameloblast produces an individual enamel rod. As the enamel rods lengthen, the layer of ameloblasts retreats. The enamel thus thickens appositionally during its formative stages. Not surprisingly, metabolic disturbances affect enamel deposition so that incremental growth lines (stria of Retzius), analogous to tree rings, generally can be seen in sections of undecalcified enamel, They make 45% to 90% angles with the enamel prisms and usually are particularly prevalent near the gingival region. Later, you will learn about other, both normal and pathological, irregularities that occur in enamel.

Dentin, as mentioned above, is a highly specialized connective tissue. The odontoblast cells responsible for its manufacture and maintenance belong to the family that includes fibroblasts and osteoblasts. They line the inner surface of the dentin facing the pulp. Each cell extends a long odontoblastic process all the way through dentin to the enamel junction. Each process lies in a channel known as a dentinal tubule. In undecalcified sections the tubules are easily seen as fine close-set, parallel, black lines when they are full of air. Where embedding material has flooded the tubules they may disappear from view.

 

Slide descriptions

D-156 Anterior tooth, transverse section (Ground)

D-154 Molar (Ground)

D-154, D-155 and D-156 are ground sections of teeth. The first two slides have longitudinal sections, the third a cross-section.

The important landmark in the crown of the tooth is the dentino-enamel junction (DEJ) where the dentin contacts the enamel. It runs between the pulp chamber and the surface of the tooth. The enamel on the outside of the DEJ usually looks yellowish or brownish. The dentin looks white or gray with fine black stria in some places. Find the DEJ at 430 X and see that it has a scalloped appearance. Increase its visibility by lowering your condenser or shutting your diaphragm way down, as you did earlier to look at ground bone. The enamel is seen to consist of vast numbers of mineral "rods" or "prisms." At some places in most of your slides air will have been trapped between individual rods, outlining them with very fine black lines. Where the mounting medium has fully penetrated the substance of the enamel, the rods may be less evident. Find an appropriately oriented section (probably best along the side of the crown) where it is apparent that the enamel rods are extraordinarily long and thin. A rod can extend all the way from the dentino-enamel junction to the surface of the enamel. In some places rods run parallel to one another (such as along the side of the crown, above). In other areas of "gnarled enamel" they are interwoven to form an especially hard tissue (e.g. look on the top surface between the cusps on the molar (illustration, magnified). In the good old days of slow hand pieces you and your patient knew when you were drilling through gnarled enamel.  Now all of this great classical dentistry is lost to new fangled technology.

You can see two types of incremental lines in the enamel. Lines of Retzius look brownish and may be more evident at low power, (example #1, example #2).  Looking very carefully at high power you can see circadian (daily) increments along individual prisms. They are about as long as a prism is wide (illustration) and will show up better if you displace your condenser lens.

The main visible structures within dentin are the dentinal tubules. In life they contain very thin processes of odontoblasts. Here, of course these soft tissues are gone and the tubules are hollow. As with the enamel rods, even when impregnated with mounting medium, they can still be seen by diffraction if the microscope condenser diaphragm is reduced. Where they are filled with air they appear black and are totally obvious, extending from the DEJ to the surface of the pulp chamber in a smooth S shaped curve. Close to the DEJ the tubules branch (second example) and the odontoblastic processes branch. How do you think that this happened, developmentally? Right near the pulp the tubules may bend in a sharper curve.

Dentin continues to be deposited slowly throughout adult life. The layer formed after the tooth erupted is called reparative dentin. You may be able to distinguish it from the primary dentin on your slide because its tubules are less orderly or curve sharply. Pathological stimuli to the odontoblasts (e.g. caries extending through the enamel or removal of the overlying enamel with a drill) increase the formation of post-eruptive dentin.

The size and number of dentinal tubules is most clearly revealed in places where they have been sectioned transversely (and examined at high power without the condenser lens). How wide would you estimate them to be from your slides? In such places you can perhaps observe that the peritubular dentin, right around the dentinal tubules is different (more highly calcified) from the matrix further away, (illustration).

Dentin grows only by apposition which, once again, leaves behind telltale incremental "growth lines" called contour lines of Owen. These are less apparent than the incremental lines but may be visible in ground sections when the tissue is properly oriented and the condenser is displaced. However, they usually show up poorly (more typical example). Most of you will have to search hard for definitive examples and some of you simply will not be able to find them on any of your slides. (In this this case ask your neighbor team to show you what they found).

A layer of cementum covers the exterior surface of the dentin in the root, in place of enamel. In many respects cementum is a form of bone. The first cementum that is laid down on a root is primary or acellular cementum.  It is only one or two microns thick and, as its name suggests is without cells or cell processes in it (illustration).  Later, under the stimulation of tension on the periodontal ligament secondary or cellular cementum is further deposited. Especially near the apex of the root, where the cementum is thickest, cementocytes are embedded in the extracellular matrix (illustration). This cellular cementum exhibits lacunae and canaliculi, looking the way you would expect in sloppy bone (illustration). The cementum 1/3 of the way down from the enamel crown usually remains without embedded cementocytes. Favorable circumstances of orientation, staining and illumination, often allow one to see that cementum has a lamellar structure basically comparable with bone.

On many teeth the layer of enamel extends down exactly to where the cementum starts. In a small percentage, the two tissues overlap or there is a tiny gap where the dentin is covered by neither (example). What is the situation for your teeth? (yea, yea, those on your slides, wise guy).

 

D-158 Maxillary tooth, monkey (decalcified, H&E)

D-152, D-153, D-158 and D-159 have conventionally decalcified and stained sections of teeth in their sockets. They complement what you saw in undecalcified sections. The first thing to do is figure out the geometry of the teeth in the sections so that you will know the orientation of sections through the dentin.

Remember, no enamel is left on these teeth, proof of the almost negligible amount of organic matrix material in that tissue. The empty space where the enamel had been is called the enamel space. In contrast, approximately 30% by weight of dentin is collagen. The dentinal tubules will be preserved, coursing through the decalcified dentin. They are best seen by lowering  the microscope condenser, which will also emphasize any incremental growth lines. Examine them at high magnification in longitudinal and transverse section. Look for the contour lines of Owen in the longitudinal orientation.

Odontoblasts can continue to deposit dentin even during adulthood, as mentioned above. This reduces the pulp space and lengthens the dentinal tubules as the odontoblastic layer retreats. Between mature (calcified) dentin and the layer of odontoblasts there is invariably a thin zone known as predentin. It is, in fact, precalcified dentinal matrix and as such, analogous to the prebone (osteoid) layer found between osteoblasts and calcified bone. Predentin is more acidophilic than that of calcified dentin.

The layer of odontoblasts within the pulp cavity should be examined with some care. These cells do not form a true epithelium, as is also the case for the sheets of osteoblasts which line the surfaces of bone. Neither of these sheets of cells has a basement membrane. Also, the cells are not really tightly lined up with each other, and while they do have junctional complexes, there can be extracellular material between individual cells. Nevertheless, the cell bodies are tightly packed and elongate to accommodate the vast number of cells.

Within the confines of the odontoblast layer the pulp cavity is filled with a loose, highly cellular connective tissue. Its blood vessels are unusually thin-walled for their size, as is feasible in this highly protected environment. As we all know only too well, nerves exist in pulp, but they are finely divided thin fibers and not easily seen except in special preparations.

Turning to the outer surface of the root, can you find acellular and cellular cementum? Again, a thin layer of acellular cementum covers the entire root surface. At the tips of the roots this layer is overlain by a thicker deposit of cellular cementum.

The periodontal ligament attaching the tooth to the alveolar bone of the tooth socket is well seen. This ligament is much more cellular than most ligaments and very much more vascular. The ends of its collagen fibers are embedded in the alveolar bone and the cementum as so-called Sharpey's fibers. Indeed, the function of cementum is to anchor the periodontal ligament. Since the periodontal ligament will haunt you for the rest of your career it probably is worth while finding places on your slide where you can see Sharpey's fibers entering into the alveolar bone and into the cementum (high mag) of the tooth.

Examine the free gingiva, the sulcular epithelium lining the sulcus, the enamel space between the gingiva and the missing enamel, and the junctional epithelium, with its epithelial attachment, (illustration). (A little line of scum from the enamel lies between the sulcus and the enamel space.) Now, follow the epithelium of the attached gingiva the other way and see that it becomes unkeratinized over the attached gingiva.

In the early life of an erupted tooth, the gingival epithelium attaches to the enamel at the base of the crown region through the epithelial attachment. The apical surfaces of these cells adhere to the enamel above them. However, with time and a recession of the gums, the epithelium becomes attached to the cementum. Later in life not only the entire crown but even part of the root may be exposed with the attachment entirely to the cementum. This migration keeps the periodontists in business. (What would the periodontal veterinarian think of these two monkeys?)




D-87 Roof of mouth (fetus) H&E

Here you can see two tooth buds growing in the fetal palate. Figure out their orientation and directions of growth. Now, identify the enamel, dentin, ameloblasts and odontoblasts.  The enamel at this early stage stains intensely because it contains a high concentration of protein. This is not the case with adult enamel. The tissues have shrunk considerably during fixation so scan around to find individual areas where the dentin and enamel remain attached (illustration), where the odontoblasts are in contact with the dentin they have produced (illustration), and where the ameloblasts are attached to their enamel surface (illustration). Study the tissues at these three areas. The odontoblasts form a closely packed sheet of cells. The ameloblasts are truly epithelial. Do they secrete from their apical or basal ends? Just outside of the ameloblasts are other epithelial cells. These cells and the ameloblasts represent two epithelia joined top to top. If this is not clear, draw a diagram of how the epithelium of the oral cavity invaginates to form the tooth bud. If you still cannot make your lab partners understand this relationship call over an instructor to report them or (perish the thought) make them look up the topic in your textbook.

When the oral epithelium dips down into the connective tissue the basal cells differentiate into a layer of ameloblasts and these induce the adjacent connective tissue cells to differentiate into odontoblasts.  The two types of cells line up across the basement membrane like two teams of Aussie rules football ("footie") players.  The odontoblasts start to secrete dentin.  This provokes the ameloblasts, in turn, to start secreting enamel.  Each ameloblast secretes one growing enamel prism.  As you look at the tooth bud you can tell which ameloblasts (and which odontoblasts) have been secreting for the longest time (how?)  What part of the tooth is the first to form?  OK now, how can you tell from looking at this slide that odontoblasts begin to secrete dentin before the corresponding ameloblasts become active? (A hint which you probably do not need, what part of the tooth bud should you look at?)

 

Yippee, That's it! N 104 is finally over. Good-by Dr. Lu, Thank you Dr. Meyer. Just the final to go and I am through with histology forever, forever . . . well, at least until the board exams.
Groan.