BIOLOGY OF THE HUMAN DENTITION II

I. Teeth

A. Enamel

The enamel is the hard substance that covers the crown of the tooth. It is highly mineralized and is totally acellular. Enamel is the hardest substance in the body. The primary mineral component is hydroxy apatite. The susceptibility of mineral component to dissolution in an acid environment is the basis for dental decay (caries). In its mature state, it is 96% mineral.

Formation of enamel is completed before eruption of the tooth into the oral cavity. It is thickest at the cusp tips (2.0-2.6 mm) and tapers off to a knife edge at the cementoenamel junction.

(CEJ).Enamel is extremely brittle and is dependent on the underlying dentin for support. It is semitranslucent and is yellow to grayish white in appearance. Enamel is a selectively permeable membrane, allowing water and certain ions to pass via osmosis.

The fundamental morphologic unit of enamel is the enamel rod. Each is formed in increments by a single enamel forming cell, the ameoloblast. Each rod traverses uninterupted through the thickness of enamel. They number 5 to 12 million rods per crown. The rods increase in diameter (4 up to 8 microns) as they flare outward from the dentinoenamel junction.

The dentinoenamel junction is the interface between the dentin and enamel. It is the remnant of the onset of enamel formation. You will see it when you do sections of teeth in the laboratory.

Enamel formation begins at the future cusp and spreads down the cusp slope. As the ameoloblasts retreat in incremental steps, they create an artifact in the enamel called the lines of Retzius. Where these lines terminate at the tooth surface they create tiny valleys on the tooth surface that travel circumferentially around the crown known as perikymata.

One of the lines of Retzius is accentuated and is more obvious that the others. It is the neonatal line which marks the division between enamel formed before birth and that which is produced after birth. You will find it in deciduous teeth and the cusps of first permanent molars. The neonatal line has application in forensic dentistry.

Enamel forms more evenly before birth than after birth. Deciduous teeth, therefore, are almost always uniform in appearance. With the permanent teeth, variations in color and degree of calcification are more likely.

A final note on ameologenesis: ameoloblasts enter their first formative state after the first layer of dentin is formed. They secrete enamel matrix as they retreat away from the DEJ. This matrix then mineralizes. Enamel is produced in a rhythmical fashion.

B. Dentin

This is the specialized connective tissue that makes up the bulk of the tooth, extending for almost its entire length. Dentin is hard, elastic, and is 70% mineral. Unlike enamel which is acellular, dentin has a cellular component which is retained after its formation by cells called odontoblasts.A characteristic of dentin is that it is permeated by closely packed tubules traversing its thickness and which contain cytoplasmic extensions that form dentin and maintain the dentin. When pulp of the tooth dies or is removed by the dentist, the dentin becomes brittle and is liable to fracture.

The color of dentin is yellowish white. The hardness of dentin is less than enamel but is greater than that of bone or cementum. Like enamel, it is is semipermeable to certain ions.

Each odontoblast gives rise to a cytoplasmic extension traversing the thickness of the dentin. The odontoblastic processes occupy the dentinal tubules. These processes were formerly called Tomes' fibres. There is one per odontoblast. In older persons, the dentinal tubules 'silt up' with mineralized substance. In a section, the dentin appears transparent and is called sclerotic dentin.

Dentin sensitivity in the live tooth is mediated by Tomes' fibres. One square millimeter of exposed dentin involves 30,000 odontoblasts and their corresponding Tomes' fibres. The prudent clinician, therefore, will take care not to overheat or dessicate the dentin during restorative procedures.

During dentinogenesis, odontoblasts secrete matrix in discrete increments as the odontoblasts retreat. This matrix, called predentin subsequently calcifies. Phases of mineralization remain visible in the dentin as the contour lines of Owen. These are analagous to the lines of Retzius in enamel. At birth, a neonatal line appears in the dentin of primary teeth and first permanent molars.

Dentin formation is a continuous process throughout the life of the tooth. Dentin has a reparative capability since the odontoblasts remain viable after eruption of the tooth. In response to excessive wear, caries, or irritants, dentin is laid down at an accelerated rate. This is reparative secondary dentin. Contrast this with enamel, which has no reparative capacity.

C. Dental Pulp

The dental pulp occupies the central portion of the tooth--the pulp cavity. It is a remnant of the embryologic organ for tooth development. We have already discussed one of its components--the odontoblast, a word that literally means 'tooth former.' In the adult dentition, the activity of the odontoblasts continues, producing what is called physiologic secondary dentin. Contrast this with the intermittant process of reparative secondary dentin. In time, the pulp chamber can be virtually obliterated.

The primary function of the pulp is to form dentin. The second function is nutritive: the pulp keeps the organic components of the surrounding mineralized tissue supplied with moisture and nutrients. The third function is sensory: extremes in temperature, pressure, or insult to the dentin or pulp is perceived as pain. A fourth function is protective--the formation of reparative secondary dentin.

Nerves and blood vessels access the pulp chamber via the apical foramen. Occasionally, there are accessory foramina.

D. Cementum.

This is the calcified connective tissue that covers the anatomic root of the tooth. It is the least visible of the tissues to the practicing dentist, yet it serves a vital function in the support of the tooth. Cementum anchors the periodontal ligament to the root of the tooth. It is bone-like and light yellow.

Cementum is the thinnest at the cementoenamel junction and is thickest at the apex. It is laid down throughout life and resting lines can be seen in histological sections. Continous cementum deposition maintains the length of the tooth as a physiologic compensation for occlusal wear. You will occasionally see heavy accretions of cementum on the roots of teeth extracted from older patients.

This tissue is important in orthodontics: it is more resistant to resorption than alveolar bone, permitting orthodontic movement of teeth without root resorption.

Histologically, there are two types of cementum: acellular which does not contain cells, and cellular which contains entrapped cells called cementocytes. Acellular cementum is usually immediately adjacent to the dentin.

Let us briefly summarize the function of cementum:

(1) it anchors the tooth to the alveolus via the periodontal ligament

(2) compensates for tooth wear

(3) contributes to continuous eruption of the teeth. Excessive cementum deposition is called hypercementosis. You will see it on extracted teeth.

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Legend: (E)=Enamel; (D)=Dentin; (C)=Cementum; (P)=Pulp

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(1)-Mineral Content: (E)=96%; (D)=70%; (C)=50%; (P)=no mineral except denticles or pulpstones.

(2)-Color: (E)=translucent yellow to grayish white; (D)=light yellow; (C)=light yellow; (P) blood red

(3)-Formative Cells: (E)=ameloblast; (D)=odontoblast; (C)=cementoblast; (P) dental papilla

(4)-Embryology: (E)=epithelial; (D)=ectomesenchyme; (C)=ectomesenchyme; (P)=ectomesenchyme

(5)-Repair: (E)=no replacement but sssome remineralization; (D)=physiological, reparative secondary dentin; (C)=new cementum deposition; (P)=can recover from mild inflammation, but severe inflammation results in death

(6)-Aging: (E)=wear, staining, dental caries and physical damage; (D)=increase in secondary and sclerotic dentin; (C)=increased amount with age, more so at the apex; (P) =reduced in size and may be obliterated

(7)-Sensitivity: (E)=none; (D)=yes, only as pain; (C)=no; (P)=yes

(8)-Cells in Mature Tissue: (E)=none; (D)=cytoplasmic extensions from the odontoblasts; (C)=cementocytes are in lacunae; (P)=odontoblasts and other cell types

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II. Investing Structures, a very brief introduction.

A. Periodontal ligament (PDL)

This is the soft specialized connective tissue located between the cementum covering the root of the tooth and the bone forming the alveolus, the bone of the tooth socket. Its average width is approximately 0.2 mm. Its principle function is to support the tooth in its socket. It also has an important function, as you have experienced if you have unexpectedly bitten into something hard. The fibers of the PDL 'suspend' the tooth in its socket; the ends of the fibers attach firmly into the cementum and bone respectively.

Mechanically, the suspension of the tooth is best understood as analogous to a hammock. Orthodontic treatment is possible because the PDL continuously responds and changes as the result of the functional requirements imposed upon it by externally applied forces.

B. Alveolar bone

Anatomically, alveolar bone is the bone of the maxilla and mandible that contains the alveoli for the teeth. It provides support and protection for the teeth. The following concept is important: the alveolar bone is dependent on the functional forces exerted upon the teeth to maintain its structure.

When the teeth are lost, the alveolar bone, in time will be lost also. This alveolar bone presents a special challenge to the dentist when making a denture for geriatric patients who have been edentulous for many years. [Special note: edentulous means 'without teeth'. Contrast this with anodontia, the congenital absense of teeth.]

C. Oral mucosa

The oral cavity is lined by a mucous membrane whose major functions are lining and protecting. It also serves as a mobile tissue that permits free movement of the lips and cheeks. A unique feature of the oral mucousa is that it is perforated by the teeth. Oral mucosa consists of:

(1) lining mucosa (mobile regions of the oral cavity)

(2) specialized mucosa (the tongue and taste buds)

(3) masticatory mucosa (gingiva and hard palate)

D. Gingiva

This is the specialized mucosa surrounding the erupted teeth. It is arbitrarily divided as follows:

(1) free gingiva, the rolled collar of tissue that surrounds the tooth

(2) attached gingiva, the stippled (like an orange peel) keratinized masticatory mucosa that is firmly bound down to the underlying bone. It is separated from the free gingiva by the free gingival groove (not present in all people) and from the alveolar mucosa by the mucogingival junction. See the diagram oon this page. These distinctions are clinically important.

E. Temporomandibular joint

This is the articulation between the mandible and the bilateral temporal bones of the skull. It is a bilateral articulation, that is to say, the right and left sides work as a unit. It is a joint of some complexity that permits hinge motion, protrusion of the mandible, and side-to-side excursion. Put your little fingers into each ear and you can feel the mandibular condyle in motion. You will learn much about the TMJ in your four years in dental school.

F. Saliva and plaque

Saliva is a complex fluid that continuously bathes the teeth. It is produced by three major paired glands (parotid, sublingual, and submandibular) and also by the minor salivary glands. Saliva keeps the mouth moist, helps in mastication and contains a digestive enzyme (amylase). Dental plaque, which we treat as a 'bad guy' is always present. It contains microorganisms, salivary proteins, shed epithelial cells, and the foods we eat. Saliva is very important in dental health. Incidentally, saliva inhibits the HIV virus.

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III. Looking at Dissected Teeth, a Practical Exercise in the Laboratory

A. The most basic dissection of a tooth is the cross section. It is also the easiest to do. Take your magnifying glass and use a bright light. Most of what you see is dentin. You may see enamel on the facial and lingual aspect depending on where you made your cut. It will be white in color. You may see a fine line of cementum. It is often brown in color.

B. The proximal sections (from themesial or distal)or facial sections are often the most revealing.With your magnifying glass, the cementum will easily be seen. Check out the size and extent of he pulp chamber. Keep this in mind when you do restorative dentistry. The sclerotic dentin is a very rough guide to the age of the patient. Clinically, it is not especially significant. When there is caries or exposed dentin, you will often see 'dead tracks' in the dentin. These follow the courses of the dentinal tubules. Once in a while you will find a tooth where the pulpal cavity is totally obliterated.

C. Facial views are especially interesting if there is proximal dental caries or an intact dental restoration. Again, check out the various dental tissues and the characteristics shkown in diagrams on this page. This view gives the best view of the pulpal horns in maxillary incisors Those pulpal horns can be troublesome when doing restorative work on youth under the age of twelve.

CJ '98