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Dental Pulp

Dental pulp is an unmineralized oral tissue composed of soft connective tissue, vascular, lymphatic and nervous elements that occupies the central pulp cavity of each tooth. Pulp has a soft, gelatinous consistency. Figure 1 (adjacent), indicates that by either weight or volume, the majority of pulp (75-80%) is water. Aside from the presence of pulp stones, found pathologically within the pulp cavity of aging teeth, there is no inorganic component in normal dental pulp. There are a total of 32 pulp organs in adult dentition. The pulp cavities of molar teeth are approximately four times larger than those of incisors.

The pulp cavity extends down through the root of the tooth as the root canal which opens into the periodontium via the apical foramen. The blood vessels, nerves etc. of dental pulp enter and leave the tooth through this foramen. This sets up a form of communication between the pulp and surrounding tissue - clinically important in the spread of inflammation from the pulp out into the surrounding periodontium.

Developmentally and functionally, pulp and dentin are closely related. Both are products of the neural crest-derived connective tissue that formed the dental papilla.

Dental pulp is a loose connective tissue with an appearance similar to mucoid CT. It contains the components common to all connective tissues:

• Cells: fibroblasts and undifferentiated mesenchymal cells (Lab Image 1 ) as well as other cell types (macrophages, lymphocytes, etc.) required for the maintenance and defense of the tissue .
  
• Fibrous matrix: collagen fibers, type I and II, are present in an unbundled and randomly dispersed fashion, higher in density around blood vessels and nerves. Type I collagen is thought to be produced by the odontoblasts as dentin, secreted by these cells, is composed of type I collagen. Type II is probably produced by the pulp fibroblasts as this type increases in frequency with the age of the tooth. Older pulp contains more collagen of both the bundled and diffuse types.
  
• Ground substance: the environment that surrounds both cells and fibers of the pulp (Lab Image 2 ) is rich in proteoglycans, glycoproteins and large amounts of water.

The large number of undifferentiated mesenchymal cells (present as perivascular cells) within the pulp facilitates the recruitment of newly differentiating cells to replace others when they are lost - specifically odontoblasts.

Odontoblasts (examined in greater detail in the module on dentin) comprise the outermost region/layer of the pulp, immediately adjacent to the dentin component of the tooth. These cells are responsible for the secretion of dentin and the formation of dentinal tubules in the crown and root.

The peripheral aspect of dental pulp, referred to as the odontogenic zone (1), differentiates into a layer of dentin-forming odontoblasts (A). Immediately subjacent to the odontoblast layer is the cell-free zone (of Weil). This region (2) contains numerous bundles of reticular (Korff's) fibers (B). These fibers pass from the central pulp region, across the cell-free zone and between the odontoblasts, their distal ends incorporated into the matrix of the dentin layer. Numerous capillaries (C) and nerves (D) are also found in this zone. Just under the cell-free zone is the cell-rich zone (3) containing numerous fibroblasts (E) - the predominant cell type of pulp. Fibroblasts of the pulp have demonstrated the ability to degrade collagen as well as form it. Perivascular cells (undifferentiated mesenchymal cells) are present in the pulp and can give rise to odontoblasts, fibroblasts or macrophages. Since odontoblasts themselves are incapable of cell division, any dental procedure that relies on the formation of new dentin (F) after destruction of odontoblasts, depends on the differentiation of new odontoblasts from these multipotential cells of the pulp. Lymphocytes, plasma cells and eosinophils are other cell types also common in dental pulp. Medial to the cell-rich zone is the deep pulp cavity (4) that contains subodontoblastic plexus (of Raschkow; parietal layer) of nerves (G).

Figure 2 indicates the 4 zones or regions of dental pulp (Lab Image 4 ) :

Vascular Supply to the Pulp

Figure 3 Illustrates the extensive vascular pattern of dental pulp - emphasizing its primary function - support and maintenance of the peripheral odontoblast layer. The odontoblasts in turn maintain the overlying layer of dentin.

The walls of pulpal vessels are very thin as the pulp is protected by a hard unyielding sheath of dentin. One or more small arterioles enter the pulp via the apical foramen and ascend through the radicular pulp of the root canal. Once they reach the pulp chamber in the crown they branch out peripherally (Lab Image 4 ) to form a dense capillary network immediately under - and sometimes extending up into - the odontoblast layer. The capillaries exhibit numerous pores, reflecting the metabolic activity of the odontoblast layer. Small venules drain the capillary bed and eventually leave as veins via the apical foramen.

Blood flow is more rapid in the pulp than in most areas of the body and the blood pressure is quite high. Arteriovenous anastomoses of arteriolar size are frequent in the pulp. For many years, investigators found it very difficult to establish the presence of lymphatics in the pulp. Most believed there was no lymphatic drainage of the teeth. Tissue fluid was speculated to have drained back into the capillary or postcapillary sites of the blood vascular system. In recent years a number of studies have demonstrated the presence of thin-walled, irregularly shaped lymphatic vessels. They are larger than capillaries and have an incomplete basal lamina facilitating the resorption of tissue fluid and large macromolecules of the pulp matrix.

The continued formation of cementum at the apical foramen can lead to occlusion of the opening. The walls of pulpal veins are first affected by the cemental constriction. Vascular congestion may occur. This ultimately leads to necrosis of the pulp.

Several large nerves enter the apical foramen of each molar and premolar with single ones entering the anterior teeth. A young premolar may have as many as 700 myelinated and 2,000 unmyelinated axons entering the apex. These nerves have two primary modalities:

1. Autonomic Nerve Fibers. Only sympathetic autonomics fibers are found in the pulp. These fibers extend from the neurons whose cell bodies are found in the superior cervical ganglion at the base of the skull. They are unmyelinated fibers and travel with the blood vessels. They innervate the smooth muscle cells of the arterioles and therefore function in regulation of blood flow in the capillary network.

2. Afferent (Sensory) Fibers. These arise from the maxillary and mandibular branches of the fifth cranial nerve (trigeminal). They are predominantly myelinated fibers and may terminate in the central pulp. From this region some will send out small individual fibers that form the subodontoblastic plexus (of Raschkow) (Lab Image 5) just under the odontoblast layer. From the plexus the fibers extend in an unmyelinated form toward the odontoblasts where they then loose their Schwann cell sheath. The fibers terminate as "free nerve endings" near the odontoblasts, extend up between them or may even extend further up for short distances into the dentinal tubule. They function in transmitting pain stimuli from heat, cold or pressure. The subodontoblastic plexus is primarily located in the roof and lateral walls of the coronal pulp. It is less developed in the root canals. Few nerve endings are found among the odontoblasts of the root.

Figure 4 illustrates the free nerve endings (F) arising from the subodontoblastic plexus (E) and passing up between odontoblasts (A) to enter the dentinal tubule where they terminate (G) on the odontoblast process (D). B = predentin, C = dentin

The origin and concepts involved in pain in the pulp-dentin complex will be examined in the module on dentin.

Figure 5 illustrates the regions where the two types of dental pulp are located:

1. Coronal pulp (A) (Lab Image 3) occupies the crown of the tooth and has six surfaces; occlusal, mesial, distal, buccal, lingual and the floor.

Pulp horns (B) are protrusions of the pulp that extend up into the cusps of the tooth. With age, pulp horns diminish and the coronal pulp decreases in volume due to continued (secondary) dentin formation - often the result of continued masticatory trauma. At the cervix of the tooth the coronal pulp joins the second type.

2. Radicular pulp (C) (Lab Image 2) extends from the cervix down to the apex of the tooth. Molars and premolars exhibit multiple radicular pulps. This pulp is tapered and conical. In a fashion similar to coronal pulp, it also decreases in volume with age due to continued dentinogenesis. Pulp passing through the apical foramen may be reduced by continued cementum formation.

Specific changes occur in dental pulp with age. Cell death results in a decreased number of cells. The surviving fibroblasts respond by producing more fibrous matrix (increased type I over type II collagen) but less ground substance that contains less water. So with age the pulp becomes:

a) less cellular

b) more fibrous

c) overall reduction in volume due to the continued deposition of dentin (secondary/reactive)

Figure 6 illustrates the normal appearance of the pulp cavity (P) at a young stage.

Figure 7 illustrates some attrition of the pulp as the result of normal aging as well as trauma from wearing of the enamel at the cusp (A). Note the pulp horn (B) is not as well defined due to responsive ingrowth of secondary dentin below the worn cusp. Cementum has begun to thicken on the root (C).

Figure 8 shows the changes in pulp cavity size by middle age. The pulp horn continues to be reduced in response to increased wearing of the overlying enamel. Anoverall reduction in pulp cavity dimensions through the continued deposition of normal secondary dentin has occurred. Histology of the pulp reveals a decreased cellularity coupled with increased fibrosis. Cementum (C) deposition continues and the apical foramen subsequently has undergone a reduction in diameter (D).

Figure 9 is illustrative of the pulp cavity in old age. Continued wearing of the enamel on the cusp has resulted in the formation of dead tracts of dentin (E). It has also stimulated the formation of reactive secondary dentin (F) that has obliterated the pulp horn and now grows into the coronal pulp cavity. The pulp cavity, coronal and radicular regions, has been markedly reduced from that in the young stages. Cementum (C) continues to be deposited and the apical foramen (D) isnow considerably narrower.

Aging decreases the ability of dental pulp to respond to injury and repair itself. The fact that the pulp is surrounded by mineralized dentin makes relatively minor pathologic events like inflammation, that cause swelling elsewhere, lead to a compression of the pulp leading to intense pain. This generally results in the death of the pulp.

Small calcified bodies are present in up to 50% of the pulp of newly erupted teeth and in over 90% of older teeth. These calcified bodies are generally found loose within the pulp but may eventually grow large enough to encroach on adjacent dentin and become attached. These bodies are classified by either their development or histology:

1. Development

Epithelio-Mesenchymal Interactions. Small groups of epithelial cells become isolated from the epithelial root sheath during development and end up in the dental papilla. Here they interact with mesenchymal cells resulting in their differentiation into odontoblasts. They form small dentinal structures within the pulp.

Calcific Degenerations. Spontaneous calcification of pulp components (collagen fibers, ground substance, cell debris, etc.) may expand or induce pulpal cells into osteoblasts. These cells then produce concentric layers of calcifying matrix on the surface of the mass - but no cells become entrapped.

Diffuse Calcification. A variation of the above whereby seriously degenerated pulp undergoes calcification in a number of locations. These bodies resemble calcific degenerations except for their smaller size and increased number.

2. Histology

Calcified bodies in the pulp may be composed of dentin, irregularly calcified tissue, or both. A calcified body containing tubular dentin is referred to as a "true" pulp stone or denticle (Lab Image 7). True pulp stones exhibit radiating striations reminiscent of dentinal tubules. Usually those bodies formed by an epithelio- mesenchymal interaction, are true pulp stones.

Irregularly calcified tissue generally does not bear much resemblance to any known tissue and as such is referred to as a "false" pulp stone or denticle (Lab Image 6). False pulp stones generally exhibit either a hyaline-like homogeneous morphology or appear to be composed of concentric lamellae.

Figure 10 shows both types of stones: A and B are false pulp stones, C is a true pulp stone. A is an "attached" stone (which may become embedded as secondary dentin deposition continues. B and C are "free" stones found within the pulp cavity.

The primary function of dental pulp is providing vitality to the tooth. Loss of the pulp following a root canal) does not mean the tooth will be lost. The tooth then functions without pain but, it has lost the protective mechanism that pulp provides.

Dental pulp also has several other functions:

• inductive: very early in development the future pulp interacts with surrounding tissues and initiates tooth formation.
  
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• formative: the odontoblasts of the outer layer of the pulp organ form the dentin that surrounds and protects.
  
• protective: pulp responds to stimuli like heat, cold, pressure, operative cutting procedures of the dentin, caries, etc.. A direct response to cutting procedures, caries, extreme pressure, etc., involves the formation of reactive (secondary) dentin by the odontoblast layer of the pulp. Formation of sclerotic dentin, in the process of obliterating the dentinal tubules, is also protective to the pulp, helping to maintain the vitality of the tooth.

1.	How much inorganic material does normal dental pulp contain? What three features common to CT compose the pulp? Which type of collagen fibers are found here?
2.	Be able to label a diagram of the architecture of the pulp. Where is the cell-free zone located? What composes the odontogenic layer? In which layer is the neural plexus located? Where is the cell-rich zone? What types of cells predominate in this layer?
3.	The dense capillary network under the odontoblasts reflect what feature of this layer? Is there a lymphatic drainage of the pulp? Where does tissue fluid drain in lieu of them?
4.	List the two types of nerve fibers found in the pulp and the specific function of each. With regard to the sensory fibers. Where to the myelinated fibers terminate? What type of pain is referred by myelinated fibers? from unmyelinated fibers?
5.	What are the two types of pulp? Do they differ in composition? Where would a pulp horn be found?
6.	What age-related changes occur in the pulp? Why?
7.	What are pulp stones? How are they thought to develop? What are the two types of pulp stones and how can you distinguish one from the other?