The Extent of the Odontoblast Process in Human Dentin HUW F. THOMAS Eastman Dental Center, Rochester, New York 14620

Impacted human third molar teeth were examined, both with the light and electron microscope, to determine the extent of the odontoblast process. The odontoblast process was found to be limited to the inner third of coronal dentin and inner half of mid-root dentin. J Dent Res 58(D):2207-2218, November 1979

The purpose of the work reported in this paper was to examine the extent of the odontoblast process in human dentin, both with the light and transmission electron microscope. The odontoblast process was followed from its junction with the odontoblast cell body through predentin and into the dentinal tubule in both crown and midroot areas of the tooth.

Introduction. Recent ultrastructural studiesl-5 have failed to confirm the presence of the odontoblast process throughout the thickness of dentin in both animals and humans. Garantl reported an absence of cellular material in the distal portion of the dentinal tubules of the rat. Holland2'3 studied the extent of the odontoblast process in the cat and found it limited in all cases to the pulpal half of coronal dentin. In a scanning electron microscope study on human premolar teeth, Brannstrom and Garberoglio4 found the odontoblast process extending no more than 0.7 mm into the dentin. Tsatsas and Frank5 examined the ultrastructure of the dentinal tubules near the dentino-enamel junction in human teeth from a variety of age groups and found no cellular material in this region. These findings contradict a long-held opinion which originated after John Tomes first described fibrils of soft tissue within the dentinal tubules.6 The term "Tomes fiber" has since often been used synonymously with "odontoblast process." This paper was one of two first-award winners in the doctoral category of the 1979 Hatton Awards Competition. Present address: Department of Pediatric Dentistry, University of Texas Health Science, Center at San Antonio. Sponsor: Odd B. Sveen This investigation was carried out while the author was a Pedodontic Fellow, Eastman Dental Center, Rochester, NY, and was in partial fulfilment of the M.S. degree, University of Rochester, Rochester, NY.

Materials and methods. Impacted third molar teeth from subjects aged from 19 to 25 years were used in this study. These teeth were chosen in order to eliminate any effect of caries or attrition on the extent of the odontoblast process. Immediately following extraction a 2mm slice was cut from the center of the entire tooth in a mesio-distal direction using a high-speed handpiece with tungsten carbide bur and water cooling. The buccal and lingual portions of each tooth were discarded leaving the central portion with the pulp chamber and canals exposed. The central portion was divided through the crown and both halves were placed immediately in Karnovsky's fixative7 pH 7.2 at a temperature of 0°C to 40C for four hours. The specimens were then assigned to one of two groups (I and II), each group consisting of specimens from ten teeth. Group I.-Following fixation, the specimens in this group were further divided into crown and mid-root regions and demineralized for 5-6 weeks in a 0.1M solution of EDTA adjusted to pH 7.4 with sodium hydroxide, and containing 4.0 percent glutaraldehyde 8 Following demineralization, representative blocks were cut from the crown and mid-root regions and post-fixed at 4°C in a 2 percent solution of osmium tetroxide buffered to pH 7.4 with sodium barbital. Dehydration was carried out in a graded series of cold ethyl alcohol followed 2207

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by several changes of propylene oxide. Il he hlocks were embedded in Fpon 812, care being taken in orientation of the blocks so that the dentin surface to be sectioned was not one which hlad been cut and exposed during the initial preparatiori of the tissue. SectiCoIs 1 to 2 miicrons thick were cut using ain LKB ultramicrotome and glass knives, stained with basic fuscin and methylene blue and examined with the lUght 5microscope. Areas for examination under the electron microscope were selected aind the blocks trimmnied accordingly. Sections ranging fromra 60 to 80 nm were cut with a dliamond knife, stainedl with uranyl acetate and lead citrate and examined with a Siemens l--lmiscope I electron microscope at 80 kv. Group II. Following fixation these teeth were further divided into crown and midlroot regions, post-fixed and dehydrated as described for teeth in GCroup I, and embedded in I '0On 812. tJltra-thin sections of the imiaterial were then ctut as for teeth in Group I. l)entin fromt the crown region of the teeth froBm both grouips was dividecd into three areas: the inner third, which inclucdedl the predentin anid odontoblast cell layer; the midlcldle third; and the outer third. Dentin from the miidl-root region (if the teeth fronm tboth groups was Jividecl into two areas: the inner half which included the predentin and odontoblast cell layer, andl the ouLter half.

ally, hlowever, the odontoblast process was difficLult to icientify Clue to the pale staining natuire of its cytoplasmi. In the midldle thLird of crown dentin, a darkly staining structture was observed within all tubules, separated fromii the walls of the tuibules by a clear area (Fig. I B). In niost

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f ig. 1A.- Inner third of crown denitin showing, the ocdontoblast layer (0) and predentinal layer (PD). Vacuolizationi (V) can be seen within the odontoblast cells. 1'lie odontoblast processes (OP) ruin frcTm the cell body t,lLircIlgh the predentin, and into the dentin (1D). (.Mae X 1400)

ResUlts. Ligl/t .llit CmCop i.- -Tlie inneic- thiird' of crown dentin showed the odonitoblusts, thie predentin anid the inner thilrd of the dentill imiatrix. IThe odontoblasts were seen as a layer of columnnar cells adjacent to the predentin (Fig. 1 A). Tfhe ntuclei of the odontoblasts were tistually located in the

basal portion of the cells, and vacuolizatioln was observed within the cell, occurring most frequently adjacent to the nuicleLus. [he path i)f sonic of the dentinal tuibules couLld tbe followed throughl the predentin into the dentin matrix, and an extension of the odontoblast cell, the odontoblast piocess, was secin cntering the tubtiles (Fig. 1A). Within1 the tulbules in the intner thircd of crown denitiin, the odontoblast process was visible along the lengtlh of the tu bule and was seen to OCCuLpy the entire tubule. Occasion-

lieI 1B3. ---1entinal ttubules ( )1) t ro t h e middle tlhird of crown dentin. Conitent of the tubules varies froni a titbe-like strtuCtLire (FS) to a solidstaining strtrctuire (SS). A space ceists betweeni this structutre and the tcbicle Wall. (Mae. X 720)

PROCSCS A IN litU,1AN DENTIN cODONTOBIIAST Vol. -58, Special Is.suec D

of the tubules this structure appeared tube-

like, but occasionally solid-staining structures were present. In some tubules a change in appearance from a solid-staining structure to a tube-like structure was seen. The structure, despite its appearance, was always separated from the tubule wall by a space. When the focus of the microscope was altered slightly, the appearance of the structure within the tubule was seen to vary. Those structures which were tube-like becamne more solid-staining in appearaince while those which were solid-stainiing became more tube-like in appearance. In the outer third of crown dentin similar structures to those described in the middle third of crown dentin were present (Fig. 1C). Both tube-like and solid-staining structures were observed. Near the denti.noenamel junction, branching, together with a decrease in the diameter of the tubules, was noted and it became difficult to identify a structure within the tubules. The appearance of sections obtained from the inner half of mid-root dentin closely reseimbled those from the inner third of crown dentin, while sections from the outer half of mid-root dentin were very similar to those obtained from the outer third of crown denitin. Pwlectron tllicroscopyj --In the ininer thircd of crown dentin the odontoblast process was observed fromi the level of the odontoblast cell body through the predentini and into the dentin matrix. The odontoblast

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cells were observed in close relationship to one another imminediately below a layer of predentin. In some areas the cell membranes of adjacent odontoblasts appeared in close contact with one another, and cell lunctions were visible (Fig. 2A). At the junction of the cell body with the odontoblast process, a constriction of the cytoplasm was seen (Fig. 2A), and the odonitoblast process was observed entering the predentinal layer. The cytoplasmic content of the odontoblast process consisted of many microfilaments and microtubules which had a preferential arrangement parallel to the long axis of the odontoblast process (Fig. 2B). ln the predentin, the cell imembrane of the odontoblast process appeared irregular in outline and was closely related to numnerotus collagen fibrils (Fig. 2C). Small darkly staining granules (Fig. 2A) and dense bodies (Fig. 2C) were seen interspersed throughout the cytoplasm. (lear vesicles were also observed which were often in close proxiimity and, in some areas, appeared to be fused with the cell membrane (Fig. 2A). No change in appearance or cytoplastic contenit was observed as the odontoblast process continued tliiough the predentin. At the predentin-dcentin junction, the odontoblast process was seen enterinig the (Ientinal tubules. The cell nmembrane of

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Fig. IC.-Outer third of crown dentin witlh dentinal tubules containing similar structures to those observed in the middle third. (Mag. X 720)

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Fig. 2A.-J unction of odontoblast (0) and odontoblast process (OP) showing a constriction of the cytoplasm (-). Cell junctions (CJ) are visible betveen adjacent odontoblasts. Intracellularly, darkly staining granules (DG1), vacuoles (V) and vesicles (VE) are visible; in some areas the vesicles appear to have fused witli the cell memibranie. (Mag. X 30,000)

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Fig. 2B.-Odontoblast process (OP) entering predentin. The cell memlbrane (CM) appears irregular in outline and is closely related to many collagen fibrils (COL). Microfilamilents (MF ) and microtubules (MN T) are the most prominent cell org,anclles. (Mag. X 34,500)

Fig. 3A.-Demineralized section showinig odontoblast process (OP) leaving predentin (PD) layer and entering dentin (D), in close association with tubule wall (W). (Mag. X 38,800)

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1 ig. 2C. -Odontoblast processes (OP) in predentin. Dense bodies (DB) and vacuoles (V) can be seen intracellularly. A very close relationship exists between nuumerous collagen fibrils and the odontoblast process. (Mag. X 20,000)

the odontoblast process closely approximated the outliie of the tubule wall in both deminleralized (Fig. 3A) anid mineralized (Fig. 3B) specimens. A short distance into the dentin a constriction of the odontoblast process was noted. This constriction was uisually found on one side and resulted in a space between the tubule wall and the odontoblast process (Fig. 4A). Collagen fibrils and a fine granular material were present in this space. 'Ihe predominant cytoplasmic organelles in this area were again microfilaments and microtubules. In some areas peritubular dentin formation was

1ig. 3B. Odontoblast process at predentin (PD) - dentin (D) julnction in mineralized tissue, closely approximlating tubule wall (W). (Mag. X 33,250)

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The extent of the odontoblast process in human dentin.

The Extent of the Odontoblast Process in Human Dentin HUW F. THOMAS Eastman Dental Center, Rochester, New York 14620 Impacted human third molar teeth...
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