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A NOTE ON THE INNERVATION OF HUMAN DENTINE

BY RALPH COCKER AND JEAN M. HATTON

King's College Hospital Dental School (University of London)

Clinical experience indicates that dentine is a highly sensitive structure, but thequestion whether nerve fibres are present in calcified dentine has been a matter ofcontroversy for many years. Wellings (1940) reviewed some of the evidence,especially the claims made by Mummery (1924), and concluded that nerve fibres doin fact enter calcified dentine, although he fully recognized the difficulty of dis-tinguishing nerve from connective tissue fibres with the silver impregnation tech-niques commonly used. Bradlaw (1939) also claimed to have traced undoubted nervefibres into the dentine although the figures which he published in support of thisclaim show fibres only in the predentine or odontogenetic zone. His fig. 6 is a pos-sible exception to this statement, and is said to show a nerve fibre entering a dentinaltubule. The section of which the figure is a photograph is, however, cut transverselyto the direction of the tubules. The fibre ends apparently in the cavity of a tubule,but whether it passed along it into the calcified dentine is impossible to say withoutevidence from sections cut parallel to the direction of the tubule.More recently, Powers (1952) found an axon in a molar tooth of a rat which could

be traced from the pulp through the predentine and into the calcified dentine in serialsections. She used the Romanes (1950) and Ungewitter (1951) techniques of silverimpregnation. She also observed numerous fibres branching and forming loops inthe predentine where, of course, they have frequently been seen in human teeth.The present position therefore seems to be that nerve fibres in the pulp and in the

predentine have been demonstrated as convincingly as is possible with histologicaltechniques, and their presence is generally accepted. It is also believed by severalauthorities that nerve fibres extend into the calcified dentine, possibly along thedentinal tubules, though, apart from Powers's observation in the rat, the histologicalevidence produced is not as satisfactory as it could be.A difficulty arises in the use of the words 'dentine' and 'predentine'. From the

literature it would seem that some authors use 'dentine' to mean the whole mass ofdentine from the layer of odontoblasts in the pulp to the outer surface. This preventsany deductions being drawn as to how far the axons, which are described, penetrateinto the dentine. From the published photomicrographs (except for Powers), theyappear to penetrate only into the odontogenetic zone (predentine) but no farther.Lewinsky & Stewart (1938) state that the only author who has claimed to havedescribed and photographed nerve fibres in the 'ossified' dentine is Tojoda (1934),but he was not able to show the continuity of these fibres with those of the pulp ashis specimens consisted only of portions of the dentine without connexion with thepulp. We feel that unless the continuity of nerve fibres can be traced from the pulpthrough the odontogenetic zone and into the calcified dentine, the histological proofof the innervation of this tissue is not yet achieved.

190 Ralph Cocker and Jean M. HattonMore recently, Powers (1952) has satisfied the criterion stated as the axon she

shows in the rat molar passes from the pulp across the predentine and penetratesinto the calcified dentine. This she says was inked-in in the photograph to bring itswhole course into view. In the circumstances it seemed desirable to publish thefollowing observations which were made in human teeth and illustrated by untouchedmicrophotographs.

MATERIAL AND TECHNIQUE

Freshly extracted and non-carious teeth were fixed for at least 3 days in 10%neutral formol saline. The mesial and distal surfaces were then ground under wateruntil only a thin slice (but containing the whole pulp) remained. The object of thiswas to reduce the amount of material requiring decalcification, so decreasing thetime required to complete the process.

Decalcification was carried out in a constant drip bath with 1% formic acid insaturated calcium phosphate (Brain, 1949, 1953) and, according to the size of thetooth, required 3-8 weeks for completion. Stronger concentrations of formic acidor any of the commoner decalcifying agents (e.g. nitric acid, trichloracetic acid)were found to be unsatisfactory since they appeared to inhibit the impregnation ofthe finer nerve fibres in subsequent operations.

After decalcification the specimens were washed for 24 hr. in running water,dehydrated, cleared and double-embedded in celloidin and paraffin wax. Sectionswere cut at 8,s, and treated with a silver impregnation technique, described byUngewitter (1951), in which urea is used to accelerate the procedure and improve thespecificity of the impregnation. It was found that the addition of 3-4 drops of purepyridine to the urea-silver nitrate solution used for impregnation improved thesharpness and general clarity of the final result, but otherwise the method wascarried out as described by Ungewitter.

OBSERVATIONS

Sections showed the usual plexiform arrangement of nerve fibres in the pulp; vari-cosities on some of the larger fibres were quite common (PI. 1, fig. 1), and seemedvery similar in appearance to those interpreted by Mohiuddin (1950) as evidence ofdegeneration. Their frequent appearance in otherwise normal pulp tissue is againstthis interpretation. Numerous fibres enter the predentine (PI. 1, fig. 2), where somerun parallel with the tubules and Tomes's fibres; branching is common in this situa-tion; some fibres are beaded, and some loop back into the pulp. All these appearanceshave been reported on many previous occasions, e.g. by Bradlaw (1939) and Powers(1952).

In some sections fibres could be traced from the pulp, through the predentine andinto the calcified dentine, reaching in a few cases as far as half-way through thethickness of the calcified dentine. These long fibres run parallel to the direction ofthe tubules, but it was not possible to determine whether they were actually withinthe tubules or between them in the matrix. In many sections where the predentinecontained numerous fine fibres, some of these could be traced into the calcifieddentine where they were very soon lost. A few only of the long fibres such as those

A note on the innervation of human dentineillustrated (PI. 1, fig. 3, 4) were found; they may of course be present in much largernumbers, since only those which lie for some distance in the plane of section can beidentified with certainty.

DISCUSSION AND CONCLUSIONS

It is well recognized that the demonstration of nerve fibres by silver impregnationmethods is open to misinterpretation, the most common cause for which is the mis-taking of connective tissue fibres for nerve axons. In the present specimens it canbe said that the axons seen in the calcified dentine and predentine all show the sharpalmost black impregnation which is characteristic of the fibres in the pulp wheretheir identification as nerves would not be seriously questioned. Moreover, theycould be traced through the predentine, between the odontoblasts, into continuitywith the fibres in the pulp, which showed a similar kind of impregnation, differingmarkedly from the golden-brown coloration of the connective tissue fibres. The latteralso pursue a far more wavy course than the axons and never show the varicositieswhich, whatever their significance, are a characteristic feature of many of the nerves.It is probable that final proof that these fibres in the calcified dentine are nervescould be obtained only by experimental methods, e.g. by the observation ofdegeneration phenomena after cutting the nerve trunks to the teeth. These methodsare not practicable with human material. In the absence of this 'final proof' it isjustifiable to conclude that fibres pass for a considerable distance into human calci-fied dentine which, so far as histological methods are capable of showing, are in factnerve fibres.

SUMMARY

1. The combined use of slow decalcification and a modified silver nitrate stainingtechnique has confirmed the course of nerve axons in the pulp and predentine ofhuman teeth.

2. In addition the method has shown for the first time a small number of axonspenetrating deeply into the calcified dentine of human teeth.

The authors would like to express their grateful thanks to George Harwood,A.I.M.L.T., of the Department of Morbid Anatomy, King's College Hospital MedicalSchool, for his very skilful photomicrography.

REFERENCES

BRADLAW, R. (1939). The histology and histopathology of the dental innervation. Proc. R. Soc.Med. 32, 1040-1053.

BRAIN, E. B. (1949). A method of preparing decalcified serial sections in paraffin wax of humanenamel and dentine in situ. Brit. dent. J. 87, 199-205.

BEmN, E. B. (1953). Personal communication.LEWINSKY, W., & STEWART, D. (1938). An account of our present knowledge of the innervation of

the teeth and their related tissues. Brit. dent. J. 65, 687-700.MOHIUDDIN, A. (1950). The fate of the nerves of the deciduous teeth. J. Anat., Lond., 84, 319-323.MUMMERY, H. J. (1924). Microscopic and General Anatomy of the Teeth, 2nd ed. Oxford.PowERs, M. M. (1952). The staining of nerve fibres in teeth. J. dent. Res. 31, 383-392.ROMANES, G. J. (1950). The staining of nerve fibres in paraffin sections with silver. J. Anat., Lond.,

84, 104-115.

191

192 Ralph Cocker and Jean M. HattonTOJODA, M. (1934). Die Innervation des menschlichen Zahnbeins. Dtsch. zahndrztl. Wschr. 37,

641 -645, 670-673.UNGEWITrER, L. H. (1951). Urea silver nitrate method for nerve fibres and nerve endings. Stain.

Tech. 26, 73-76.WELLINGS, A. W. (1940). The entrance of nerves into the dentine. Proc. R. Soc. jMed. 33, 563-576.

EXPLANATION OF PLATE

(All human material.)

Fig. 1. Axons lying in the pulp showing apparent branching and varicosities. 8/jt thick; x 560.Fig. 2. Many fine axons passing from the pulp between the odontoblasts and into the predentine.

25A thick; x 560.Fig. 3. An axon passing from the pulp through the predentine and continuing deeply into the

calcified dentine. 8/z thick; x 469.Fig. 4. Another axon passing from the pulp through the predentine and continuing deeply into the

calcified dentine. 8p thick; x 469.

Journal of Anatomy, Vol. 89, Part 2

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