- Email: [email protected]
The effect of preventing eruption on the proliferative basal tissues of the rat lower incisor
Archs oralBid.Vol.17,pp.1279-1288, 1972. Pergamon Press. Printed ia GreatBritain.
THE EFFECT OF PREVENTING ERUPTION ON THE PROLIFERATIVE BASAL TISSUES OF THE RAT LOWER INCISOR B. K. B. BERKOVITZ Department of Anatomy (Oral Biology), The Medical School, University Walk, Bristol BS8 lTD, England
Summ~y-Eruption was prevented in the continuously growing lower right incisor of 9 male and 10 female Wistar rats by the surgical procedure of either complete or partial root transection, thereby allowing for a separation of the process of tooth eruption from that of basal cell proliferation. The experimental tooth was cut down to the gingival margin immediately following operation to eliminate masticatory stress. Animals were killed 120 days after operation, following which the jaws were dissected out, radiographed and then sectioned. Following cessation of eruption, the outline of the basal portion of the “root” showed one of the following a~angements: {I) in 10 jaws it was in the form of a smooth curve similar to that of controls, (2) in 3 jaws it was more sharply curved than the controls so that the pulp opening faced directly upwards, (3) in the remaining 6 jaws it was completely recurved so that the pulp opening was directed anteriorly. Histological examination revealed the presence of continued enamel and dentine formation in this basal region with the latter tissue completely obliterating the normal pulp cavity except for a small cone-shaped area at the base. These results clearly demonstrate that the proliferative basal region of the rat incisor is capable of generating a force when eruption is prevented, sufficient to cause a significant change in root morphology. In one complete root transection specimen, the proximal segment continued to erupt forwards and penetrated the side of the socket and then the cheek to reach the exterior.
THE ACTIVITY of the continuously growing basal region of the rat incisor has been studied following retardation or prevention of tooth eruption achieved by hypophysectomy (SCHOURand VAN DYKE, 1932; BECKYet al., 1946; BAUME et al., 1954), crowning of incisors (ARCEand ERAUSQUIN, 1944; WEINREB, MICHAELI and SILBERMAN, 1969), magnesium-deficient diet (IRVING, 1940; BECKSand FURUTA, 1941; GAGNON, SCHOUR and PATRAS, 1942), lathyritic diet (SCIAKY and UNGAR, 1961; SARNAT and SCIAKY, 1965; THOMAS, 1965) and in the rabbit incisor following vaccinia injection (KRESHOVERand HANCOCK, 1956). A striking observation in the upper incisors was the appearance of extensive regular “concertina-like” folds in the enamel and subjacent dentine in the basal region, evident on radiographs (BECKS et al., 1946). There was little recorded evidence of any significant resorption of bone at the base of these sockets, which may have been expected if basal cell proliferation continued during the period of the reduced eruption, and which would have resulted in a change in the normal morphology of the socket. 1279
1280
B. K. B. BERICOVITZ
In the above studies, a similar folding associated with the lower incisors was observed only occasionally by BECKSer al. (1946), while ECCLES(1965) did not record any basal folding when eruption was severely retarded following pinning or crowning of the rat lower incisor. Following root transection, SARNATand SCHOUR(1944) and BRYER(1957) noted that the proximal fragment of the rat lower incisor showed a continuation of growth in length as observed by the compression basally of the odontogenic epithelium and the basal incisor tissue. NESS(1956) observed a crumpling of basal tissue following pinning of three rabbit lower incisors for a period of up to 48 days; the base of the socket of two of these incisors had moved backwards I mm. A review of the literature has revealed that, apart from the work of SARNATand SCHOUR(1944) and NESS (1956)Jhere is little evidence of any significant change in the morphology of the basal region of the socket of the rodent incisor when eruption has been severely retarded, and the subsequent interpretation of the results of such studies has been complicated by the fact that the experimental tooth was, in nearly every case, left in functional occlusion. The present study has been carried out to determine the extent of continued proliferative activity in the basal region of the rat lower incisor when eruption is prevented for a prolonged period of time. Any significant change in the normal morphology of the base of the socket should be observable on radio~aphical examination. In every case, the experimental tooth is to be cut down to the gum margin following the surgical procedures necessary to prevent eruption, thereby eliminating problems related to masticatory stress. The findings may indicate whether the proliferative basal tissues are capable of generating any kind of force sufficient for them to be considered in the mechanism responsible for effecting tooth eruption (e.g. SIGHER, 1942). MATERIALS AND METHODS It has been shown that, following root transection in the rat lower incisor, the proximal segment is retained in a basal position while the distal segment usually continues erupting and is eventually exfoliated from the oral end of the socket (BERKOV~Z, 1971). This procedure has been utilized in the present study to obtain cessation of eruption of the proximal segment which will also be isolated from any rn~ti~tory stresses. Root transection was carried out on the lower right incisor of 5 male and 7 female young adult Wistar rats, using the same surgical procedure as previously described (BERKOVTTZ, 1971). The “crown” of the tooth was cut down to the gingival margin by means of a Carborundum disc immediately following the transection. In addition to the complete transection procedures described above, partial root transection was carried out on the lower right incisor tooth in a further 5 male and 3 female young adult Wistar rats, such that the bur only removed a segment of tooth substance from the enamel half of the tooth. The tooth “crown” was cut down to the gingival margin by means of a Carborundum disc immediately following the operation. Following these experimental procedures, animals were fed on Modified IXet 41 B (Oxoid) and water ad libitum, and were killed 120 days after operation. At the end of the experiment, the average weight of the male rats was 425 g, and the females 250 g. In addition to the adjacent impeded left lower incisor tooth of these experimental animals, the jaws of unoperated male and female Wistar rats of similar age and weight were obtained for use as controls. At the end of the experiment, jaws were radiographed and 60 m.a.s. Ilford Industrial F was the film used.
with an A.E.I. Raymax 50 unit at 40 KV.
EFFECT
OF PREVENTING
ERUPTION
ON TISSUES
OF THE RAT
LOWER
INCISOR
1281
Jaws were fixed in Bouin’s solution, decalcified in EDTA (pH 7.4), embedded in paraffin wax and serially sectioned longitudinally in a microtome set to cut at a thickness of 10 pm. Sections were stained with haematoxylin and eosin.
RESULTS
Of the 12 animals which underwent complete root transection, the proximal segment exhibited no forward movement in 9, as evidenced by radiographical examination, while in 2 of the remaining animals the proximal segment had erupted forwards 3 mm over the experimental period of 4 months. The findings in the remaining animal will be described later. Of the 8 animals which were subjected to partial root transection, the tooth was completely prevented from further eruption in 5 animals, while in the remaining 3 it had erupted forwards 4, 5 and 7 mm, respectively, over the experimental period of 4 months. Thus, the experimental procedures had totally prevented eruption of the tooth in 14 cases, while eruption was greatly reduced in the remaining 5 animals. Radiographic examination of the operated jaws indicated that the pulp chambers of all the experimental non-erupting teeth had been filled in by radiopaque material, later identified histologically as dentine, except for a small cone-shaped area at the base (compare Figs. 1 and 2). In the control situation, the radiological outline of the “root”portion of the incisor approximately formed an arc of a circle with a radius of curvature of 15 mm with the pulp opening being angled backwards and upwards (Fig. 1). In the experimental teeth following cessation of eruption, the outline of the basal portion of the “root” showed one of the following arrangements : (1) In 10 jaws (3 complete and 7 partial root transections), it was in the form of a smooth curve similar to that of controls (Fig. 2). (2) In 3 jaws (2 complete and 1 partial root transection), it was more sharply curved than the controls so that the pulp opening faced directly upwards (Fig. 3). (3) In the remaining 6 jaws (all complete root transected teeth), it was completely recurved so that the pulp opening was directed anteriorly (Fig. 4). This basal segment of the “root” had a radius of curvature of approximately 4 mm and its anterior limb was extended forwards by about 2-3 mm. Histological examination confirmed that, following cessation of eruption, dentine had continued to be formed, resulting in a constriction of the pulp cavity, and from the presence of an enamel space lined by ameloblasts, it was concluded that enamel had also continued to be formed (Figs. 5 and 6). The above results have excluded one male rat whose incisor tooth had been completely root transected; by the end of the experimental period, in addition to the distal segment which had erupted to the very edge of the socket, this specimen exhibited an incisor tooth segment on the side of the face, which had penetrated the cheek tissues to reach the exterior. Because of its curvature (radius of curvature = 5 mm), the tip of this incisor was about to repenetrate the cheek tissue (Fig. 7). Radiological examination revealed that this “additional” incisor tooth was derived from the A0.B.1719-c
1282
B. K. B.
BERKOVITZ
proximal segment which had erupted forwards following transection (Fig. 8), but had penetrated the side of the socket half-way along its length and close to the site of the original surgical wound. The total amount of eruption for this proximal segment was approximately 1S mm for the 4 month period. Extensive regular “concertina-like” folding of the enamel and subjacent dentine, described by other authors in the basal region of the upper incisor following cessation of eruption achieved by other methods (e.g. BECKS et al., 1946), was not observed in the present study on the lower incisor, though a less conspicuous folding was evident (Fig. 9). DISCUSSION
The surgical techniques used in the present study have prevented eruption of the rat lower incisor for a period of 120 days without apparent damage to the proliferative basal tissues. In approximately half of the animals used, there was no significant change in the morphology of the basal region of the socket, though the pulp cavity had been almost totally filled in by continued dentine formation. In the remaining animals, however, the morphology of the basal region of the socket was modified by continued formation of dental tissues and bone resorption which, in the majority of these animals, resulted in the basal formative region of the tooth recurving upon itself so that the pulp opening faced anteriorly. This observation that some form of curvature was maintained, rather than the tooth simply extending directly backwards, may indicate differential growth rates between the enamel-forming and cement-forming elements of the tooth and/or differential rates of bone resorption in these regions; the radius of curvature of this basal segment was much smaller than that for the control situation. It remains to be determined whether a similar surgical technique would produce a similar result for the rat upper incisor, though there is a significant difference in the anatomical relationships between the two situations. These results clearly demonstrate that the proliferative basal region of the rat incisor is capable of generating a force when eruption is prevented sufficient to cause a significant change in root morphology. Whether the force generated in this situation could account for normal tooth eruption, as has been proposed (e.g. SICHER,1942), remains undetermined, and arguing against this view is the finding that removal of this basal zone does not inhibit tooth eruption (e.g. BERKOVITZand THOMAS,1969). A similar view was forwarded by NESS (1964), who concluded that, though evidence suggested that proliferation of basal epithelium was capable of exerting an axial force, other evidence showed that it did not account for the eruption of the rodent incisor. The present study does not provide evidence either for or against a more recent theory that the periodontal ligament is responsible for the eruptive mechanism (for review see TEN CATE, 1969). The following interpretations have been given to account for the extensive regular “concertina-like” foldings seen in the basal zone of the rat incisor following cessation of eruption : (I) SICHER(1942) thought that hypophysectomy resulted in a dissociation of epithelial and pulpal growth, pulpal growth being inhibited but epithelial growth not being disturbed to the same degree, the eventual result being folding of the
EFFECT
OF PREVENTING
ERUPTION
ON TISSUES
OF THE
RAT
LOWER
INCISOR
1283
proliferating epithelial structures. (2) BAUMEet al. (1954) suggested that the folding was produced by the forces of mastication, a view expanded by WEINREBet al. (1969) who stressed the impor~nt relationship between masticatory load and the physiological state of the periodontium. The findings in the present study following removal of incisal stress, when only slight traces of folding in the basal enamel region were evident, would lend support to the latter interpretation of WEINREBet al. (1969). It is not yet possible to account for the non-eruption of the partial root transected teeth used in the present study, bearing in mind that, following complete root transection, the distal segment continues to erupt, most probably under the influence of the periodontal ligament (BERKOVITZ,1971). An explanation could be that the procedure of both partial and complete root transection interferes with the process of eruption primarily in the segment proximal to the level of the wound; in the former procedure the lack of eruption will affect the whole tooth, while in the latter procedure (i.e. complete root transection) the distal segment is physically free to erupt independently of the severely affected proximal segment. In one case following complete root transection, the proximal segment erupted forwards and out of the side of the socket, penetrating the cheek. This condition is reminiscent of the examples of “additional” teeth produced by ENGLISH, SCHLACKand ELLINGER(1954) and GO~GIEL (19671, when separation of the continuously growing rat incisor into two segments was achieved by irradiation. Acknowledgemen&-technical assistance.
wish to thank Mrs. P. WALTONand Miss D. OVENDEN for
R4sun&--Par une section radiculaire transversafe complete ou partielfe, permettant de s&arer le processus d’kuption dentaire de celui de la p#lif~ration ceffulaire basale, f’eruption de f’incisive inferieure droite, a croissance continue, chez 9 rats Wistar m&Ies et 9 femelfes, a pu hre em@ht!e. La couronne de la dent experimentale est sectionn&e au ras de la gencive pour civiter fes traumatismes masticatoires. Les animaux sont sacrifies 120 jours apres f’eruption; fes maxiffaires sont radiographies puis coupes. Apres atret de I’eruption, la partie basale de la “racine” presente: (1) au niveau de 10 mandibules, une forme courbe regufiere identique aux temoins; (2) au niveau de 3 mandibufes, une forme beaucoup plus recourbee que ceffe des temoins, de sorte que l’ouverture pulpaire est dirig6e vers fe haut; (3) au niveau de 6 mandibules, une forme entierement recourb& de teffe sorte que f’ouverture pufpaire est anterieure. L’examen histofogique montre la formation d’emaif et de dentine dans la region basafe: f’ivoire obfitere totalement la cavite pufpaire normafe a f’exception d’une zone, en forme de cone, situ&s dans la region basale. Ces resultats demontrent nettement que la region proliferative basale de f’incisive de rat est susceptible de foumir une force suffisante pour provoquer un changement morphologique de la racine, forsque 1’6ruption est empkhee. Dans un cas de section transversafe totafe de la racine, fe segment proximal a effect& son &uption, en p&&rant dans la surface fat&ale de f’alv4ole puis dans la joue, pour atteindre la surface cutan& Zusammenfassung-Durch volfstlndige oder teifweise Wurzefdurchtrennung wurde bei 9 mlnnfichen und 10 weibfichen Wistarratten das stgndige Wachstum des unteren rechten Schneidezahnes unterbrochen, wodurch sich der Vorgang des Zahndurchbruches von dem der basalen ~IfproIifemtion unte~heiden fief% Der Versuchszahn wurde unmittefbar nach der Operation bis zum Zahnfleischrand gekiirzt, urn Kaubefastungen auszuschaften. Die Tiere wurden 120 Tage nach der Operation getotet, danach
1284
B. K. B. BERKOV~TZ die Kiefer freiprapariert, radiographie~ und danach geschnitten. Nach Unterbr~hung des Durchbruches zeigte der UrnriD des basaien Abschnittes der “Wurzel” eine der folgenden Anordnungen: (1) bei 10 Kiefern zeigte sich die Formeiner den Kontrollen Ihnlichen sanften Kurve; (2) bei 3 Kiefern war er starker als bei den Kontrollen gekriimmt, so da8 die Pulpaoffnung nach oben zeigte; (3) bei den tibrigen 6 Kiefem war er vollstlndig soweit gekrtimmt, dal3 die Pulpaiiffnung in anteriorer Richtung lag. Histologisch ergab sich fortgesetzte Schmelz- und Dentinbildung in diesem basalen Abschnitt, wobei das Dentin das normale Pulpakavum mit Ausnahme eines schmalen keilfiirmigen Abschnittes an der Basis vollstandig obliterierte. Diese Ergebnisse weisen eindeutig darauf hin, daD die proliferative Basalregion des Rattenschneidezahnes befahigt ist, bei verhindertem Zahndurchbruch eine Kraft zu entwickeln, die ausreicht, eine deutliche Veranderung in der WurzeImorphoIogie zu verursachen. In einem Fall einer volfstlndigen Wu~eld~htrennung brach das proximaie Segment weiter durch und durchd~ die Seite der Alveole und dann die Wange bis nach a&en. REFERENCES
ARCE, C. and ERAUSQUIN,J. 1944. Supresibn experimental de la eruption en 10s incisivos de la rata. Rev. odont., B. Aires 32,525-540. BAUME,L. J., BECKS,H., RAY, J. C. and EVANS,H. M. 1954. Hormonal control of tooth eruption. IL. The effects of hypophysectomy on the upper rat incisor following progressively longer intervals. J. dent. Res. 33,91-103. BECKS,H., COLLINS,D. A., SIMPSON, M. E. and EVANS,H. M. 1946. Changes in the central incisors of hypophysectomised female rats after different postoperative periods. Archs Path. 41,457-475. BECKS, H. and FURUTA,W. J. 1941. Effect of magnesium deficient diets on oral and dental tissues. II. Changes in the enamel structure. 1. Am. dent. Ass. 28, 1083-1088. BERKOVITZ,B. K. B. 1971. The effect of root transection and partial root resection on the unimpeded eruption rate of the rat incisor. Archs oral Viol. 16,1033-1043. BERKOYITZ,B. K. B. and THOMAS,N. R. i969. Unimpeded eruption in the root-resected lower incisor of the rat with a preliminary note on root transection. Archs oral Biol. 14,771-780. BRYER,L. W. 1957. An experimental evaluation of the physiology of tooth eruption. Inr. dent. J. 7, 432-478. ECCLES.J. D. 1965. The effects of reducing function and stopping eruption on the periodontium of the rat incisor. J. dent. Res. 44, 860-868 ENGLISH.J. A.. SCHLACK.C. A. and ELLINGER.F. 1954. Oral manifestations of ionizina radiation. II. Effeit of 200 kV X-ray on rat incisor teeth when administered locally to the head & the 1500 rad dose range. J. dent. Res. 33, 377-388. GAGNON, J., SCHOUR, I. and PATRAS,M. C. 1942. Effect of magnesium deficiency on dentine apposition and eruption in incisor of rat. Proc. Sot. exp. Biol. Med. 49,662-666. GOWGIEL,J. M. 1967. Observations on the phenomena of tooth eruption J. dent. Res. 46, 132% 1330. IRVING, J. T. 1940. The influence of diets low in magnesium upon the histoIogica1 appearance of the incisor tooth of the rat. J. Physfol., Land. 99,817. KRESHOVER,S. J. and HANCOCK, JR., J. A. 1956. The pathogenesis of abnormal enamel formation in rabbits inoculated with vaccinia. J. dent. Res. 35,684-692. NESS, A. R. 1956. The response of the rabbit mandibular incisor to experimental shortening and to the prevention of its eruption. Proc. R. Sot. B. 146,129-I54. NESS, A. R. 1964. Movement and forces in tooth eruption. In: Advances in Oral Biology (edited by ST,\PLE, P. H.) pp. 33-75. Academic Press, New York. SARNAT, B. G. and SCHOUR, 1. 1944. Effect of experimental fracture on bone, dentin and enamel. Archs Surg. 49, 23-38. SARNAT,H. and SCIAKY,I. 1965. Experimental lathyrism in rats: effects of removing incisal stress. Periodontics 3, 128-134. SCHOUR, I. and VAN DYKE, H. B. 1932. Changes in the teeth following hypophysectomy. I. Changes in the incisor of the white rat. Am. J. Anat. 50, 397-433. SCIAKY,I. and UNGAR, H. 1961. Osteolathyrism in the incisors of rats. Ann. Dent. 20,42-50. SICHER, H. 1942. Tooth eruption: the axial movement of continuously growing teeth. J. dent. Res. 21,201-210.
EFFECT
OF PREVENTING
TEN CATE, A. R. 1969.
ERUPTION
ON TISSUES
OF THE RAT LOWERINCISOR
1285
The mechanism of tooth eruption. In: Biology of the periodontium (edited by New York. Ph.D. Thesis. University of
MELCHER, A. H. and BOWEN, W. H.), pp. 91-103. Academic Press, THOMAS, N. R. 1965. The process and mechanism of tooth eruption.
Bristol. WEINREB, M. M.,
MICHAELI, Y. and SILBERMAN,G. 1969. Role of attrition and occlusal contact in the physiology of the rat incisor. IV. Prevention of attrition in the articulating incisor. J. dent. Res. 48,120-l 30.
PLATES 1-3 OVERLEAF
B. K. B. BERKOVITZ
1286
FIG. 1. Radiograph
PLATE1. of an adjacent lower left incisor used as a control in the present experiment. Scale = 5 mm.
FIG. 2. Radiograph of the lower right incisor 4 months after eruption had been prevented following partial root transection. The arrow indicates the site of operation. Compared with Fig. 1 the pulp cavity in the proximal portion has continued to fill in due to dentine formation. but the outline of the basal portion of the “root” is similar to that seen in Fig. 1. Scale = 5 mm. FIG. 3. Radiograph of the lower right incisor 4 months after root transection. Both tooth segments have erupted forwards approximately 3 mm. The arrow indicates the site of operation. The outline of the proximal segment is more sharply curved so that the pulp opening faces directly upwards. Scale = 5 mm.
tFFECT
OF PREVENTING
ERUPTION
ON TlSSUES
OF THE
RAT
LOWER
INCISOR
PLATE
A.O.B.
f.p.
i78h
I
PLATE
?
EFFECTOF PREVENTINGERUPTIONON TISSUESOF THE RAT LOWERINCISOR
PLATE 2. FIG. 4. Radiograph of the lower right incisor 4 months after eruption of the proximal segment had been prevented following root transection. The arrow indicates the site of operation. The base of the proximal segment is recurved and the pulp opening faces anteriorly. The distal segment lies at the very edge of the socket. Scale = 5 mm. FIG. 5. Composite of photomicrographs of jaw seen in Fig. 4. Arrow indicates portion of distal segment which has erupted to the oral end of the socket. D = dentine. P = pulp. Scale = 5 mm. FIG. 6. High-power view of recurved basal portion of tooth from jaw seen in Fig. 4 showing the distribution of dental tissues. D = dentine. P = pulp. E = enamel space. A = ameloblasts. Scale = 0.5 mm.
1287
1288
B. K. B. BERKOVITZ
PLATE3. FIG. 7. Photograph showing an incisor tooth segment, 4 months after root transection, having penetrated the cheek tissue to reach the exterior at A and because of its curvature about to repenetrate the cheek at B. Scale = 5 mm. FIG. 8. Oblique radiograph of the lower right incisor seen in Fig. 7,4 months transection. The distal segment lies at the very edge of the socket while the segment has continued to erupt forwards to penetrate the side of the mandible along the length of the socket. Arrow indicates site of operation. Scale
after root proximal half-way = 5 mm.
FIG. 9. Radiograph of the lower right incisor 4 months after eruption of the proximal segment had been prevented following root transection, and some folding of the basal labial tissues is evident. Arrow indicates site of operation. The distal segment has been exfoliated from the socket. Scale = 5 mm.
tFFt(‘T
OF PKEVENTING
ERUPTION
ON TISSUES
OF THE
RAT
LOWER
INCISOR
PLATE
A.O.B.
3
f.p. 1288
THE EFFECT OF PREVENTING ERUPTION ON THE PROLIFERATIVE BASAL TISSUES OF THE RAT LOWER INCISOR B. K. B. BERKOVITZ Department of Anatomy (Oral Biology), The Medical School, University Walk, Bristol BS8 lTD, England
Summ~y-Eruption was prevented in the continuously growing lower right incisor of 9 male and 10 female Wistar rats by the surgical procedure of either complete or partial root transection, thereby allowing for a separation of the process of tooth eruption from that of basal cell proliferation. The experimental tooth was cut down to the gingival margin immediately following operation to eliminate masticatory stress. Animals were killed 120 days after operation, following which the jaws were dissected out, radiographed and then sectioned. Following cessation of eruption, the outline of the basal portion of the “root” showed one of the following a~angements: {I) in 10 jaws it was in the form of a smooth curve similar to that of controls, (2) in 3 jaws it was more sharply curved than the controls so that the pulp opening faced directly upwards, (3) in the remaining 6 jaws it was completely recurved so that the pulp opening was directed anteriorly. Histological examination revealed the presence of continued enamel and dentine formation in this basal region with the latter tissue completely obliterating the normal pulp cavity except for a small cone-shaped area at the base. These results clearly demonstrate that the proliferative basal region of the rat incisor is capable of generating a force when eruption is prevented, sufficient to cause a significant change in root morphology. In one complete root transection specimen, the proximal segment continued to erupt forwards and penetrated the side of the socket and then the cheek to reach the exterior.
THE ACTIVITY of the continuously growing basal region of the rat incisor has been studied following retardation or prevention of tooth eruption achieved by hypophysectomy (SCHOURand VAN DYKE, 1932; BECKYet al., 1946; BAUME et al., 1954), crowning of incisors (ARCEand ERAUSQUIN, 1944; WEINREB, MICHAELI and SILBERMAN, 1969), magnesium-deficient diet (IRVING, 1940; BECKSand FURUTA, 1941; GAGNON, SCHOUR and PATRAS, 1942), lathyritic diet (SCIAKY and UNGAR, 1961; SARNAT and SCIAKY, 1965; THOMAS, 1965) and in the rabbit incisor following vaccinia injection (KRESHOVERand HANCOCK, 1956). A striking observation in the upper incisors was the appearance of extensive regular “concertina-like” folds in the enamel and subjacent dentine in the basal region, evident on radiographs (BECKS et al., 1946). There was little recorded evidence of any significant resorption of bone at the base of these sockets, which may have been expected if basal cell proliferation continued during the period of the reduced eruption, and which would have resulted in a change in the normal morphology of the socket. 1279
1280
B. K. B. BERICOVITZ
In the above studies, a similar folding associated with the lower incisors was observed only occasionally by BECKSer al. (1946), while ECCLES(1965) did not record any basal folding when eruption was severely retarded following pinning or crowning of the rat lower incisor. Following root transection, SARNATand SCHOUR(1944) and BRYER(1957) noted that the proximal fragment of the rat lower incisor showed a continuation of growth in length as observed by the compression basally of the odontogenic epithelium and the basal incisor tissue. NESS(1956) observed a crumpling of basal tissue following pinning of three rabbit lower incisors for a period of up to 48 days; the base of the socket of two of these incisors had moved backwards I mm. A review of the literature has revealed that, apart from the work of SARNATand SCHOUR(1944) and NESS (1956)Jhere is little evidence of any significant change in the morphology of the basal region of the socket of the rodent incisor when eruption has been severely retarded, and the subsequent interpretation of the results of such studies has been complicated by the fact that the experimental tooth was, in nearly every case, left in functional occlusion. The present study has been carried out to determine the extent of continued proliferative activity in the basal region of the rat lower incisor when eruption is prevented for a prolonged period of time. Any significant change in the normal morphology of the base of the socket should be observable on radio~aphical examination. In every case, the experimental tooth is to be cut down to the gum margin following the surgical procedures necessary to prevent eruption, thereby eliminating problems related to masticatory stress. The findings may indicate whether the proliferative basal tissues are capable of generating any kind of force sufficient for them to be considered in the mechanism responsible for effecting tooth eruption (e.g. SIGHER, 1942). MATERIALS AND METHODS It has been shown that, following root transection in the rat lower incisor, the proximal segment is retained in a basal position while the distal segment usually continues erupting and is eventually exfoliated from the oral end of the socket (BERKOV~Z, 1971). This procedure has been utilized in the present study to obtain cessation of eruption of the proximal segment which will also be isolated from any rn~ti~tory stresses. Root transection was carried out on the lower right incisor of 5 male and 7 female young adult Wistar rats, using the same surgical procedure as previously described (BERKOVTTZ, 1971). The “crown” of the tooth was cut down to the gingival margin by means of a Carborundum disc immediately following the transection. In addition to the complete transection procedures described above, partial root transection was carried out on the lower right incisor tooth in a further 5 male and 3 female young adult Wistar rats, such that the bur only removed a segment of tooth substance from the enamel half of the tooth. The tooth “crown” was cut down to the gingival margin by means of a Carborundum disc immediately following the operation. Following these experimental procedures, animals were fed on Modified IXet 41 B (Oxoid) and water ad libitum, and were killed 120 days after operation. At the end of the experiment, the average weight of the male rats was 425 g, and the females 250 g. In addition to the adjacent impeded left lower incisor tooth of these experimental animals, the jaws of unoperated male and female Wistar rats of similar age and weight were obtained for use as controls. At the end of the experiment, jaws were radiographed and 60 m.a.s. Ilford Industrial F was the film used.
with an A.E.I. Raymax 50 unit at 40 KV.
EFFECT
OF PREVENTING
ERUPTION
ON TISSUES
OF THE RAT
LOWER
INCISOR
1281
Jaws were fixed in Bouin’s solution, decalcified in EDTA (pH 7.4), embedded in paraffin wax and serially sectioned longitudinally in a microtome set to cut at a thickness of 10 pm. Sections were stained with haematoxylin and eosin.
RESULTS
Of the 12 animals which underwent complete root transection, the proximal segment exhibited no forward movement in 9, as evidenced by radiographical examination, while in 2 of the remaining animals the proximal segment had erupted forwards 3 mm over the experimental period of 4 months. The findings in the remaining animal will be described later. Of the 8 animals which were subjected to partial root transection, the tooth was completely prevented from further eruption in 5 animals, while in the remaining 3 it had erupted forwards 4, 5 and 7 mm, respectively, over the experimental period of 4 months. Thus, the experimental procedures had totally prevented eruption of the tooth in 14 cases, while eruption was greatly reduced in the remaining 5 animals. Radiographic examination of the operated jaws indicated that the pulp chambers of all the experimental non-erupting teeth had been filled in by radiopaque material, later identified histologically as dentine, except for a small cone-shaped area at the base (compare Figs. 1 and 2). In the control situation, the radiological outline of the “root”portion of the incisor approximately formed an arc of a circle with a radius of curvature of 15 mm with the pulp opening being angled backwards and upwards (Fig. 1). In the experimental teeth following cessation of eruption, the outline of the basal portion of the “root” showed one of the following arrangements : (1) In 10 jaws (3 complete and 7 partial root transections), it was in the form of a smooth curve similar to that of controls (Fig. 2). (2) In 3 jaws (2 complete and 1 partial root transection), it was more sharply curved than the controls so that the pulp opening faced directly upwards (Fig. 3). (3) In the remaining 6 jaws (all complete root transected teeth), it was completely recurved so that the pulp opening was directed anteriorly (Fig. 4). This basal segment of the “root” had a radius of curvature of approximately 4 mm and its anterior limb was extended forwards by about 2-3 mm. Histological examination confirmed that, following cessation of eruption, dentine had continued to be formed, resulting in a constriction of the pulp cavity, and from the presence of an enamel space lined by ameloblasts, it was concluded that enamel had also continued to be formed (Figs. 5 and 6). The above results have excluded one male rat whose incisor tooth had been completely root transected; by the end of the experimental period, in addition to the distal segment which had erupted to the very edge of the socket, this specimen exhibited an incisor tooth segment on the side of the face, which had penetrated the cheek tissues to reach the exterior. Because of its curvature (radius of curvature = 5 mm), the tip of this incisor was about to repenetrate the cheek tissue (Fig. 7). Radiological examination revealed that this “additional” incisor tooth was derived from the A0.B.1719-c
1282
B. K. B.
BERKOVITZ
proximal segment which had erupted forwards following transection (Fig. 8), but had penetrated the side of the socket half-way along its length and close to the site of the original surgical wound. The total amount of eruption for this proximal segment was approximately 1S mm for the 4 month period. Extensive regular “concertina-like” folding of the enamel and subjacent dentine, described by other authors in the basal region of the upper incisor following cessation of eruption achieved by other methods (e.g. BECKS et al., 1946), was not observed in the present study on the lower incisor, though a less conspicuous folding was evident (Fig. 9). DISCUSSION
The surgical techniques used in the present study have prevented eruption of the rat lower incisor for a period of 120 days without apparent damage to the proliferative basal tissues. In approximately half of the animals used, there was no significant change in the morphology of the basal region of the socket, though the pulp cavity had been almost totally filled in by continued dentine formation. In the remaining animals, however, the morphology of the basal region of the socket was modified by continued formation of dental tissues and bone resorption which, in the majority of these animals, resulted in the basal formative region of the tooth recurving upon itself so that the pulp opening faced anteriorly. This observation that some form of curvature was maintained, rather than the tooth simply extending directly backwards, may indicate differential growth rates between the enamel-forming and cement-forming elements of the tooth and/or differential rates of bone resorption in these regions; the radius of curvature of this basal segment was much smaller than that for the control situation. It remains to be determined whether a similar surgical technique would produce a similar result for the rat upper incisor, though there is a significant difference in the anatomical relationships between the two situations. These results clearly demonstrate that the proliferative basal region of the rat incisor is capable of generating a force when eruption is prevented sufficient to cause a significant change in root morphology. Whether the force generated in this situation could account for normal tooth eruption, as has been proposed (e.g. SICHER,1942), remains undetermined, and arguing against this view is the finding that removal of this basal zone does not inhibit tooth eruption (e.g. BERKOVITZand THOMAS,1969). A similar view was forwarded by NESS (1964), who concluded that, though evidence suggested that proliferation of basal epithelium was capable of exerting an axial force, other evidence showed that it did not account for the eruption of the rodent incisor. The present study does not provide evidence either for or against a more recent theory that the periodontal ligament is responsible for the eruptive mechanism (for review see TEN CATE, 1969). The following interpretations have been given to account for the extensive regular “concertina-like” foldings seen in the basal zone of the rat incisor following cessation of eruption : (I) SICHER(1942) thought that hypophysectomy resulted in a dissociation of epithelial and pulpal growth, pulpal growth being inhibited but epithelial growth not being disturbed to the same degree, the eventual result being folding of the
EFFECT
OF PREVENTING
ERUPTION
ON TISSUES
OF THE
RAT
LOWER
INCISOR
1283
proliferating epithelial structures. (2) BAUMEet al. (1954) suggested that the folding was produced by the forces of mastication, a view expanded by WEINREBet al. (1969) who stressed the impor~nt relationship between masticatory load and the physiological state of the periodontium. The findings in the present study following removal of incisal stress, when only slight traces of folding in the basal enamel region were evident, would lend support to the latter interpretation of WEINREBet al. (1969). It is not yet possible to account for the non-eruption of the partial root transected teeth used in the present study, bearing in mind that, following complete root transection, the distal segment continues to erupt, most probably under the influence of the periodontal ligament (BERKOVITZ,1971). An explanation could be that the procedure of both partial and complete root transection interferes with the process of eruption primarily in the segment proximal to the level of the wound; in the former procedure the lack of eruption will affect the whole tooth, while in the latter procedure (i.e. complete root transection) the distal segment is physically free to erupt independently of the severely affected proximal segment. In one case following complete root transection, the proximal segment erupted forwards and out of the side of the socket, penetrating the cheek. This condition is reminiscent of the examples of “additional” teeth produced by ENGLISH, SCHLACKand ELLINGER(1954) and GO~GIEL (19671, when separation of the continuously growing rat incisor into two segments was achieved by irradiation. Acknowledgemen&-technical assistance.
wish to thank Mrs. P. WALTONand Miss D. OVENDEN for
R4sun&--Par une section radiculaire transversafe complete ou partielfe, permettant de s&arer le processus d’kuption dentaire de celui de la p#lif~ration ceffulaire basale, f’eruption de f’incisive inferieure droite, a croissance continue, chez 9 rats Wistar m&Ies et 9 femelfes, a pu hre em@ht!e. La couronne de la dent experimentale est sectionn&e au ras de la gencive pour civiter fes traumatismes masticatoires. Les animaux sont sacrifies 120 jours apres f’eruption; fes maxiffaires sont radiographies puis coupes. Apres atret de I’eruption, la partie basale de la “racine” presente: (1) au niveau de 10 mandibules, une forme courbe regufiere identique aux temoins; (2) au niveau de 3 mandibufes, une forme beaucoup plus recourbee que ceffe des temoins, de sorte que l’ouverture pulpaire est dirig6e vers fe haut; (3) au niveau de 6 mandibules, une forme entierement recourb& de teffe sorte que f’ouverture pufpaire est anterieure. L’examen histofogique montre la formation d’emaif et de dentine dans la region basafe: f’ivoire obfitere totalement la cavite pufpaire normafe a f’exception d’une zone, en forme de cone, situ&s dans la region basale. Ces resultats demontrent nettement que la region proliferative basale de f’incisive de rat est susceptible de foumir une force suffisante pour provoquer un changement morphologique de la racine, forsque 1’6ruption est empkhee. Dans un cas de section transversafe totafe de la racine, fe segment proximal a effect& son &uption, en p&&rant dans la surface fat&ale de f’alv4ole puis dans la joue, pour atteindre la surface cutan& Zusammenfassung-Durch volfstlndige oder teifweise Wurzefdurchtrennung wurde bei 9 mlnnfichen und 10 weibfichen Wistarratten das stgndige Wachstum des unteren rechten Schneidezahnes unterbrochen, wodurch sich der Vorgang des Zahndurchbruches von dem der basalen ~IfproIifemtion unte~heiden fief% Der Versuchszahn wurde unmittefbar nach der Operation bis zum Zahnfleischrand gekiirzt, urn Kaubefastungen auszuschaften. Die Tiere wurden 120 Tage nach der Operation getotet, danach
1284
B. K. B. BERKOV~TZ die Kiefer freiprapariert, radiographie~ und danach geschnitten. Nach Unterbr~hung des Durchbruches zeigte der UrnriD des basaien Abschnittes der “Wurzel” eine der folgenden Anordnungen: (1) bei 10 Kiefern zeigte sich die Formeiner den Kontrollen Ihnlichen sanften Kurve; (2) bei 3 Kiefern war er starker als bei den Kontrollen gekriimmt, so da8 die Pulpaoffnung nach oben zeigte; (3) bei den tibrigen 6 Kiefem war er vollstlndig soweit gekrtimmt, dal3 die Pulpaiiffnung in anteriorer Richtung lag. Histologisch ergab sich fortgesetzte Schmelz- und Dentinbildung in diesem basalen Abschnitt, wobei das Dentin das normale Pulpakavum mit Ausnahme eines schmalen keilfiirmigen Abschnittes an der Basis vollstandig obliterierte. Diese Ergebnisse weisen eindeutig darauf hin, daD die proliferative Basalregion des Rattenschneidezahnes befahigt ist, bei verhindertem Zahndurchbruch eine Kraft zu entwickeln, die ausreicht, eine deutliche Veranderung in der WurzeImorphoIogie zu verursachen. In einem Fall einer volfstlndigen Wu~eld~htrennung brach das proximaie Segment weiter durch und durchd~ die Seite der Alveole und dann die Wange bis nach a&en. REFERENCES
ARCE, C. and ERAUSQUIN,J. 1944. Supresibn experimental de la eruption en 10s incisivos de la rata. Rev. odont., B. Aires 32,525-540. BAUME,L. J., BECKS,H., RAY, J. C. and EVANS,H. M. 1954. Hormonal control of tooth eruption. IL. The effects of hypophysectomy on the upper rat incisor following progressively longer intervals. J. dent. Res. 33,91-103. BECKS,H., COLLINS,D. A., SIMPSON, M. E. and EVANS,H. M. 1946. Changes in the central incisors of hypophysectomised female rats after different postoperative periods. Archs Path. 41,457-475. BECKS, H. and FURUTA,W. J. 1941. Effect of magnesium deficient diets on oral and dental tissues. II. Changes in the enamel structure. 1. Am. dent. Ass. 28, 1083-1088. BERKOVITZ,B. K. B. 1971. The effect of root transection and partial root resection on the unimpeded eruption rate of the rat incisor. Archs oral Viol. 16,1033-1043. BERKOYITZ,B. K. B. and THOMAS,N. R. i969. Unimpeded eruption in the root-resected lower incisor of the rat with a preliminary note on root transection. Archs oral Biol. 14,771-780. BRYER,L. W. 1957. An experimental evaluation of the physiology of tooth eruption. Inr. dent. J. 7, 432-478. ECCLES.J. D. 1965. The effects of reducing function and stopping eruption on the periodontium of the rat incisor. J. dent. Res. 44, 860-868 ENGLISH.J. A.. SCHLACK.C. A. and ELLINGER.F. 1954. Oral manifestations of ionizina radiation. II. Effeit of 200 kV X-ray on rat incisor teeth when administered locally to the head & the 1500 rad dose range. J. dent. Res. 33, 377-388. GAGNON, J., SCHOUR, I. and PATRAS,M. C. 1942. Effect of magnesium deficiency on dentine apposition and eruption in incisor of rat. Proc. Sot. exp. Biol. Med. 49,662-666. GOWGIEL,J. M. 1967. Observations on the phenomena of tooth eruption J. dent. Res. 46, 132% 1330. IRVING, J. T. 1940. The influence of diets low in magnesium upon the histoIogica1 appearance of the incisor tooth of the rat. J. Physfol., Land. 99,817. KRESHOVER,S. J. and HANCOCK, JR., J. A. 1956. The pathogenesis of abnormal enamel formation in rabbits inoculated with vaccinia. J. dent. Res. 35,684-692. NESS, A. R. 1956. The response of the rabbit mandibular incisor to experimental shortening and to the prevention of its eruption. Proc. R. Sot. B. 146,129-I54. NESS, A. R. 1964. Movement and forces in tooth eruption. In: Advances in Oral Biology (edited by ST,\PLE, P. H.) pp. 33-75. Academic Press, New York. SARNAT, B. G. and SCHOUR, 1. 1944. Effect of experimental fracture on bone, dentin and enamel. Archs Surg. 49, 23-38. SARNAT,H. and SCIAKY,I. 1965. Experimental lathyrism in rats: effects of removing incisal stress. Periodontics 3, 128-134. SCHOUR, I. and VAN DYKE, H. B. 1932. Changes in the teeth following hypophysectomy. I. Changes in the incisor of the white rat. Am. J. Anat. 50, 397-433. SCIAKY,I. and UNGAR, H. 1961. Osteolathyrism in the incisors of rats. Ann. Dent. 20,42-50. SICHER, H. 1942. Tooth eruption: the axial movement of continuously growing teeth. J. dent. Res. 21,201-210.
EFFECT
OF PREVENTING
TEN CATE, A. R. 1969.
ERUPTION
ON TISSUES
OF THE RAT LOWERINCISOR
1285
The mechanism of tooth eruption. In: Biology of the periodontium (edited by New York. Ph.D. Thesis. University of
MELCHER, A. H. and BOWEN, W. H.), pp. 91-103. Academic Press, THOMAS, N. R. 1965. The process and mechanism of tooth eruption.
Bristol. WEINREB, M. M.,
MICHAELI, Y. and SILBERMAN,G. 1969. Role of attrition and occlusal contact in the physiology of the rat incisor. IV. Prevention of attrition in the articulating incisor. J. dent. Res. 48,120-l 30.
PLATES 1-3 OVERLEAF
B. K. B. BERKOVITZ
1286
FIG. 1. Radiograph
PLATE1. of an adjacent lower left incisor used as a control in the present experiment. Scale = 5 mm.
FIG. 2. Radiograph of the lower right incisor 4 months after eruption had been prevented following partial root transection. The arrow indicates the site of operation. Compared with Fig. 1 the pulp cavity in the proximal portion has continued to fill in due to dentine formation. but the outline of the basal portion of the “root” is similar to that seen in Fig. 1. Scale = 5 mm. FIG. 3. Radiograph of the lower right incisor 4 months after root transection. Both tooth segments have erupted forwards approximately 3 mm. The arrow indicates the site of operation. The outline of the proximal segment is more sharply curved so that the pulp opening faces directly upwards. Scale = 5 mm.
tFFECT
OF PREVENTING
ERUPTION
ON TlSSUES
OF THE
RAT
LOWER
INCISOR
PLATE
A.O.B.
f.p.
i78h
I
PLATE
?
EFFECTOF PREVENTINGERUPTIONON TISSUESOF THE RAT LOWERINCISOR
PLATE 2. FIG. 4. Radiograph of the lower right incisor 4 months after eruption of the proximal segment had been prevented following root transection. The arrow indicates the site of operation. The base of the proximal segment is recurved and the pulp opening faces anteriorly. The distal segment lies at the very edge of the socket. Scale = 5 mm. FIG. 5. Composite of photomicrographs of jaw seen in Fig. 4. Arrow indicates portion of distal segment which has erupted to the oral end of the socket. D = dentine. P = pulp. Scale = 5 mm. FIG. 6. High-power view of recurved basal portion of tooth from jaw seen in Fig. 4 showing the distribution of dental tissues. D = dentine. P = pulp. E = enamel space. A = ameloblasts. Scale = 0.5 mm.
1287
1288
B. K. B. BERKOVITZ
PLATE3. FIG. 7. Photograph showing an incisor tooth segment, 4 months after root transection, having penetrated the cheek tissue to reach the exterior at A and because of its curvature about to repenetrate the cheek at B. Scale = 5 mm. FIG. 8. Oblique radiograph of the lower right incisor seen in Fig. 7,4 months transection. The distal segment lies at the very edge of the socket while the segment has continued to erupt forwards to penetrate the side of the mandible along the length of the socket. Arrow indicates site of operation. Scale
after root proximal half-way = 5 mm.
FIG. 9. Radiograph of the lower right incisor 4 months after eruption of the proximal segment had been prevented following root transection, and some folding of the basal labial tissues is evident. Arrow indicates site of operation. The distal segment has been exfoliated from the socket. Scale = 5 mm.
tFFt(‘T
OF PKEVENTING
ERUPTION
ON TISSUES
OF THE
RAT
LOWER
INCISOR
PLATE
A.O.B.
3
f.p. 1288