Observations on the Enamel Spindle found in Dogs and Cats. Motoki YAMADA* andhajime OHAZAMA**

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1 Observations on the Enamel Spindle found in Dogs and Cats by Motoki YAMADA* andhajime OHAZAMA** Introductory We often find the dentinal fibers which protrude themselves into the enamel around thedentino-enamel junction of humanteeth. Of them, what becomes enlarged after it has entered into the enamel is referred to as enamel spindle and what is simply composed of the dentinal fibers is known as simple process. There are two schools of scholars; one considers these two to be same in essence and the other regarding them quite apart in character. There have been made public many research reports on thesubject by NOEL and RETZIUS as early as in 1839 tokirino whose report came out in The various causes assigned to enamel spindle include developmental defects, artifacts, matters resulting from the absorption of dentin, nervous endings, dentinal tubules or dentinal fibers entering into enamel. However, at present the majority of scholarly opinions believes that an enamel spindle consists of dentinal fibers protruding into enamel and that it already exists inside the ameloblasts before the dentin is formed. Yet it is not definite as to why an enamel spindle assumes two different external forms. On the other hand, with reference to the significance of this spindle not very much is known. Many scholars consider the spindle to be a nutritional duct to enamel substance. FUJITA assigns in his book the role of an antenna to it to receive external stimuli on the part of teeth. Through our use of dogs and cats to study their second dentin, we have come to fi nd that there exist no enamel spindles in them but we find only those which are similar to simple processes. Experimental Procedure The longitudinal ground sections were prepared of the teeth of dogs and cats which were collected at random, and put to observational purposes. Part of them were treated with carbol-fuchsin stain and others were used with no staining. The total number of teeth used was 85, consisting of 20 incisors, 10 canines and 21 molars of dogs and 13 incisors, 15 molars and 6 milk teeth of cats. Dept. of Radiology, and Dept. ofaizatoiny Nihon Univ. School of Dentistry 95

2 96 Findings As the present observations were based on the useof ground sections, we were forced to examine thick specimens and, for this reason, the dentino-enamel junction appeared in various elevations depending upon the focusing of microscope during the observations. Thus difficulties were felt in determining whether an end of dentinal fiber terminated with the enamel or it existed inside thedentin. Therefore, particular caution was exercised in this regard. The first thought that struck us was that in the teeth of dogs and cats nothing in the nature of an enamel spindle exists. But instead we found that, as is the case with human teeth, there are cases in which the ends of dentinal fibers terminated in the enamel (Fig. 1, A, C). Yet even inthese cases they are not exactly like that of human in that a greater part of dentinal fibers terminate soon after they go over the boundaries. Wefailed in detecting anything which protruded into enamel from dentin. With reference to the milk teeth of cats, quite many of dentinal fiber ends go deeply into the enamel as against the permanent teeth (Fig. 1 B). For this reason, the dentino-enamel junction in milk teeth appear as if a bush. Although it was not possible to compare numerically, the depth of protrusions goes as deep as15-20ƒê which is about1.5 or 2 times that of permanent teeth (Fig2). But they are nodifferent from those of permanent teeth in basic character. Otherwise there has been noticed nothing which resembles an enamel spindle of human teeth. Discussions It is thought peculiar that in human teeth the dentinal fibers protrude themselves into the enamel from dentin. But it is a thing of common occurrence in the teeth of fish and especially in porgy species, the dentinal fibers reach well into the outer layer of enamel. There is an exception, however, in such families as Semicossyhus reticulatus (ISOKAWA). Therefore, it is by no means extraordinary that in the mammalians the dentinal fibers protrude themselves into enamel. Yet studies with other animals are ratherscanty one finds that a few researchers such as MEYER, ORBAN and CAUSH give descriptions of dentino-enamel junction in a few animals. However, they are satisfied with pointing out that a protrusion of dentinal fibers exists in them and do not attempt any comparative study in detail. In regard to human teeth, EIDMANN, LAM, SCHLACK andkirino among others have shown that the protrusion is developed by the entrance of dentinal fibers into ameloblasts before the hard dental tissues are formed. And this protrusion further develops into an enamel spindle or a simple process. As has been described above, we have found that in the teeth of dogs and cats there is nothing resembling an enamel spindle in human teeth. But we are not satisfied with a simple statement that in them the development of enamel spindles is very backward but believe that the matter should be further explicated in the light of comparative anatomy. KIRK maintains that in human teeth the locality of generation of protrusions differs according to the kinds of teeth but, as far as our findings go, we have not found an evidence of this. No adequate explanation is still wanting as to adifference between the milk and permanent teeth in this regard.

3 Although FUJITA assigns the role of an antenna to an enamel spindle to receive external stimuli on the part of teeth, there are offered two opposing views in the teeth of carnivorous animals which possess thin enamel layers. One view is that in them as the enamel layer is thin, external stimuli are easily conveyed to the teeth without a spindle. The other view runs that as the enamel layer is thin, it must possess a spindle so that external stimuli are quickly conveyed. After all are said for and against this, we are bound to admit that we do not know definitely about the significance of an enamel spindle. Conclusions As a result of microscopic observations of 85 teeth of dogs and cats for the state of dentinal fibers protruding into the enamel, we have obtained the followingconclusions: (1) As against human teeth, there exists nothing resembling an enamel spindle in the teeth of dogs and cats. (2) The dentinal fibers which protrude into the enamel in the teeth of dogs and cats are just a few in number. (3) In the milk teeth of cats more dentinal fibers protrude into the enamel than is the case with the permanent teeth. The protrusion is also far deeper. (4) The behavior of the dentinal fiber protruding into the enamel is that it starts from the enamel bordering upon dentin. (5) No adequate explanation is possible so far in regard to the significance of an enamel spindle. References 1) ABBOT,F.: Congenital defects in enamel. D. Cosmos33: (1891) 2) BEUST, T.B.: Intrafollicular enamel developmenṭ J. A. D.A.15: (1928) 3)BODECKER, C.F.: Enamel lamella.j.a.d.a.15: (1928) 4)BODECKER, C.F.: The treatment of sensitive teeth. D. Cosmos70: (1928) 5)FISH, E.W.: The circulation of lymph in dentin and enameḷj.a.d.a.14: (1927) 6)DEPENDORF, Beitrage zur Kenntnis der Innervierung der menschlichen Zahnpulpaund des Dentins. Dtsch. Mschr. f. Zhk.31: (1913) 7) HOPEWELL-SMITH,A.: Concerning humanenamel: factș explanations and applications I, II. D. Cosmos68: (1926),69: , (1927) 8)KIRINO, T. & TAKAHASHI,K.: Studies on the human enamel spindle. Kaibo Z.35: Suppl.V.: 4 (1960) 9) KIRK, L. E.van: Variations in structure of human enamel and dentin. J. A. D. A.15: (1928) 10) KIRK, L. E.van: A further study of the frequency of occurrence of certain structural varia - tions in human enamel, dentin and cementum. J.A.D.A.16: (1929) 11) LEIGH, R.W.: Studies of the enameḷj.a.d.a.14: , ) McCREA, M. W. & Robinson,H.B.G.: 15: (1936) Dentinal projection in marspial enamel. J. D. Res. 13) MEYER,W.:Strittige (1925) Fragen in der Histologie des Zahnschmelzeṣ Vjschr. f. Zhk.41: 14) URBAN,B.: Der histologische Bau des Kaninchengebisseṣ Vjschr. f. Zhk.41: (1925) 15) URBAN,B.: Zur Histologie des Schmelzesund des Schmelz-Dentingrenzẹ 42: (1926) Vjschr. f. Zhk. 97

4 98 16) SCHLACK, C. A. : Investigations into the etiology of enamel spindles in human teeth. J. D. Res. 19 : (1940) 17) SMREKER, E.: Fortsetzungen von Zahnbeinkanalchen in den Schmelz menschlichen Vjschr. f. Zhk. 39 : (1923) 18) WILLIAMS, J. L. : A contribution to the study of pathology of enamel. D. Cosmos , , (1897)

5 99 Fig. 1. Histological aspects of dentino-enamel junctions A. Feline upper third molar B. Feline lower second milk molar C. Canine lower tusk

6 100 Fig. 2. Higher Magnification of dentino-enamel junctions A. Higher enlargement of a portion seen in C of Fig. 1 :,about X1000) B. Higher magnification of a region seen in B of Fig. 1,about x 1000

Development of teeth. 5.DM - Pedo

Development of teeth. 5.DM - Pedo Development of teeth 5.DM - Pedo Tooth development process of continuous changes in predetermined order starts from dental lamina A band of ectodermal cells growing from the epithelium of the embryonic