Coll. Antropol. 27 (2003) 1: 373–380 UDC 572.72:616.314.11 Original scientific paper

Comparative Morphology of Incisor Enamel and Dentin in Humans and Fat Dormice (Glis glis)

Dean Konjevi}1, Tomislav Keros2, Hrvoje Brki}3, Alen Slavica1, Zdravko Janicki1 and Josip Margaleti}4

1 Chair for Game Biology, Pathology and Breeding, Veterinary Faculty, University of Zagreb, Zagreb, Croatia 2 Croatian Veterinary Institute, Zagreb, Croatia 3 Department for Dental Anthropology, School of Dental Medicine, University of Zagreb, Zagreb, Croatia 4 Department of Forest Protection and Wildlife Management, Faculty of Forestry, University of Zagreb, Zagreb, Croatia

ABSTRACT

The structure of teeth in all living beings is genetically predetermined, although it can change under external physiological and pathological factors. The author’s hypoth- esis was to indicate evolutional shifts resulting from genetic, functional and other dif- ferences. A comparative study about certain characteristics of incisors in humans and myomorpha, the fat dormouse (Glis glis) being their representative as well, comprised measurements of enamel and dentin thickness in individual incisor segments, evalua- tion of external enamel index, and also assessment of histological structure of enamel and dentin. The study results involving dormice showed the enamel to be thicker in lower than in the upper teeth, quite contrary to enamel thickness in humans. In the up- per incisors in dormice the enamel is the thickest in the medial layer of the crown, and in the cervical portion of the crown in the lower incisors. The thickness of dentin in dor- mice is greater in the oral than in the vestibular side. These findings significantly differ from those reported in reference literature, but they are based on the function of teeth in dormice. Histological characteristics of hard dental tissues in dormice are similar to those in humans, with exception of uniserial structure of enamel and appearance of dentinoenamel junction.

Key words: enamel, dentin, fat dormouse, humans

Received for publication February 3, 2003

373 D. Konjevi} et al.: Incisors in Humans and Fat Dormice, Coll. Antropol. 27 (2003) 1: 373–380

Introduction non, caused by different directions of winding and interlacing of enamel rods. The fat dormouse is referred to as our Close to dentinoenamel junctions are ena- autochthonous game classified in the or- mel tufts and enamel spindles. The enamel der of Rodentia, the family of Muscaridi- tufts are filled with organic substance nidae, and the genus of Gliridae. Based enamelin and they are formed within dif- on the shape of the cranium, morphology ferently directed groups of enamel prisms of masticatory muscles and structure of in dentinoenamel junction. enamel and dentin, the rodents are clas- The natural structure of dentin is sified into three sub-groups and the Myo- made of odontoblastic processes located morpha sub-group includes the dormice. and protected within dentinal canaliculi With regard to diversity of functions, the that extend from periphery of the pulp to upper and the lower incisors leave differ- dentinoenamel junction and sometimes ent prints, so that the teeth of rodents further into the enamel and enamel spin- have prints characterized by grooves1. dles. Within the tubules there is peri- The function of gnawing is enabled by tubular dentin and the tubules are con- extremely hard enamel on incisors of dor- nected by intertubular dentin. The Von mice. The enamel is highly mineralized Ebner's incremental lines show the den- and hence the hardest tissue in living or- tin depositing rate, while the prenatal ganisms. Its physical characteristics com- dentin is separated from the postnatal prise thickness, hardness, density, per- one by the so-called neonatal line3–7. meability, color and elasticity. The thick- The purpose of the comparative study ness of enamel is not the same in each is to define the evolutional shifts in hu- part of the dental crown and dental types, mans and in animals indicating their but there are individual differences as similarities and differences that have re- well. In humans, for example, the enamel sulted from genetic, functional and other layer is gradually thinned toward the cer- distinguishing factors. Even though com- vical part of a tooth, while its density di- parative studies are of great importance minishes from the surface toward the for establishing the evolutional shifts, we dentinoenamel junction. Therefore in hu- have found only few articles about com- mans the surface of the enamel is more parative studies between human and ro- abundantly mineralized than its deeper dent teeth in the recent literature8–10. layers, enabling better protection of teeth from undesirable external influences. The color of enamel is determined by its thick- Materials and Methods ness, density and degree of mineralisa- The study sample comprised 30 fat tion1–5. dormice (20 females and 10 males) cap- The fundamental units of enamel tured in the region of Gorski Kotar. Body structure are referred to as enamel rods length and body masses were taken for enclosed in a prism sheath and separated all animals, the parameters were then by interprismatic spaces. In the enamel marked and specimens frozen. The prep- of humans the lines of Retzius, or incre- aration of study samples comprised sepa- mental lines, are visible and they reflect ration of the lower jaw from the rest of the rate of depositing. In the cervical part the cranial skeleton by special scissors of the tooth along the dentinoenamel across the temporomandibular joint, sep- junction the dia or para-zones, the so- aration of the left from the right half, and called Hunter-Schreger bands, are found removal of the remaining soft tissue. The and they are considered optical phenome- Sunshine diamond F6 199,014 turbo

374 D. Konjevi} et al.: Incisors in Humans and Fat Dormice, Coll. Antropol. 27 (2003) 1: 373–380

Kavo Estetic 1,024 drill (Kaltenbach & Results Voigt D – 7950), at 1,500 turns per second was used to cut the upper jaw immedi- The measurement results of fat dor- ately before the premolars. All the speci- mice physical features show that their mens were stored into properly signed body lengths ranged from 29.7 to 15.4 cm, compartments and preserved in 5% for- the mean value being 18.5cm and stan- malin. dard deviation 2.61 cm. The maximal body mass (weight) was 154 g and the When making thin cuts one side of the minimal was 84 g, while the mean value lower jaw and parts of the upper jaw were being 114.83 g and standard deviation dispensed in methacrylate in special 16.36 g. molds using the Varidur 10 powder (di- The thickness of enamel in the cervi- benzyl peroxide) and Varidur fluid (tetra- cal portion of the upper teeth ranged from hydrofurfuryl 2 methacrilate). All the 40.7 to 36.5 mm, the mean value being sections were made by Isomet 1000 cutter 38.18 mm and standard deviation 1.19 (Buehler), at 175–200 turns per second mm. The same measurement values for and 0.7 mm cut thickness. The obtained lower teeth ranged from 55.41 to 41.4 mm, sections were photographed by Olympus the mean value being 43.95 mm and stan- BH-2 microscope at the magnification of dard deviation 2.85 mm. The enamel 40, 100 and 200. The specimens were thickness in the medial part of the tooth measured by special micrometer scale, crown in the upper teeth ranged from using the 10 times magnifying eyepiece 40.8 to 37.0 mm, the mean value being and 40 times magnifying lens. 38.59 mm and standard deviation 1.25 The enamel thickness was measured mm. The same measurement values for in the cervical, medial and incisal parts of lower teeth ranged from 51.8 to 41.0 mm, the tooth crown. From the thickness of the mean value being 44.48 mm and stan- exterior and interior enamel layers, ex- dard deviation 2.51 mm. ternal index was calculated11. The ob- The thickness of enamel in the incisal tained measurements served to the calcu- edge of the upper teeth ranged from 40.7 lation of enamel external index based on to 35.0 mm, the mean value being 38.63 the following equation: and standard deviation 2.86, while in the lower teeth the range was from 52.5 to External index = (outer layer 41.0 mm, the mean value being 45.0 mm thickness/total enamel thickness) ´ 100 and standard deviation 2.86 mm. Figure 1 shows the correlation between The thickness of dentin was measured the thickness of external enamel layer in the incisal and medial thirds of the and the external index. The thickness of tooth crown, while in the medial third of external enamel layer ranged from 15.2 the crown the measurements were made to 11.3 mm, the mean value being 13.25 on the vestibular and oral sides of teeth. mm. This Figure shows that external in- dex of 30.68% is conditioned by the small- Many authors studied enamel and den- est thickness of outer enamel layer (11.3 tin characteristics of human incisors2,3,12–14. mm). The mean number of samples shows The results of these were compared with external index from 33 to 37%. dormice incisors in our investigation. Figure 2 shows the correlation be- The results were statistically analy- tween the thickness if internal enamel zed by SPSS program for Windows Re- layer and the external index. The thick- lease 6.1. ness of internal enamel layer ranged

375 D. Konjevi} et al.: Incisors in Humans and Fat Dormice, Coll. Antropol. 27 (2003) 1: 373–380

16 Series1

15

14

13

12

11

The thickness of superficial enamel layer

10 30 31 32 33 34 35 36 37 38 39 External index (%) Fig. 1. Correlation between the thickness of superficial enamel layer (mm) and external index (%).

28 Series1 27.5

27

26.5

26

25.5

25

ess of internal enamel24.5 layer

Thickn 24

23.5

23 30 31 32 33 34 35 36 37 38 39 External index (%) Fig. 2. Correlation between the internal enamel layer (mm) and external index (%). from 27.4 to 23.4 mm, the mean value be- dentin thickness in the lower incisors ing 25.16 mm. This Figure shows that ex- ranged from 1.540 to 1.700 mm, the mean ternal index of 29.47% is conditioned by value being 1.635. The dentin thickness the thickness of inner enamel layer of is in the vestibular part of the tooth cham- 27.4 mm. ber in the upper teeth ranged from 0.490 to 0.723, the mean value being 0.620 mm The results of dentin thickness mea- and from 0.530 to 0.770 mm in the lower surements are as follows. The dentin teeth, the mean value being 0.663 mm. In thickness in the incisal third of the upper the oral part of the tooth chamber the incisors ranged from 1.545 to 1.750 mm, dentin thickness ranged from 0.525 to the mean value being 1.665 mm. The 0.765 mm in the upper incisors, the mean

376 D. Konjevi} et al.: Incisors in Humans and Fat Dormice, Coll. Antropol. 27 (2003) 1: 373–380 value being 0.654 mm, while it ranged vidual categories of the teeth. The thick- from 0.560 to 0.790 mm in the lower inci- ness of enamel in human incisors varies sors, the mean value being 0.666 mm. from 2 to 2.5 mm12,13. In humans, the Figure 3 shows the correlation be- enamel is the thickest in molar tubercles, tween dentin thickness in the upper and about 2.6 mm on the average, followed by lower teeth in the vestibular part. Figure the premolar tubercles, about 2.3 mm on 4 shows the same correlation for the oral the average, while in the incisal edges of part of the teeth. This Figure shows that the upper incisors the average thickness for the mean number of samples thick- of the enamel is 2.0 mm15–20. ness of dentin in the oral part of both up- Mishima and Kozawa8 compared mor- per and lower incisors is the same from phology of calcospherites between contin- the values of 0.650–0.700 mm. The bigger uously growing teeth and uncontinously values in the upper incisors are in corre- growing teeth. While Carlos et al.21 used lation with minor values in the lower in- ground sections to perform comparative cisors and the opposite. analysis of the structure of the denti- noenamel junction (DEJ) among diverse mammalian species. Discussion The measurement results concerning Our studies about physical character- the enamel thickness in dormice show istics of enamel and dentin comprised in that it is slightly thicker in the upper the first place the measurements of ena- than in the lower incisors (40.8 mm vs. mel layer thickness in individual seg- 55.5 mm), which is not quite in accordance ments of incisors, calculation of external with the data found in the available refer- enamel index, and measurements of den- ence literature1–7. However, it is in con- tin thickness in different tooth segments. formity with their functional adjustment. The thickness of enamel in dormice The fact is that the animal gnaws by and also in humans differs with regard to lower incisors so that their enamel layer individual segments of the tooth crown, is undergoing constant attrition and re- and it is also variable with regard to indi- newal, whereas the upper incisors serve

16 Series1

15

14

13

12

11

The thickness of superficial enamel layer

10 30 31 32 33 34 35 36 37 38 39 External index (%) Fig. 3. Correlation between dentin thickness in the upper and lower incisors vestibulary (mm).

377 D. Konjevi} et al.: Incisors in Humans and Fat Dormice, Coll. Antropol. 27 (2003) 1: 373–380

28 Series1 27.5

27

26.5

26

25.5

25

ess of internal enamel24.5 layer

Thickn 24

23.5

23 30 31 32 33 34 35 36 37 38 39 External index (%) Fig. 4. Correlation between dentin thickness in the upper and lower incisors orally (mm). only to hold the bite. The layer of enamel All these characteristics have their ef- in the upper incisors is the thickest in the fect on the structure of teeth. Based on medial portion of dental crown and is the morphology of the jaw and its mus- more or less equally thick in the cervical cles, especially the masseter muscle, and part and the incisal edge (40.8–37.0 mm the structure of enamel, rodents are usu- vs. 40.7–35.0 mm). In lower incisors the ally divided into three sub-groups. The thickest layer of enamel is found in the subgroup of Myomorpha includes the dor- cervical portion (55.5–41.4 mm), followed mice, i.e. the Gliridae. The enamel in dor- by incisal edge region (52.5–41.0 mm), mice surrounds the external anterior sur- and then in the medial part of the dental face of the incisors. The remaining hard crown (51.8–41.0 mm). It is commonly tissue is made of dentin. Abundant infor- considered to be the result of the gnawing mation about the characteristics of hu- function, since constant attrition leads to man teeth indicates the importance of the greater wearing out of the enamel in the issue from the aspect of basic sciences, incisal edge and medial third than in the but also from the point of view of each in- cervical portion. The outer enamel layer dividual dental medical specialty17,23–31. is less thick than the internal one, which The data about structural properties of is caused by structural specifities of DEJ teeth in rodents, dormice in particular, in dormice. In our study samples the are rather scarce and hence the incentive thickness of dentin in vestibular side of to undertake our study11,15,16,18–20. the upper incisors was smaller than in the lower ones (0.490–0.723 mm vs. 0.530 Our studies have also shown that his- –0.770 mm). Also, the thickness of dentin tological structure of human teeth when in the oral side was greater in lower than compared with that in dormice is not sig- in the upper teeth (0.525– 0.765 mm vs. nificantly different, except in the follow- 0.560–0.790 mm). These findings are not ing detail: unlike the Lagomorpha, which, in accordance with those found in rele- similar to humans, have multiserial struc- vant reference literature, but this may be ture of the enamel, the Sciuromorpha attributed to special function of incisors and the Myomorpha (dormice), which are in dormice2,3,6,22. of specific interest to us, have uniserial

378 D. Konjevi} et al.: Incisors in Humans and Fat Dormice, Coll. Antropol. 27 (2003) 1: 373–380 structure of enamel with parazones and of protein is usually inhibited by the ac- diazones in the thickness of only one ena- tion of forces during dimensional changes mel prism. The prisms in these zones occurring in the course of detachment of overlap and inter-cross with the prisms of single enamel segments, preventing thus the adjacent zones under the angle of resorption of organic matrix, which is nearly 90 degrees. In horizontal sections therefore permanently retained in this the prisms are obliquely intersected so part of the enamel. Hence the junction of that from the surface of the cross-section enamel and dentin in dormice, for greater they turn toward the left and toward the mineralization of DEJ, might be stronger right. For reasons of uniserial enamel and provide better protection of dentin structure in dormouse, the diazones and during constant attrition of the enamel in the parazones are less visible than in the course of gnawing15,18–20,23. man. Another particularity of the enamel It should also be pointed out that the in dormouse is in smaller number of studies undertaken herewith should be enamel tufts in dentino-enamel junction. supplemented by the studies of dental The enamel tufts are regions in the ena- pulp, cementum and parodontal ligaments mel with a lesser degree of mineraliza- in the dormice in order to obtain complete tion resulting from the disorders in pro- insight into the relationship between their tein removal in certain parts of the structure and function when compared enamel during maturation. The removal with human teeth.

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379 D. Konjevi} et al.: Incisors in Humans and Fat Dormice, Coll. Antropol. 27 (2003) 1: 373–380

D. Konjevi}

Chair for Game Biology, Pathology and Breeding, Veterinary Faculty, University of Zagreb, Heinzelova 55, 10000 Zagreb, Croatia

KOMPARATIVNA MORFOLOGIJA CAKLINE I DENTINA SJEKUTI]A U LJUDI I SIVIH PUHOVA (GLIS GLIS)

SA@ETAK

Gra|a zuba je u svih `ivih bi}a genetski predodre|ena, premda se mo`e mijenjati utjecajem vanjskih fiziolo{kih i patolo{kih ~imbenika. Ovaj rad je imao za cilj uputiti na razvojne promjene kao rezultat genskih, funkcionalnih i drugih razlika. Stoga su poredbene studije zna~ajne jer upu}uju na evolucijske pomake proistekle iz genskih, funkcijskih i inih raznolikosti. Poredbeno istra`ivanje nekih osobitosti sjekuti}a u ljudi i myomorpha, kojih je predstavnik i sivi puh (Myoxus glis), obuhvatilo je mjerenje de- bljine cakline i dentina u pojedinim dijelovima sjekuti}a te izra~unavanje vanjskog ca- klinskog indeksa, ali i prou~avanje histolo{ke gra|e dentina i cakline. Rezultati istra- `ivanja u puhova su pokazali da je caklina deblja u donjih nego gornjih zuba, do~im je u ~ovjeka obratno. U gornjih sjekuti}a puhova caklina je najdeblja u srednjem sloju kru- ne, a u donjih u vratnom dijelu krune. Debljina dentina u puhova je ve}a na oralnoj nego na vestibularnoj strani. Svi se nalazi bitno razlikuju od onih iz literature, no ima- ju upori{te u funkciji zuba u puhova. Histolo{ke osobitosti tvrdih zubnih tkiva puhova sli~ne su onima u ~ovjeka s izuzetkom uniserijalne gra|e cakline i izgleda dentinsko -caklinske granice.

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