Enamel microstructure and molar wear in the greater galago Otolemur crassicaudatus (mammalia primates).код для вставкиСкачать
AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 92~217-233(1993) Enamel Microstructure and Molar Wear in the Greater Galago, Otolemur crassicaudatus (Mammalia, Primates) MARY c. MAAS Department of Biological Anthropology and Anatomy, Duke University Medical Center, Durham, North Carolina 2771 0 KEY WORDS Dental function, Tooth wear, Enamel function ABSTRACT This study describes the molar enamel microstructure of the greater galago, based on SEM study of four individuals. Galago molar enamel consists primarily of radially oriented Pattern 1 prisms. However, the most superficial enamel is characterized by regions of poorly developed prisms or nonprismatic enamel, and Pattern 3 prisms can be found at depths intermediate and deep to the enamel surface. Orientations of prism long axes relative to wear surfaces differ among functionally distinct regions (cuspal facets, Phase UII facets, and crushing basins). Consequently, orientations of enamel crystallites relative to these surfaces also differ. Because crystallites are the structural unit involved in enamel abrasion, these differences in orientation may have important effects on molar wear patterns, Crystallite orientations differ most between cuspal facets and Phase ID1 facet surfaces. Cuspal facets are characterized by near surface-parallel interprismatic and surface-oblique prismatic crystallites. Previous experimental studies suggest that this arrangement is most resistant to wear when surface-normal (compressive)loads predominate. In contrast, prismatic and interprismatic crystallites intercept Phase UII facet surfaces obliquely, an arrangement expected to resist abrasion when surface-parallel (shearing) loads predominate. Superficial enamel is preserved at most basin surfaces, indicating that these regions are subject to comparatively little abrasive wear. These results support the hypothesis that galago occlusal enamel is organized so as to resist abrasion of different functional regions, a property that may prove important in maintaining functional efficiency. However, this largely reflects constraints of occlusal topography on a microstructure typical of many mammals and thus does not appear to represent a structural innovation. 0 1993 Wiley-Liss, Inc. Analysis of dental functional morphology has contributed significantly to our understanding of the adaptive radiations of primates, A critical aspect of primate dental function is the well-established relationship between molar occlusal morphology and diet (e.g.,Butler, 1973; Kay, 1975,1977; Kay and Hiiemae, 1974a,b; Kay et al., 1978; Maier, 1980, 1984; Seligsohn, 1977; Seligsohn and Szalay, 1974,1978).However, teeth are subject to wear, which may in turn affect their function. In some cases tooth wear serves to maintain the original occlusal fordfunction complex; in other cases occlusal form 0 1993 WILEY-LISS, INC. changes with wear, but occlusal effectiveness is enhanced (Every, 1974; Lanyon and Sanson, 1986; Rosenberger and Kinzey, 1976; Seligsohn and Szalay, 1974, 1978). Enamel structure influences the wear properties of teeth (e.g., Boyde, 1984a,b)and studies of a variety of mammals have demonstrated that the relationship between structure and wear can be important in maintaining dental functional efficiency (e.g., Boyde and Fortelius, 1986; Ferreira et Received June 29,1992; accepted April 28, 1993 218 M.C. MAAS al., 1989; Rensberger and Koenigswald, 1980; Stern et al., 1989; Walker, 1984; Young et al., 1987). This study of enamel microstructure and dental wear in the largeeared greater galago, Otolemur crassicaudatus’, represents a first look at the relationship between enamel structure and molar function in the primates. It is also the first such study in an animal that possesses a modified tribosphenic dentition but which lacks highly derived specializations of tooth form or enamel structure, which have been the focus of previous studies. The primary objective is to document the microstructural organization of galago molar enamel, particularly its arrangement at occlusal surfaces of functionally distinct regions. MOLAR MORPHOLOGY AND FUNCTION The diet of Otolemur crassicaudatus (the largest of the galagos) consists primarily of gums, insects, and fruit; the relative proportions of these foods may vary according to season and locality (Crompton, 1984; Nash, 1986). Previous studies report that 0. crassicaudatus molars emphasize crushing and grinding features and de-emphasize features associated with shear (Kay and Hiiemae, 1974b; Maier, 1980). For example, even relative to other galagos, cusp acuity is reduced, crest edges are rounded, and basins are broad and shallow (Seligsohn, 1977). Likewise, development and orientation of molar wear facets reflect this relative emphasis on compression and grinding and deemphasis of vertical shear during mastication. In mammals with primitive tribosphenic molars (e.g., Didelphis) close contact between teeth during the chewing cycle occurs in the single-phase power stroke, which terminates as teeth reach centric occlusion. In galagos and other primates the power stroke incorporates a second phase, during which close contact between teeth continues as up- ‘The most recent revision of galago systematics recognizes the genus Otolemur, including 0. crassicaudatus (the large-eared greater galago) and 0.gurnetti (the small-eared greater galago) (Olson, 1979, 1986).Olson’s classification is followed here. However, as noted by Nash et al. (1989), many workers prefer to recognize the overall similarity of the group by placing all species in a single genus, Galago. per and lower molars move out of centric occlusion (Phase I1 of the power stroke). Thus, compared to those on tribosphenic molars, Phase I facets on Otolemur molars are oriented differently or are reduced, and Phase I1 facets are well developed (Kay and Hiiemae, 1974b). STRUCTURALPROPERTIESOF TOOTH ENAMEL Tooth enamel is compositionally and structurally heterogeneous. Its composition, by weight 96% hydroxyapatite crystallites, 1%protein, and 3% water (e.g., Ten Cate, 1980), ensures that although enamel is the hardest tissue in the body it also is tough and resistant to brittle fracture. However, enamel’s anisotropic structural organization is the principal determinant of its wear properties and its response to occlusal force (Yettram et al., 1976). Enamel structure can be viewed as a hierarchy of increasingly complex levels: crystallites, prisms, enamel types, and “schmelzmuster,”the overall organization of enamel in a tooth (Koenigswald and Clemens, 1992). Hydroxyapatite crystallites are the smallest structural units. Groups of similarly oriented crystallites in turn form prisms and interprismatic enamel. Prisms are approximately 3 to 5 km in cross-sectional diameter and are separated from one another by regions of interprismatic enamel (Fig. 1A,B). The cross-sectional shape and packing arrangement of prisms, the completeness of prism boundaries, and the orientations of prismatic and interprismatic crystallites in mature (fully mineralized) enamel reflect the morphology of the developing enamel surface, and are the basis of the standard classification scheme for prisms (Patterns 1 , 2 , and 3 of Boyde [1964, 1976bl). The boundaries between prisms and interprismatic enamel are defined by abrupt changes in crystallite orientation, but crystallites within each are roughly parallel to one another. It is important to understand that prism cross-sectional size and shape, as well as the orientation of crystallites relative to a wear surface are in part dependent on the plane at which the surface intercepts prism long axes. Thus, a wear surface oriented perpendicular to prism long GALAGO ENAMEL MICROSTRUCTURE 219 Fig. 1. Block diagrams of enamel types found in Otolernur crassicaudatus molars. a: Prismatic enamel with complete prism boundaries (cf. Pattern 1). The anterior face of the block is sectioned perpendicular to prism long axes.The three sides of a triangle (x,y, and d) join the centers of three contiguous prisms and are used to calculate prism cross-sectional shape parameters for enamel Patterns 1 and 3 (see text for explanation of prism parameters). The hexagon demarcates the area (one prism and one half of the surrounding interprismatic enamel) included in the measurement of amelo- blast secretory area (ASA). Prism area (PA) is the measured area of the prism within the hexagon. b: Prismatic enamel with cervically incomplete prism boundaries (cf. Pattern 3). The anterior face of the block is sectioned perpendicular to prism long axes. c: Prismatic enamel with complete prism boundaries (cf. Pattern 1). The anterior face of the block is sectioned in plane approximately 60" to prism long axes. d: Nonprismatic enamel. In all cases, orientations of hydroxyapatite crystallites are based on SEM micrographs of mature galago enamel (see text). axes may intercept both prismatic and interprismatic crystallites slightly oblique to their long axes (Fig. lA), whereas a surface oriented at an angle oblique to prism long axes might intercept prismatic crystallites perpendicular to their long axes and interprismatic crystallites more nearly parallel to their long axes (Fig. 1C). The next hierarchical level recognized in enamel structure is the enamel type, which is defined by the arrangement and orientation of prisms and interprismatic enamel. Enamel types typical of primate teeth include radial, decussating, and nonprismatic enamel. In radial enamel, prisms are parallel to one another in their course from the enamel-dentine junction (EDJ) to the tooth surface and typically are inclined slightly cuspally. In decussating enamel, prisms form groups with alternating orientations. 220 M.C. MAAS Abrasive particles trapped between occlusal surfaces may produce microscopic wear features on these facets. The primary force vectors during chewing are directed parallel to the surface and encompass both shearing and grinding, although both include a surface-normal force component (Kay and Hiiemae, 1974b).At the termination of Phase I of the power stroke, cusp tips interlock with crushing basins (centric occlusion) and force is directed normal to the surface. Striations on Phase I and I1 facets are probably formed by surface-parallel forces as teeth move into and out of centric occlusion, but the presence of pits on Phase I1 facets may be due to abrasion by compressive force at the end of Phase I (Hylander et al., 1987). Abrasive tooth wear during mastication occurs through three general mechanisms, each of which is generated by a combination TOOTH WEAR AND of surface-parallel and surface-normal comENAMEL STRUCTURE ponents of load, but with one or the other Tooth wear occurs when enamel is frac- generally dominating (Vingsbo, 1988). The tured or abraded by occlusal forces, typically three mechanisms are 1) microcutting by forces generated during mastication. Masti- abrasive particles, where the predominant cation begins with the initial stage of food component of force is parallel to the surface breakdown (puncture-crushing) and pro- (i.e., shearing and grinding); 2 ) adhesion, ceeds through a later stage (chewing),when which involves transfer of enamel between food breakdown is sufficient to allow teeth to tooth surfaces after shear deformation and come into close contact. Puncture-crushing fracture of one surface, and where, as in miand chewing are part of a continuum of pro- crocutting, the surface-parallel component gressive food breakdown and therefore will of force predominates; and 3) impact fracvary in proportion and extent depending on ture, which is associated with predominance the form of teeth and the physical properties of the surface-normal component of force of food. However, though somewhat artifi- (compression)(Roulet, 1987;Vingsbo, 1988). cial, the categorization of mastication into It has been shown experimentally that force two processes does reflect potentially impor- directed normal to the surface (compression) tant difference in the mechanics of mastica- produces short, wide microwear features tion and jaw movement (Hiiemae and (pits), whereas force directed parallel to the surface (shear) produces elongate miCrompton, 1985). During puncture-crushing the primary di- crowear features (striations) (Maas, 1988, rection of force is compressive, and wear oc- 1991). However, Teaford and Runestad curs predominantly at cusp tips (Hiiemae (1992) recently suggested that adhesion (a and Crompton, 1985). In the galago, punc- wear regime in which surface-parallel force ture-crushing results in wear that charac- components predominate) may account for teristically blunts cusp tips and tops of the formation of very small pits (< 4 p.m in crests (Kay and Hiiemae, 1974b). During diameter) on tooth surfaces. Abrasion typically appears to remove chewing (the later “stage”of the masticatory cycle), teeth come into closer contact, move- enamel in units smaller than entire prisms, ment is largely controlled by tooth shape, at the level of the crystallite or groups of crystallites (Boyde and and planar wear facets (Phase I and Phase subparallel I1 facets) are formed by precise positioning Fortelius, 1986; Maas, 19911,but high-magof teeth (Hiiemae and Crompton, 1985). nitude forces can result in larger-scale frac- The long axes of prisms in one group are roughly parallel to one another but at an angle to the prisms in the adjacent group. In nonprismatic enamel (Fig. 1D) there are no abrupt changes in crystallite orientation and thus no prism boundaries and no differentiation between prismatic and interprismatic enamel (e.g., Carlson, 1990). Crystallite orientations, prism patterns, and enamel types vary from region to region within individual teeth and according to the depth of enamel from the tooth surface (e.g., Carlson and Krause, 1985; Grine et al., 1987;Maas, in press). The description of the three-dimensional variation of enamel types throughout a tooth comprises the fourth level in the hierarchical scheme of enamel structure, the schmelzmuster (Koenigswald and Clemens, 1992). GALAGO ENAMEL MICROSTRUCTURE ture and loss of enamel, in units of prisms or groups of prisms. This is because cracks generated by chewing stress propagate preferentially along prism boundaries, where crystallites are less densely packed (Hassan et al., 1981; Rasmussen et al., 1976). The likely structural constraint on wear from both abrasion and large-scale fracture is the orientation of structural units, either individual crystallites or prisms (Boyde and Fortelius, 1986; Maas, 1991; Powers et al., 1973; Rasmussen et al., 1976). Enamel is least resistant to surface-parallel force when crystallites or prisms are oriented with long axes parallel to the wear surface (Boyde, 1984a,b; Rensberger and Koenigswald, 1980; Walker, 1984) and most resistant when crystallites are oriented perpendicular to the wear surface. I n contrast, enamel should be least resistant to compressive force when prism or crystallite long axes are normal to the surface and most resistant when their long axes are oriented parallel to the surface. At a large scale this is because resultant tensile forces act in a direction perpendicular to prism long axes and tend to pull prisms apart (Pfretzschner, 1986). Since work of fracture is increased by change in direction of prism boundary planes (Boyde, 1976a; Rasmussen et al., 19761, tensile cracks are resisted by change in prism orientation, as in prism decussation (Pfretzschner, 1986). Similarly, change in crystallite orientation, as between prisms and interprismatic enamel should resist tensile cracks at a smaller scale. MATERIALS AND METHODS Molar teeth were extracted from four adult Otolemur crassicaudatus cadavers (one 4-year-old, one 6-year-old, and two 9-year-old individuals) obtained from the Duke University Primate Center. All four were captive animals whose diet consisted of fresh fruit, crickets, and monkey chow. The molars were removed from frozen specimens and cleaned. The teeth were prepared for documentation of four aspects of dental structure: 1) cuspal microwear and age-related differences in occlusal morphology; 2) distribution of enamel types within teeth (schmelzmuster); 3) depth-related variation in prism cross-sectional morphol- 22 1 ogy; and 4) structure and organization of enamel in functionally distinct regions (cuspal facets, Phase I/II facets, and crushing basins). For study of molar wear and occlusal morphology, occlusal views of upper and lower molars were traced using a camera lucida and total occlusal area and area of exposed dentine were measured and recorded with a computerized digitizing program (Sigmascan, Jandel Scientific). Dental impressions (President J e t Regular Body dental impression material, Coltene) were made of first and second molars and two sets of epoxy resin (Araldite GY 502 and HY 956, CIBAGEIGY) casts were produced. One set of casts was sectioned buccolingually through the paracone and protocone or the hypoconid and entoconid and used to assess differences in occlusal shape. The plane of section was defined as the plane through the centers of the two cusps (or the centers of the dentine islands a t cusp tips) and perpendicular to a plane passing through the average rootcrown junction around the circumference of the tooth (the cervical plane) (Fig. 2A: C-C; Fig. 2B: D-D). The four measures of occlusal shape recorded from sectioned casts included 1)the angle between the buccal slope of the protocone or entoconid and the cervical plane, 2) the angle between the lingual slope of the paracone or hypoconid and the cervical plane, 3) the height of the paracone or hypoconid relative to the cervical plane, and 4) the height of protocone or entoconid relative to the cervical plane (Fig. 3). The second set of casts was used in a survey of cuspal facet microwear. These casts were not sectioned but were sputter-coated with gold palladium and examined at low power (200-500x1 in a scanning electron microscope (SEMI. For study of overall enamel organization (schmelzmuster) two pairs (right and left) of upper and lower second molars from two individuals were embedded in epoxy resin (Epo-Tek, Epoxy Technology, Inc.) and sectioned. Each embedded tooth was sectioned twice, in planes through centers of two cusps and perpendicular to the cervical plane. The right upper and lower molars were sectioned mesiodistally through the buccal cusps and through the lingual cusps (Fig. 2, A-A and M.C. MAAS 222 Mesial A A Mesial CLingual D Buccal D- A a Fig. 2. Occlusal view of upper (a) and lower (h) second molars illustrating orientations of longitudinal planes of section. Mesiodistal section A-A passes through buccal cusps (paracone-metacone or protoconidhypoconid) and mesiodistal section B-B passes through lingual cusps (protocone-hypocone or metaconid-ento- P a r a c o n e h Fig. 3. Camera lucida sketch of upper (a) and lower (b) molars sectioned through plane D-D (Buccolingual section through distal cusps). The heavy black line indicates the horizontal cervical plane (root-crown junction). Numbers indicate four measurements of occlusal shape. 1: Angle between the buccal slope of the protocone or entoconid and the cervical plane. 2: Angle between the lingual slope of the paracone or hypoconid and the cervical plane. 3: Height of the paracone or hypoconid relative to the cervical plane. 4: Height of protocone or entoconid relative to the cervical plane. B-B) and the left upper and lower molars were sectioned buccolingually through the mesial cusps and through the distal cusps Distal B b conid). Buccolingual section C-C passes through mesial cusps (paracone-protocone or protoconid-metaconid) and buccolingual section D-D passes through distal cusps (metacone-hypoconeor hypoconid-entoconid). All section planes are oriented perpendicular to the horizontal cervical plane (root-crownjunction). (Fig. 2, C-C and D-D). Sectioned specimens were mounted on SEM stubs and successively polished with 800 and 1200 grit silicon carbide abrasive paper and 0.05 p.m alumina powder (Al,03). To assess depth-related variation in prism structure, prism cross-sectional morphology was documented from surface-tangentially ground sections of right lower first molars of two individuals. Each molar was sectioned longitudinally through the centers of the hypoconid and entoconid, in a plane perpendicular to the cervical plane, and the total radial thickness of the enamel was measured at the longitudinally sectioned face along a line perpendicular to the enamel-dentine junction. This represents the total thickness of enamel and was used to calculate the relative depth from the tooth surface for each tangentially ground section. The tangentially ground sections were prepared and studied at three different depths relative to the tooth surface: each specimen was first ground to a depth of less than 113 of the total thickness (superficial enamel) and documented, then reground to a depth of V 2 the original radial thickness (intermediate enamel) and documented, and finally ground 223 GALAGO ENAMEL MICROSTRUCTURE and documented at a depth greater than 2/3 of the original radial thickness (deep enamel). Whole upper and lower second molars from two individuals were selected for study of enamel structure at occlusal surfaces of three functionally distinct regions: cuspal facets, Phase I/II facets, and crushing basins. These specimens were cleaned and mounted directly on aluminum SEM stubs. The teeth were sketched using a camera lucida to record positions of wear facets, in order to facilitate location of wear surfaces in the SEM. All enamel specimens (polished sectioned molars, surface-tangentially ground molars, and whole molars) were mounted on SEM stubs, etched with a solution of 5% HC1 for up to 10 seconds, rinsed, and air-dried. The specimens were sputter-coated with gold palladium and examined in a JEOL 35C SEM at 10 kV in secondary electron mode. Sectioned molars were photographed at low magnification (200-300 X ) to document enamel types and schmelzmuster and photographed at higher magnification (2,000x ) to record crystallite orientation. Surfacetangentially ground molars and occlusal surfaces of whole molars were photographed . position of SEM photographs at 2 , 0 0 0 ~The were recorded on camera lucida drawings of specimens made prior to etching. As described above, cuspal facets and Phase I and Phase I1 facets were mapped on the drawings of whole teeth, but in most cases the boundaries of wear facets were clearly identifiable on the etched surfaces. Five parameters were calculated to quantify differences in prism cross-sectional arrangement at different depths relative to the enamel surface and in different functional regions: 1)CD (distance between tenters of adjacent prisms); 2) K (apicocervical compression or distention of prisms); 3) PA (prism area, the area contained within a prism boundary); 4) ASA (secretory area of one ameloblast, or enamel-secreting cell); and 5) PNASA (a measure of the relative amount of prismatic to interprismatic enamel). CD and K are calculated from x, y, and d, the sides of a triangle formed between the centers of three contiguous prisms (Fig. lA), using the equations for Pattern 1 and Pattern 3 prisms (Fosse, 1968a,b) modified by Grine et a]. (1987): CD K= 2dhd =- v3’ 112d fi 9 hd where h, = Vl/2 4d2f - (d2 + y2 - x2)’ d For each parameter average values were computed for samples of ten prisms from each specimen, calculated from x, y, and d values for ten triangles. PA is calculated as the average area for the ten prisms located at the occlusal (crown) apex of each of the ten triangles. ASA is the average of ameloblast secretory areas for the same ten prisms and is measured as the area within a hexagon drawn around one prism and half of the interprismatic enamel surrounding the prism (see Fig. 1A). Both ASA and PA were measured directly from micrographs of prism cross-sections, using the digitizing program Sigmascan. The parameters CD, K, ASA, PA, and PA/ ASA were determined for the superficial, intermediate, and deep enamel of surface-tangential sections, and for occlusal enamel of etched whole molars at cuspal facets, Phase I/II facets, and crushing basins. The prism shape parameters for surface-tangential sections are assumed to represent a crosssectional plane oriented perpendicular to prism long axes and therefore are expected to reflect actual differences in prism shape, given a constant plane of section. In contrast, the cross-sectional parameters of functionally distinct occlusal surfaces will reflect differences in prism shape due to differences in the angle at which prism long axes intercept the surface, and thus differences in the plane of “section”(see Fig. lA,C). RESULTS Tooth wear The primary differences in molar morphology among the galagos examined in this study pertain to reduction of cusp and crest M.C. MAAS 224 TABLE 1. Measures of tooth wear and occlusal shape for Otolemur crassicaudatus molars Upper molars Age Tooth - (year) Protocone angle' Paracone ande2 Cusp height PAPR (mm)3,4 Dentine' 128" 138" 130" 138" 125" 122" 2.011.7 1.911.7 2 ,011.7 1.911.8 2.311.3 2.111.5 2.7% 7.9% 3.5% 4.0% 5.0% 4.6% Hypoconid angle' 140" 157" 144" 143" 131" 134" 129" Cusp height HYD/ENTD (mm)3,4 2.111.9 2.011.7 2.912.7 2.312.3 2.111.8 2.011.6 1.811.5 Dentines 1.4% 2.070 1.0% 1.3% 4.1% 2.3% 7.0% LM1 LM2 LM1 LM2 LM1 4 4 6 6 9 I.M2 9 130" 133" 129" 143" 134" 133" Age (year) 4 4 6 6 9 9 9 Entoconid angle' 145" 129" 135" 147" 139" 151" 149" Lower molars Tooth LM1 LM2 LM1 LM2 LMl LM2 LM2 'Angle of huccal slope of protocone or entoconid and the cervical plane 'Angle of lingual slope of paracone or hypoconid and the cervical plane. 3Height of paracone (PA) or hypoconid (HYD)measured from cervical plane to cusp tip 4Height of protocone (PR) or entoconid (ENTD) measured from cervical plane to cusp tip. '(Total area exposed dentineitotal tooth area) x 100. lar sections, is simple, consisting primarily of radial enamel and, close to the outer surface, a much smaller proportion of nonprismatic enamel (Figs. 4,5). There is no indication of prism decussation; prism long axes are parallel to one another and run outward from the EDJ with a slight cuspal inclination. The superficial layer of nonprismatic enamel is thickest towards the root-crown junction. Incremental lines are most prominent in these regions (Figs. 4A, 5A). The nonprismatic layer is thinner near cusp tips and in crushing basins. In these areas the superficial enamel also is characterized by regions of poorly developed prisms. Examination of enamel in sections ground tangential to the tooth surface shows that, regardless of depth from the surface, prisms are packed in a hexagonal arrangement of horizontally offset rows (Fig. 6). Prisms in superficial enamel always are demarcated from interprismatic enamel by complete, continuous boundaries (cf. Pattern l),but at intermediate and deep levels boundaries of some prisms appear incomplete cervically (cf. Pattern 3) (Fig. 6B,C). At the incomplete cervical boundaries, the change in orientation between prismatic and interprismatic Enamel structure crystallites is gradual (see Fig. 1B). In the Galago molar schmelzmuster, as deter- two individuals examined here, prisms with mined from SEM study of longitudinal mo- complete boundaries (cf. Pattern 1) appear acuity in older animals, a pattern reported in previous studies of wear-related variation in galago molar morphology (Seligsohn, 1977). Surprisingly, neither cusp height nor percent exposed dentine show a clear association with age of individuals (Table 1).In the small sample examined here (four individuals), these measures differ most between lower molars of the two 9 year olds, but in general, all four individuals have similar molar morphologies. The differences that do occur appear to reflect individual morphological variation rather than age-related wear. The amount of exposed dentine represents less than 10% of the entire occlusal area, even for the oldest individual (Table 1). The dentine is exposed primarily a t cusp tips. Worn cusps are characterized by facets consisting of flat ridges of enamel surrounding small islands of exposed dentine. These cuspal facets are oriented horizontally (parallel to the cervical plane of the molar). Microscopic wear features on the enamel ridges of cuspal facets consist of both scratches and pits, but pits, defined by a length to width ratio of less than 4:l (e.g., Teaford, 1988), are the predominant microwear features. GALAGO ENAMEL MICROSTRUCTURE Fig. 4. Photographic montage of Otolernur crassicaudatus RM2 illustrating overall enamel organization. The molar is sectioned buccolingually through the paracone (left) and protocone (right). Insets A-D illustrate the relative orientation of prismatic. and interprismatic crystallites in different regions. A: Buccal enamel a t 225 transition between prismatic and nonprismatic enamel. B: Paracone cuspal facet. C :Talon basin. D: Buccal slope of protocone (wear facet 9). Note differences in prism orientation, relative to wear surfaces at cuspal facet (B) and wear facet 9 (D). Scale bars, 10 pm. Fig. 5. Photographic montage of LM, illustrating overall enamel organization. The molar is sectioned buccolingually through the entoconid (left) and hypoconid (right). Insets A-D illustrate the relative orientation of prismatic and interprismatic crystallites. A. Nonpris- matic lingual enamel. B: Entoconid cuspal facet. C: Talonid basin. D: Lingual slope of hypoconid (Phase I1 wear facet 9). Note differences in orientation of prisms relative to the wear surface a t cuspal facet (B) and Phase I1 facet (D). Scale bars, 10 pm. to predominate, as has been reported previously for galago enamel (Martin et al., 1988; Shellis and Poole, 1977). However, the occasional occurrence of prisms with cervically incomplete boundaries clarifies the apparently contradictory report that galago, along with other prosimians, has a Pattern 3 prism arrangement (Shellis, 1984). Interprismatic crystallites appear parallel to one another in etched specimens of mature galago enamel. This differs somewhat from the descriptions of interprismatic crystallite orientation for Pattern 1 enamel, based on developmental models (Boyde, 197613). The extent t o which this difference is an artifact of sectioning planes or represents a variation in enamel structure can only be addressed by studies of the developing enamel surface in galagos. In intermediate and deep enamel the an- 226 M.C. MAAS Intermediate Deep 10 12 14 18 16 20 ASA Intermediate Deep 3.8 3.4 4.2 4.6 CD - Superficial - H Intermediate Deep - I I , 3.0 , , 4.0 . . 5.0 . . I , 6.0 PA Superficial 0.6 0.8 1.0 1.2 1.4 K Intermediate Deep 20 30 40 50 PAiASA ( x 100 ) Fig. 6. Photomicrographs of etched enamel from the buccal surface of the M, hypoconid ground tangential to the surface at three depths relative to the EDJ. A Superficial enamel. B: Intermediate enamel. C: Deep enamel. Scale bar, 10 pm. gle between long axes of interprismatic and prismatic crystallites is typically large (at least 45").In superficial enamel, in contrast, the angle between prismatic and interprismatic crystallites generally is less than 45". The specimens examined here show almost complete overlap in most metrical parameters of prism cross-sectional morphology. However, although the small sample Fig. 7. Ranges of enamel prism parameters for superficial, intermediate, and deep enamel. Bars represent the range of values for twenty prisms ( N = 2 individuals). ASA: Ameloblast secretory area. CD: Central distance. PA Prism area. K: Prism compression/ distention. PNASA: ratio of prism area (PA) to ameloblast secretory area (ASA), a measure of the relative proportions of prismatic and interprismatic enamel. See text for explanation of parameters. sizes preclude any meaningful statistical assessment of depth-related variation, a few parameters do show some depth-related trends (Fig. 7). Horizontal distance between prism centers (CD) and ameloblast secretory GALAGO ENAMEL MICROSTRUCTURE 227 Fig. 8. Photomicrographs of Otolemur crassicaudatus enamel structure a t functionally distinct occlusal surfaces of LM'. A Phase I Facet 3. B: Phase I1 Facet 10. C: Protocone cuspal facet. D: Center of talon basin. Scale bars, 10 (rm. area (ASA) increase from the surface to the EDJ. Prism area (PA) is greatest for intermediate and deep enamel and least for superficial enamel, while the opposite maintains for apico-cervicalcompression (K). The ratio of prism area to ameloblast secretory area (PNASA) shows no depth-related pattern. In many derived enamels, such as those of some ungulates (Pfretzschner, 1991), depthrelated differences in prism morphology are an important aspect of enamel microstructure. In contrast, in galago the differences in prism morphology at different depths (with the exception of the poorly developed prisms occasionally found in superficial enamel) are relatively minor and the similarities are more striking than the differences. It should be emphasized, however, that intraspecific depth-related metrical differences do exist and these must be taken into account in metrical analyses of interspecific variation in enamel microstructure. Occlusal enamel structure The most pronounced differences in occlusal enamel structure among functional regions are found between cuspal facet enamel and Phase UII facet enamel. Prisms are oriented such that crystallites in both prismatic and interprismatic enamel intercept the occlusal surfaces of Phase I and Phase I1 facets at oblique angles (Figs. 4D, 5D, 8A,B, 9A,B). The arrangement at cusp tips is distinctly different: prism long axes are oriented such that their component crystallites are nearly perpendicular to the surface, but interprismatic crystallites lie more nearly parallel t o the surface (Figs. 4B, 5B, 8C, 9C). In both regions the angle between prismatic and interprismatic crystallites is 228 M.C. MAAS Fig. 9. Photomicrographs of Otolenur crussicaudutus enamel structure at functionally distinct occlusal surfaces of LM,. A: Phase I Facet 3. B: Phase I1 Facet 10. C: Hypoconid cuspal facet. D: Center of talonid basin. Scale bars, 10 )*m greater than 45", as is typical of intermediate and deep enamel. Crushing basins exhibit the least abrasive wear of all functional surfaces. Consequently, their occlusal surfaces typically show characteristics of superficial enamel, either nonprismatic enamel or enamel with poorly developed prism structure and little difference in orientation between prismatic and interprismatic crystallites (Figs. 4C, 5C, 8D, 9D). Prisms, if present, are oriented with long axes slightly oblique to the crushing basin surface. Rather than reflecting differences in the intrinsic structure of the enamel, the differences in the orientation of crystallites and prisms among surfaces of cuspal facets, Phase I/II facets, and crushing basins reflect differences in the orientation of the wear surfaces. At cuspal facets the wear plane is slightly oblique to prism long axes, while at Phase ID1 facets the wear plane is perpendicular to prism long axes. Thus, although the orientation of prisms relative to the EDJ generally is constant throughout the molar, the wear plane at cuspal facets will intercept prismatic crystallites nearly perpendicular to their long axes and interprismatic crystallites more nearly parallel to long axes (see Fig. 1C). Similarly, Phase I/II facet wear planes are nearly perpendicular to prism long axes and therefore will section both interprismatic and prismatic crystallites obliquely, even though the relative orientation of prismatic t o interprismatic crystallites is no different than at cuspal facets (see Fig. 1A). Thus the difference in crystallite orientation relative t o surfaces of cuspal facets and Phase I or Phase I1 facets is constrained by tooth form (i.e., the orientation of wear surfaces) and does not represent a fundamental difference in enamel organization between these two functional regions. There is no clear pattern to variation in GALAGO ENAMEL MICROSTRUCTURE TABLE 2. Descriptive parameters of prism morphology for functionally dLstinct regions of molar occlusal surfaces of Otolemur crassicaudatus (average values based on N = 2 individualsj PNASA Location LM2 Phase VII facets Facet 1 Facet 3 Facet 4 Facet 6 Facet 9 Facet 10 Cuspal facets Paracone Metacone Protocone Hypocone CD K (X100) 4.5 5.0 4.3 3.6 3.9 3.5 0.8 0.9 0.7 0.8 0.7 0.7 38.4 27.2 59.0 46.4 53.3 57.9 4.7 4.1 4.2 3.5 0.6 0.8 0.8 0.7 38.3 34.1 46.4 41.9 4.6 4.2 3.9 4.8 4.1 4.2 4.2 3.5 0.8 0.7 0.9 0.8 0.9 0.7 0.8 0.9 38.4 39.7 45.9 59.0 52.4 46.4 68.5 48.6 3.7 0.7 56.1 4.3 4.6 5.5 3.3 0.7 0.7 1.3 0.9 39.5 38.7 25.5 42.5 LM, Phase UII facets Facet 1 Facet 2 Facet 3 Facet 4 Facet 5 Facet 6 Facet 9 Facet 10 Crushing basins Talonid basin Cuspal facets Protoconid Metaconid Hypoconid Entoconid prism shape or packing parameters among functionally distinct regions of molar teeth (Table 2). Average values of parameters CD and K vary randomly with respect to functional regions. The ratio of prismatic to interprismatic enamel (PMASA), however, generally is lowest for surfaces of cuspal facets, which therefore include a greater proportion of interprismatic crystallites. Enamel at crushinglgrinding surfaces (including both Phase I1 facets and crushing basins) has the highest prismatic to interprismatic ratio. DISCUSSION There was little difference in molar morphology among the different aged individuals examined here, but whether this is typical of galagos remains to be tested in more extensive samples, and by longitudinal wear studies. Determination of the degree to which the relative wear of cuspal facets, Phase VII facets, and crushing basins is de- 229 pendent on diet (e.g., relative proportions of food requiring more or less puncture-crushing or the amount of abrasives in food) will be of particular importance in future analyses of galago tooth wear. Despite the limited sample size, the tooth wear pattern described here is in general agreement with previous reports of galago molar wear and suggests that galagos conserve molar form, and presumably function, with wear. The hypothesis suggested here that molar wear patterns in the greater galago are influenced by the differences in occlusal enamel among functional regions is predicated on several assumptions concerning the relationship of masticatory force vectors to different regions of the occlusal surface. Mechanical models of primate molar functional morphology indicate that 1)Phase I and Phase I1 facets are subject to surfaceparallel loads as opposing surfaces slide past one another, as well as surface-normal loads (e.g., Kay and Hiiemae, 1974a), 2) cuspal facets are subject to high compressive loads, primarily during puncture-crushing (e.g., Hiiemae and Crompton, 19851, and 3) crushing basins are subject to lower magnitude compressive loads and transitory surface-parallel loads as molars move into and out of centric occlusion during the power stroke of chewing (Kay and Hiiemae, 197413).It must be emphasized that this simple model of molar force vectors obscures some of the complexity of mastication. For example, Phase I and Phase I1 of the power stroke include components of force both normal to and parallel to the surface (Kay and Hiiemae, 1974b), but the relative proportions of these force components undoubtedly vary from chewing stroke to chewing stroke. Another complication for the chewing forceabrasion model is that compressive force generated during terminal Phase I, a s teeth come into centric occlusion, may be involved in abrasion of Phase I1 wear surfaces (Hylander et al., 1987). Thus this terminal Phase I pulping action may account for the development of Phase I1 facets (Fortelius, 1990). Clearly the relationships among occlusal forces, stress, and wear a t discrete regions of molar teeth are still very much a matter of hypothesis. Consequently, the relatively simple model of masticatory vectors 230 M.C. MAAS outline above is intended only to provide a reasonable starting point for analysis of the mechanical relationship between enamel structure and chewing. The distinctive arrangement of enamel a t cuspal facets, where crystallites approximating surface-parallel in orientation comprise a relatively large proportion of the occlusal surface, appears best-designed to resist abrasion by compressive loads. In contrast, the enamel organization a t Phase I/II facets and basins is such that crystallites intercept the surface obliquely. This arrangement is expected to be more resistant to abrasion by shearing force than if crystallite orientation were parallel to the tooth surface (and thus to the direction of force) (Boyde, 1984a,b; Boyde and Fortelius, 1986; Rensberger and Koenigswald, 1980). Crystallites also are oriented somewhat oblique to the surface of crushing basins. This orientation is expected to be particularly resistant to wear generated by compression, which, according to the assumptions of the chewing model, should occur at basin surfaces. The relative lack of wear at basin surfaces suggests that compressive loads in these regions may be too low to cause significant abrasion. In considering the functional role of enamel structure, it is important to point out that the Otolemur schmelzmuster (prismatic radial enamel and a nonprismatic surface layer of varying thickness) may be primitive for mammals with prismatic enamel (Koenigswald and Clemens, 1992). Because galago morphology is more derived than many of these primitive mammals, this suggests that body size, rather than molar form, is the critical factor constraining molar stress mechanics. It further implies that the mechanical stresses on galago teeth are not markedly different in degree from those of primitive mammals, despite the galago’s more derived molar morphology. It can be hypothesized that galago molars (like those of other small-bodied mammals) lack derived enamel characters because they are not subject to as high or concentrated occlusal forces as molars of larger-bodied herbivores, where stress-related fracture presents a significant biomechanical problem (Koenigswald et al., 1987). However, despite the likelihood that occlusal stresses will not differ appreciably between tribosphenic molars and the modified tribosphenic molars of galagos, the differences in molar form still may have important effects on abrasive wear patterns. It has been suggested here that differences in molar form imply differences in orientation of crystallites relative to wear surfaces, and thus differences in resistance to wear. The recent study of enamel structure and function in Didelphis by Stern et al. (1989) allows a comparison of the relationship of microstructure and wear between a n animal with primitive tribosphenic molars (Didelphis) and the galago. Cuspal facets in Didelphis, like Otolemur, are characterized by interprismatic enamel with crystallites oriented nearly parallel to the surface. However, the two animals show very different wear patterns. Cusps of Didelphis molars are much more extensively worn than those of galago molars. Unlike the blunted cusps and ridges of worn galago molars, worn Didelphis molar cusps and ridges form sharp cutting edges, a result of the rapid wear of apices of cusps and crests in conjunction with wear resistance of wear crest and cusp slopes (Stern e t al., 1989). The different wear patterns in these two animals are a general indication of differences in molar function. More specifically, they reflect differences in molar force vectors and the effects of those vectors on the underlying enamel structure. For example, wear at galago cuspal facets appears to result primarily from compressive force, as indicated by the predominance of microwear pits at these surfaces. In contrast, Stern et al. (1989) suggested that the much more extensive molar cusp wear in Didelphis is the result of abrasion by food particles driven parallel to the surface. It should be noted, however, that if adhesion between opposing surfaces accounts for formation of small microwear pits (Teaford and Runestad, 1992), then surface-parallel force also may be a factor in galago cuspal abrasion. Another factor contributing to the differences in wear patterns, and dental function between the two animals may be relative abrasion resistance of cuspal facets and Phase I facets. In Didelphis, Phase I GALAGO ENAMEL MICROSTRUCTURE 231 facets are thought to be more resistant to abrasion than cuspal facets, producing a sharp cutting edge. In Otolemur, because both surfaces appear to be similarly resistant t o abrasion, there is no sharp edge between the two functional surfaces. Thus, gross wear patterns, microwear, and microstructure all interact to contribute to the general differences in molar function reflected in the different molar forms of these two animals. sense. Nevertheless, the fortuitous combination of enamel structure and molar morphology results in enamel that appears to resist abrasion at different functional surfaces and therefore may contribute to molar functional efficiency through the conservation of occlusal morphology. An understanding of the functional role of such simple enamels represents a first step in the understanding of the evolutionary significance of derived enamels in primates. CONCLUSIONS The galagos examined in this study show little age-related variation in molar morphology. While recognizing the limits of sample size, and recognizing that wear rates will vary with physical properties of the diet, these results suggest that galago molars generally are resistant to abrasive wear. The combination of radial prisms and superficial nonprismatic enamel found in galago molars is typical of many small-bodied mammals and lacks features associated with fracture resistance (e.g., prism decussation) found in more derived enamels. There are pronounced, qualitative differences in enamel microstructure at occlusal surfaces of cuspal facets and Phase L'II facets. However, these differences do not reflect intrinsic ultrastructural differences. Instead, the differences at the surfaces of cuspal facets and Phase L'II facets reflect differences in the orientation of the respective wear surfaces relative t o prism, and thus crystallite, long axes. Crushing basins show less abrasive wear than either cuspal facets or Phase I/II facets. Consequently, superficial enamel typically is preserved at basin surfaces. The microstructural differences between basin occlusal surfaces and those of cuspal or Phase L'II facets therefore reflect differences in the amount of abrasion, and not regional microstructural characteristics. The galago schmelzmuster is typical of many primates (Martin et al., 1988) and is found in a number of primitive mammals (Koenigswald and Clemens, 1992; Koenigswald et al., 1987). Thus, although it may represent an important component of the galago molar structure/function complex, it cannot be considered adaptive in the strict ACKNOWLEDGMENTS I thank Professor W. von Koenigswald, Dr. H.-U. Pfretzschner, and Mr. M.R.L. Anthony for stimulating discussions of ideas presented in this paper, and Mr. J. Muennig for drafting and photography. I also thank two anonymous reviewers for their insightful comments. This research was supported by a Duke University Medical Center Small Research Grant, and BNS 9020788 from the National Science Foundation. 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