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Enamel microstructure and molar wear in the greater galago Otolemur crassicaudatus (mammalia primates).

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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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greater, mammalia, otolemur, wears, primate, molar, enamel, crassicaudata, microstructure, galago
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