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Intestinal dimensions of mice divergently selected for body weight

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THE ANATOMICAL RECORD 250:268–280 (1998)
Crown Morphology, Enamel Distribution, and Enamel Structure
in Mouse Molars
S. PETTER LYNGSTADAAS,1,2* CHRISTINA B. MØINICHEN,1 AND STEINAR RISNES1
1Department of Oral Biology, Faculty of Dentistry, University of Oslo, Oslo, Norway
2Department of Pathology, Faculty of Dentistry, University of Oslo, Oslo, Norway
ABSTRACT
Background: Biomolecular research and genetic manipulations have stressed the importance of thorough knowledge of normal
organ morphology. Mouse molar teeth are convenient models for studying
basic interactions in organ development and morphogenesis. The aim of
the present study was to provide basic information on their morphology.
Methods: Intact and sectioned/ground molars of mice of various ages
were observed with SEM.
Results: Enamel-free areas (EFA) were present on cusp tips at time of
eruption. The dominating structural configuration in enamel was prism
decussation in inner enamel and parallel prisms in outer enamel. Prism
decussation tended to be absent at cusp ridges and in the bottom of
grooves. In the former location, the distinction between prisms and
interprism was often obscured in the middle enamel zone due to decreased difference in orientation of their crystals. A thin layer of enamel,
often aprismatic, covered the distal aspect of cusps in maxillary molars
and the mesial aspect of cusps in mandibular molars. The enamel abutting
on EFA was often aprismatic. Aprismatic enamel exhibited incremental
lines with a periodicity of about 1 mm and was often traversed by cracks.
The enamel surface was porous in the bottom of grooves. Parts of mouse
molar enamel were incompletely mineralized at the time of eruption.
Conclusions: SEM is a convenient method for combined studies of crown
morphology and enamel structure. Based on morphological criteria, a
modification of the cusp nomenclature is proposed. Enamel thickness
and structure in mouse molars show regional variations. Fundamental
similarities exist between mouse molar cusps and mouse incisors.
Mouse molar enamel undergoes posteruptive maturation. Anat. Rec.
250:268–280, 1998. r 1998 Wiley-Liss, Inc.
Key words: tooth; molar; crown morphology; dental enamel; scanning
electron microscopy; mouse
Over the last decade, methods for generating transgenic mouse lineages and gene knockouts have established the mouse as the most widely used animal model
for the study of vertebrate genetics, embryogenesis,
organogenesis, and tumorgenesis. An intimate knowledge of the normal development and anatomy of the
mouse is important in order to evaluate and interpret
observations from such research. Recent advances in
biomolecular research (e.g., Chen et al., 1994; Brookes
et al., 1995; Lyngstadaas et al., 1995; Matzuk et al.,
1995; Thesleff et al., 1995a) have shed new light on the
molecular biology of tooth formation. There is an increasing interest in using mouse molars as a model system
for studying organ development and morphogenesis
and biomineralization both in vitro and in vivo (Slavkin,
1991; Thesleff et al., 1995b; Lyngstadaas et al., 1995).
Mouse molar teeth are paired mineralized organs
formed at the interface between the ectoderm lining the
r 1998 WILEY-LISS, INC.
oral cavity and the underlying mesenchyme, and their
development is controlled by basic molecular mechanisms common to most organs (Thesleff et al., 1995b).
Teeth consist of the mineralized tissues dentine, cementum, and enamel. Dentine and cementum are calcified
connective tissues closely related to bone (Linde and
Goldberg, 1993). Enamel constitutes the outermost
covering of adult amphibian, reptilian, and mammalian
teeth. Biomineralization comes to an extreme in this
tissue, the hardest and most durable in the vertebrate
Contract grant sponsor: Research Council of Norway; Norwegian
Cancer Society.
*Correspondence to: S. Petter Lyngstadaas, Laboratory for Molecular Biology, Department of Pathology, Dental Faculty, University of
Oslo, PO Box 1109, Blindern, N-0317 Oslo, Norway.
E-mail: spl@odont.uio.no
Received 22 April 1996; Accepted 14 October 1997
269
MOUSE MOLAR ENAMEL
body. Mature enamel contains 95% mineral by weight.
The mineral component is mainly hydroxyapatite in
the form of closely packed ultramicroscopic crystals,
larger than those in other mineralized tissues (Sasaki
et al., 1990). The crystals are organized by differential
orientation into a basic pattern of prisms (rods) and
interprism (interrod) which is common to all mammals
but which shows intergeneric variations in design. The
morphology of mouse molars has been described at the
light microscopy level (Gaunt, 1955, 1956; Cohn, 1957;
Lumsden, 1979). To our knowledge, no systematic
examination of these teeth by scanning electron microscopy (SEM) has previously been reported. One reason
for this may be the small size of these teeth, making
sectioning and preparation for scanning electron microscopy technically demanding. Another reason may be
that the ultrastructure of mouse molar teeth is assumed to be similar to the rat (Skobe, 1978; Risnes,
1979a). We report here a scanning electron microscope
study of the mouse molar dentition, with emphasis on
crown morphology, enamel distribution, and enamel
structure/ultrastructure.
MATERIALS AND METHODS
Balb/C albino mice were kept in a temperature
(20–22°C) and humidity (45–55%) controlled environment and maintained on a 14 h light/10 h dark cycle
with ad libitum access to food and water. Husbandry
was in accordance with Norwegian regulations, meeting the NIH Guidelines for the Care and Use of
Laboratory Animals. A total of 17 mice aged 14, 17, or
35 days were obtained from different litters and sacrificed by cervical dislocation. Their upper and lower jaws
were dissected out and fixed in 70% ethanol. After
fixation, all soft tissue was carefully removed by dissection, light brushing, and ultrasonication. Jaw segments
containing all three upper or lower molar teeth were cut
out with a rotating diamond wheel (Risnes, 1981),
air-dried, and mounted on brass cylinders with a cyanoacrylate glue. The specimens were sputter-coated with
50 nm gold-palladium and observed in a Philips SEM
515 operated at 15 kV, using a specially designed holder
allowing multiangular viewing of the specimens (Risnes,
1982). The right maxillary and mandibular molar segments from a 35-day-old mouse were subjected to
repeated cycles of transverse grinding, cleaning, etching, drying, and sputter-coating, allowing observation
of seven serially ground planes through each molar
segment. In addition, some molar segments were ground
longitudinally in a mesiodistal direction, some after
first having been embedded in Epont. Grinding was
performed under a stereomicroscope using grits 800
and 1200 of 3Mt waterproof silicone carbide paper in a
specially designed apparatus (Risnes, 1985). After grinding, the teeth were cleaned by careful brushing under
running tap water and etched three times for 10 s in
0.1% nitric acid. After air-drying overnight, the specimens were sputter-coated with 30 nm gold-palladium
and observed in the SEM.
After acid-etching, immature enamel can be distinguished from mature enamel in the SEM by a difference
in texture, especially by a difference in the distinctness
of crystals (Risnes, 1990; Risnes et al., 1996).
In the present study, we used a modified version of
Gaunt’s (1955) cusp terminology.
RESULTS
Crown Morphology
At an age of 17 days (Fig. 1a–d), the first and second
molars had just erupted but had not reached occlusion.
The cusps were virtually unworn. The third molars had
not yet erupted. At an age of 35 days, all molars had
reached occlusion, and their cusps were somewhat
worn (Fig. 1e–h).
The maxillary molars, M1, M2, and M3, exhibited
three rows of cusps, lingual (L), buccal (B), and central,
all markedly tilted in a distal direction and separated
by two shallow grooves oriented mesiodistally and by
deep, mesially convex grooves traversing buccolingually (Fig. 1a,c–e,g,h). In the mandibular molars, M1,
M2, and M3, the cusps formed two rows, lingual (L) and
buccal (B), separated by a shallow groove oriented
mesiodistally and by deep, mesially convex grooves
traversing buccolingually (Fig. 1b–d,f–h). On M1 cusps,
B1, B2, L1, and L2 formed a cuspal complex surrounding a common dentin surface. The cusps of the mandibular molars, except cusps B1 and L1 on M1, had a
pronounced mesial tilt. A rudimentary central cusp,
cusp 4, was located at the most distal part of the
mandibular molars.
The three maxillary and the three mandibular molars showed a monotonic form gradient in which the
greatest complexity was anteriorly; morphologically,
the second molars closely resembled the posterior twothirds of the first molars, whereas the third molars
resembled the posterior half of the second molars (Fig.
1a,b,e,f). M2 had lost the cusps corresponding to B1 and
1 of M1, but a rudimentary B1 was appreciable (Fig. 1c).
M2 had lost the cusps corresponding to B1 and L1 of M1,
but also here a rudimentary B1 was discernable (Fig.
1c).
Enamel Distribution
The distribution of enamel was observed both in
unsectioned (Figs. 1, 2) and in sectioned teeth (Figs.
3, 4).
Enamel-free areas (EFA) at the tip of the cusps were
obliquely oriented, facing distally in the upper jaw and
mesially in the lower jaw (Fig. 1a–h), except on cusps
B1 and L1 of M1, where the EFA faced distally and was
continuous with the EFA of the adjacent cusps B2 and
L2 (Fig. 1b,f). At the time of eruption, before occlusal
attrition had started, the EFA were partly surrounded
by a rampart of enamel protruding somewhat beyond
the dentin (Figs. 1a–d, 2a,b). Distinct pits could occupy
variable portions of the EFA (Figs. 1a,b and 2b). Generally, the EFA reached to the entrance of the main
transverse grooves (Fig. 2a,b). However, the EFA could
extend for short distances into the transverse groove,
especially in relation to the buccal and lingual cusps,
where the EFA often showed tongue-like extensions
(Fig. 2a,b). At an age of 35 days, attrition had increased
the size of the EFA, reduced their obliquity, and reduced
the height of the cusps (Figs. 1e–h).
Serial transverse grinding (Fig. 3a–e, g–k) and
selected longitudinal sections (Fig. 3f,l) demonstrated
the distribution of enamel in more detail. The distal
aspect of the cusps in the maxillary molars and the
mesial aspect of the cusps in the mandibular molars
were covered with a rather thin layer of enamel (Figs.
270
S.P. LYNGSTADAAS ET AL.
Fig. 1. Crown morphology of mouse molars. Arrows relate to details
shown in subsequent figures. a–d: Age 14 days. 324. a: Occlusal view
of right maxillary molars. For cusp designation, see Fig. 1e. b:
Occlusal view of right mandibular molars. For cusp designation, see
Fig. 1f. c: Buccal view of right molars. Bullets mark rudimentary
cusps B1 on M2 and M2. d: Lingual view of right molars. e–h. Age 35
days. 3 24. e: Occlusal view of right maxillary molars. Buccal cusps
(B1, B2, B3), lingual cusps (L1, L2), and central cusps (1, 2, 3) are
counted from mesial to distal. f: Occlusal view of right mandibular
molars. Buccal cusps (B1, B2, B3) and lingual cusps (L1, L2, L3) are
counted from mesial to distal. Distally, there is one central cusp (4). g:
Buccal view of right molars (not the same as those shown in Fig. 1e,f).
h: Lingual view of right molars (same as those shown in Fig. 1g).
3d, e, f, i, j, k, l, 4). All other aspects of the crown were
more abundantly covered, but the thickness of the enamel
varied somewhat from region to region, reaching about
110 µm at the summit of the convex aspect of the cusps
(Fig. 3f). In maxillary molars, the central cusps were
connected by low ridges elevating the bottom of the
transverse grooves halfway between the buccal and
lingual aspects (Fig. 3d,e). Adjacent to these ridges, the
MOUSE MOLAR ENAMEL
271
Fig. 2. Enamel-free areas (EFA). a: Cusp L2 of M1 shown in Fig. 1a. EFA extends tongue-like (arrows)
into groove aspect of cusp. E, enamel. Age 14 days. 3312. b: Cusp L3 of left M1 (corresponding cusp is seen
in right M1 in Fig. 1b). The dentin surface of EFA is scalloped with a centrally placed pit (Pi). EFA extends
tongue-like (arrows) into groove aspect of cusp. E, enamel. Age 17 days. 3300.
thin enamel covering the distal aspects of the cusps was
somewhat thickened (Fig. 4). A similar ridge existed
between cusps L1 and 2 in M1 and M2 (Fig. 3d,e).
Enamel Structure
Considering the whole thickness of enamel, we observed three different configurations in mouse molars:
1) decussating prisms in the inner enamel and parallel
prisms in the outer enamel (Fig. 5a), 2) parallel prisms
only (Fig. 5a), and 3) aprismatic enamel (Fig. 5b). Of
these, the first was the most common configuration.
Figure 6 demonstrates schematically the distribution of
prism decussation in mouse molars as seen in midcoronal, mesiodistal longitudinal planes and in midcuspal
transverse planes. Prism decussation tended to be
absent at the summit of the convex aspect of the cusps,
in the bottom of grooves/invaginations, and in the thin
enamel covering the distal aspect of the cusps in the
maxillary molars and the mesial aspect of the cusps in
the mandibular molars. Cervical to the level of the
cusps, prism decussation was present around the whole
circumference of the crown on all molars.
Enamel With Prism Decussation
The expression of prism decussation varied considerably, from fully developed over large areas (Figs. 5a,
7a–d) through fully developed in isolated areas (Fig. 7f)
to vague attempts in isolated areas (Fig. 7g). Generally,
prism decussation occupied the inner one-half to twothirds of the enamel thickness, while parallel prisms
occupied the outer one-third to one-half (Figs. 5a, 7a,c).
Toward the cemento-enamel junction, the latter component decreased (Fig. 7b).
Prism decussation was of the uniserial lamellar type
(Figs. 5a, 7a–f). The single-layered lamellae of prisms
were generally arranged transversely to the long axis of
the tooth, evidenced by the fact that the lamellae were
cut transversely in longitudinal tooth sections (Figs. 5a,
7a,b) and ran roughly parallel with the enamel-dentin
junction in transverse tooth sections (Fig. 7d). On the
side aspects of the cusps, as seen in the transverse
plane, the prism lamellae deviated away from the
enamel-dentin junction in a direction away from the cusp
summit (Fig. 8a,b), indicating that the prism lamellae
were oriented obliquely relative to the ground transverse plane but transversely relative to the slope of the
cusp. Figure 9 is a schematic representation of the
orientation of the prism lamellae in mouse molars
deduced from observations of serially ground transverse planes.
The lamellae were generally inclined occlusally at an
angle of about 55–65° relative to the enamel-dentin
junction (Figs. 5a, 7a), an angle which tended to
increase to 80° or more toward the cemento-enamel
junction (Fig. 7b). The variability in the inclination of
prism lamellae was reflected by the variable appearance of prisms in transverse tooth sections (Fig. 7c,d).
The occlusal inclination of the prisms showed a distinct
increase as they passed from the inner zone with
decussation to the outer zone without decussation
(Figs. 5a, 7a); in the latter, the angle between prisms
and enamel-dentin junction varied between 10 and 40°.
Transverse tooth sections through regions with occlusally inclined decussating lamellae showed that the
interprism in these regions was arranged in radial
sheets delimiting radial lines of prisms containing
prisms belonging to different prism lamellae (Fig. 7d,f).
272
S.P. LYNGSTADAAS ET AL.
Fig. 3. Sectioned and/or ground planes through molars of mice, age
35 days. Arrows relate to details shown in subsequent figures. Inserts
show position of sectioned/ground plane. For cusp designation, see Fig.
1e,f. 324. a–e: Serial transverse grinding of right maxillary molars
shown in Fig. 1e. f: Midcoronal mesiodistal longitudinal section
through maxillary right molars seen from lingual side. g–k: Serial
transverse grinding of right mandibular molars shown in Fig. 1f. l:
Midcoronal mesiodistal longitudinal section through mandibular right
molars seen from lingual side.
This was most conspicuous close to the enamel-dentin
junction where the transverse inclination of prisms was
less pronounced. Little or no interprism was observed
between prisms belonging to adjacent lamellae (Fig.
7e). However, in areas where the enamel was somewhat
hypomineralized, a substance occupying the position of
the interprism surrounded the prisms completely (Fig.
7a). Generally, the crystals constituting the interprism
were oriented roughly at right angles to the prisms and
their constituent crystals (Fig. 7e). Thus, in longitudi-
273
MOUSE MOLAR ENAMEL
prisms coursed toward the enamel surface (Figs. 5a,
10a). Concomitantly, the orientation of the interprism
crystals shifted from a distinct cervical inclination to an
orientation approximately perpendicular to the enamel
surface (Figs. 5a, 10a). In this way, the distinction
between prisms and interprism became less obvious,
due to a diminished difference in orientation of their
constituent crystals (Fig. 10b).
In transverse tooth sections, the prisms in the inner
enamel tended to be arranged in radial lines delimited
by radially directed, rather coarse sheets of interprism
(Figs. 7f, 10c). Crosscut prism profiles varied greatly in
size and shape but were mostly ovoid. Toward the
middle or outer part of the enamel, the distinction
between prisms and interprism became less evident
(Fig. 10c). This was apparently due to a diminished
difference in orientation between their constituent crystals (Fig. 10d), in agreement with the observations in
the longitudinal plane. This blurred configuration could
persist to the enamel surface, but sometimes a distinct
prism-interprism pattern was reestablished, either in
the form of a honeycomb pattern similar to that found
in the outer part of the enamel with prism decussation
or in the form of rounded prisms embedded in abundant
interprism (Fig. 10e). The superficial 3–5 µm of the
enamel was prism-free (Figs. 5a, 10a).
Aprismatic Enamel
Fig. 4. Groove region between cusps 1 and 2 of M1 from fourth
transversely ground plane (see location in Fig. 3d). Thin enamel (E),
which covers distal aspect of cusp 1, is thickened at the ridge
connected to cusp 2 and shows a break in its continuity (arrow)
corresponding to a tongue-like extension of EFA similar to that shown
in Fig. 2a. D, dentin. 3212.
nal sections the crystals of the interprism exhibited a
distinct cervical inclination (Fig. 7a). Toward the outer
enamel without prism decussation, the orientation of
the interprism crystals changed, becoming less cervically inclined and almost perpendicular to the enamel
surface (Figs. 5a, 7a). In transverse tooth sections the
interprism of the outer enamel formed a distinct honeycomb pattern in which the ovoid ‘‘cells’’ were occupied
by prisms (Figs. 7c, 8b). The superficial 3–5 µm of the
enamel was devoid of prisms (Figs. 5a, 7a–c).
Enamel With Parallel Prisms Only
Regions with parallel prisms throughout the whole
thickness of enamel were typically, but with some
exceptions, found at the summit of the convex aspect of
the cusps (Figs. 5a, 6, 8a, 10a,c) and also at the bottom
of grooves/invaginations (Fig. 6).
Generally, the prisms were inclined occlusally (Figs.
5a, 10a). Their angle relative to the enamel-dentin
junction varied considerably from region to region and
could increase from about 20° at the tip of a cusp to
about 90° at the bottom of a groove. An increase in
occlusal inclination of prisms in the outer enamel as
found in regions with prism decussation was not observed. On the contrary, the angle between prisms and
the enamel-dentin junction tended to increase as the
The thin enamel covering the distal aspect of the
cusps in the maxillary molars and the mesial aspect of
the cusps in the mandibular molars was partly aprismatic, prisms being especially absent in its thinnest
parts (Figs. 5b, 11a). Also, the enamel situated adjacent
to the enamel-free areas tended to be aprismatic (Fig.
11b,c). The crystals were generally oriented perpendicular to the enamel-dentin junction and the enamel
surface. Cracks (Fig. 11b,c) were a more frequent
finding in aprismatic than in prismatic enamel and
were visible also on the enamel surface (Fig. 11d). In
transition zones between prismatic and aprismatic
enamel, scattered prisms were embedded in abundant
interprism (Fig. 11c). In the aprismatic enamel, lines
resembling incremental lines, with a periodicity of
about 1 µm, were frequently observed (Fig. 11a,b).
Enamel Maturation
Generally, the molar enamel of 14–17-day-old mice
appeared fully matured with distinct crystals after
acid-etching. Locally, however, areas of hypomineralized enamel could be found. Although areas of hypomineralized enamel could persist also in 35-day-old mice,
especially in the bottom of the grooves (Fig. 7g), there
was less of it than in younger mice.
DISCUSSION
Crown Morphology
The crown morphology of mouse molars presented
here relates to the BALB/c strain. Grüneberg (1965)
found minor variations in normal crown morphology
among various inbred strains and also among wild
mice, while aberrations of a more major character were
observed in certain mutants. There are fundamental
similarities but also minor differences between the
274
S.P. LYNGSTADAAS ET AL.
Fig. 5. Types and typical locations of enamel structure configurations. Longitudinal plane, age 35 days. a: Occlusally to contact area of
1
M and M2 (see location in Fig. 3f). M1 shows prism decussation in
inner enamel (IE) and parallel prism in outer enamel (OE), while M2
shows parallel prisms throughout whole thickness of enamel. Superficial enamel (SE) is prism-free. DEJ, dentinoenamel junction. R, resin.
3680. b: Thin aprismatic enamel (E) covers distal aspect of cusp 1 in
M1 (see location in Fig. 3f). D, dentin; R, resin. 31,250.
Fig. 6. Distribution of prism decussation (hatched areas) in mouse
molars. Dentin is stippled. The left part of figure shows midcoronal
mesiodistal longitudinal planes through maxillary (MAX.) and mandibular (MAN.) right molars seen from lingual side. The right part of
figure shows midcuspal transverse planes through maxillary and
mandibular right molars seen from occlusal aspect. The plane through
maxillary molars corresponds to the fourth transversely ground plane
shown in Fig. 3d. The plane through mandibular molars represents
parts of several transversely ground planes (i.e., those of Fig. 3h–k).
Arrows indicate direction of force during mastication.
crown morphology of mouse and rat molars (Kurahashi
et al., 1968).
The nomenclature of mouse molar cusps proposed by
Gaunt (1955) involves an aspect of cusp homology based
on interpretations of the sequence of development of
cusps during odontogenesis (Gaunt, 1955, 1956, 1961b).
From reconstructions of the molar tooth germs based on
serial-sectioning of mouse jaws at various ages, Gaunt
concluded that second molars were homologous with
the distal part of first molars and that third molars
were homologous with the mesial part of second molars.
From a morphological point of view, however, we found
that third molars bore a stronger resemblance to the
distal than to the mesial part of second molars. Especially the three cusps in the mandibular third molar
were more readily comparable with the three distal
cusps (L3, B3, and 4) than the two mesial (L2, B2) and
the distolingual (L3) cusps on the second molar. We
therefore propose that the cusp nomenclature should be
modified for the third molars, from L2, B2, and L3 to
L3, B3, and 4 in the lower jaw (Fig. 1f) and from 2, L2,
and B2 to 3, L2, and B3 in the upper jaw (Fig. 1e). The
available odontogenetic evidence is not sufficiently conclusive and consistent to reject the possibility that each
MOUSE MOLAR ENAMEL
of the two posterior molars are formed through a
reduction of the mesial part of the preceding tooth.
From Gaunt’s (1955) reconstructions of the molar germs,
it appears that the middle and distal parts of the first
molars develop at about the same time and only slightly
before the mesial part, both with respect to initial cusp
height and onset of dentin and enamel formation. The
identification by Gaunt (1955) of two central cusps (2
and 3) on the maxillary third molar is questionable. It
seems unlikely that the maxillary and not the mandibular third molar should retain all the major components,
although rudimentary, of the second molar.
Enamel Distribution
Enamel does not cover murine molar tooth crowns
completely. It is well known that enamel-free areas
(EFA) are present at the tip of the cusps both in the rat
(Addison and Appleton, 1921; Mellanby, 1939) and in
the mouse (Mahn, 1890; Gaunt, 1956, 1961a; Cohn,
1957). Prior to occlusal attrition, the cusp tips were
concave, sometimes accentuated by a distinct pit in the
EFA, the apical pit of Gaunt (1956, 1961a). The elevated rim of dentin and enamel is rapidly worn away
during attrition.
The formation of the enamel-free areas on the cusps
of mouse and rat molars have been addressed by
several investigators. Addison and Appleton (1921),
Mellanby (1939), Gaunt (1956), and Cohn (1957) concluded that the cells of the inner enamel epithelium
adjacent to enamel-free regions did not differentiate
into secretory ameloblasts and consequently failed to
produce enamel. However, more recent investigations
have demonstrated that the epithelial cells over the
enamel-free areas elaborate a thin, noncontinuous layer
of organic matrix which contains amelogenin (Nakamura et al., 1991; Inai et al., 1992), which becomes
mineralized by deposition of fine crystals (Sakakura et
al., 1989) and which on demineralization leaves an
organic residue devoid of collagen (Sakakura et al.,
1989). Contrary to Sutcliffe and Owens (1980), who
with the SEM were able to observe what seemed to be
remnants of such an enamel layer on the EFA of rat
molars, we could not find it on molars of 14-day-old
mice. It has been suggested that this rudimentary
enamel layer, together with the adjacent and subjacent
dentin, is subjected to resorption prior to eruption
(Sutcliffe and Owens, 1980, 1981), which is supported
by the scalloped appearance of the dentin surface in the
EFA observed in the present study. It is possible that
the tendency toward enamel production on EFA is
greater and/or that the tendency toward its resorption
is slighter in the rat than in the mouse. This is
corroborated by Johannessen’s (1961) finding that
enamel was often present on the supposed EFA of
unworn rat molars.
We have demonstrated the presence of a thin layer of
enamel covering the aspect of the cusps overhanging
the transverse grooves, facing distally in the maxillary
molars and mesially in the mandibular molars. These
aspects have previously been described as enamel-free
(Gaunt, 1956).
Enamel Structure
The structure of mouse molar enamel has many
features in common with rat molar enamel (Skobe and
275
Stern, 1978; Risnes, 1979a). The characteristic uniserial lamellar prism pattern (Korvenkontio, 1934) present in the incisor enamel (Møinichen et al., 1996)
dominates also in the molars. As in the rat (Risnes,
1979a), there are features which distinguish the structure of molar enamel from that of the incisor, including
the presence of extensive regions without prism decussation and the variability in distinctness and distribution of prism decussation.
The similarity of molar enamel to incisor enamel
becomes especially evident when cusps are sectioned
transversely. A transversely sectioned molar cusp (e.g.,
Fig. 8b) resembles a transversely sectioned incisor
(Møinichen et al., 1996) where a layer of enamel with
prism decussation in the inner enamel and parallel
prisms in the outer enamel covers a markedly convex
dentin surface. Contributing to the similarity is the fact
that the prism rows tend to be oriented transversely to
the cusp axis just as they are oriented transversely to
the long axis of the incisor (Risnes, 1979b; Møinichen et
al., 1996). Furthermore, the reconstructions of mouse
molar development by Gaunt (1956) showed that the
front of enamel formation on a cusp at a certain stage is
C-shaped, with the arms of the C pointing occlusally.
This is in accordance with the finding of Smith and
Warshawsky (1976) that the front of enamel formation
in rat incisors is correspondingly C-shaped, with the
arms of the C pointing incisally. Finally, the eccentric
distribution of enamel on the cusps, with abundant
enamel on the convex aspect and lack of a prism-free
area and its tongue-like extension or only very thin
enamel on the opposite aspect, resemble the distribution of enamel on the incisors. Thus, it may be argued
that murine incisors and molar cusps are analogous.
Also, functional considerations support the incisor-cusp
analogy. With a mandibular power stroke directed
mesially and somewhat lingually as in the rat (Hiiemä,
1967; Weijs, 1975), the cusps of both upper and lower
molars will meet the food and antagonist in a direction
similar to that of the incisors with respect to enamel
distribution. In the mouse, the distribution of prism
decussation in molar enamel relative to the direction of
the mandibular power stroke is different and in fact
rather the opposite of the corresponding relationship in
Arvicolidae (Koenigswald, 1982) (Fig. 6). In Arvicolidae, prism decussation is absent, while in the mouse it
is partly present in the trailing edge enamel. In Arvicolidae, the leading edge enamel contains prism decussation, while in the mouse the leading edge enamel facing
the transverse grooves is thin and lacks prism decussation. These differences may be related to the different
cuspal arrangement on the molars of mouse and Arvicolidae, causing different distribution of stress zones in
the enamel.
In the present study, it was observed that the distinction between prisms and interprism is often obscured in
the middle and outer enamel in regions without prism
decussation, due to a diminished difference in the
orientation of their crystals. It appears that this is
mainly due to a change in the orientation of the
interprism crystals from a cervical one to one perpendicular to the enamel surface but enhanced by a
concomitant erection of the prisms. A similar change in
interprism crystal orientation occurs at the transition
from inner to outer enamel in regions with prism
276
S.P. LYNGSTADAAS ET AL.
Fig. 7.
MOUSE MOLAR ENAMEL
277
Fig. 8. Transversely ground planes through molar cusps. Age 35
days. a: First ground plane through cusp 1 of M1 (see location in Fig.
3a). Prism decussation in inner enamel (IE) is only vaguely expressed
at summit of convex aspect of cusp. On side aspects of cusp, prism
lamellae deviate away from dentin (D) in direction away from summit
of convex aspect of cusp. EFA, enamel-free area; OE, outer enamel.
3252. b: First ground plane through cusp L1 of M1 (see location in Fig.
3g). Prism decussation is present in inner enamel (IE) over whole
convex aspect of cusp. D, dentin; EFA, enamel-free area; OE, outer
enamel. 3252.
Fig. 7. Enamel with prism decussation. Age 35 days. a,b,g: Longitudinal plane. c–f: Transverse plane. a: Enamel on mesial aspect of left
M3 (see corresponding location in Fig. 3p) shows prism decussation in
inner enamel (IE) and parallel prisms in outer enamel (OE). Superficial enamel (SE) is prism-free. There is a great difference in crystal
orientation between interprism (white arrows) and prisms (black
arrows) both in inner and outer enamel. To the right is a region with
hypomineralized enamel. Here, prism domains (p) are completely
surrounded by interprism (ip). DEJ, dentinoenamel junction. Occlusal
direction toward left. 31,930. b: Cervical enamel mesially on left M1
(see corresponding location in Fig. 3l). Inner enamel (IE) and outer
enamel (OE) fades away; only prism-free superficial enamel (SE)
remains at end of enamel attenuation. DEJ, dentinoenamel junction.
R, resin. 31,050. c: Contact region between M1 and M2 in fourth
transversely ground plane (see location in Figure 3j). Both molars
show prism decussation in inner enamel (IE). In outer enamel (OE),
prisms tend to be arranged in honeycomb pattern, especially evident
in M1. Superficial enamel (SE) is prism-free. DEJ, dentinoenamel
junction. 3745. d: Inner enamel with prism decussation in fifth
transversely ground plane of M1 (mesiobuccal location in Fig. 3e).
Transverse inclination of prisms increase from dentinoenamel junction (DEJ) and outwards. Radial sheets of interprism (arrows) are
especially evident close to dentin. Various prism pattern aberrations
are present. 31,200. e: Decussating prisms (p) with abundant interprism (ip) between prisms of the same lamella and little or no
interprism between prisms of adjacent lamellae. DEJ, dentinoenamel
junction. 35,000. f: Isolated area of prism decussation in fifth transversely ground plane of M1 (see location on mesial aspect of cusp B2 in
Fig. 3e). Radial sheets of interprism (arrows) are more readily visible
in areas without than with prism decussation. DEJ, dentinoenamel
junction. 3680. g: Bottom of groove between cusps 1 and 2 of M1 (see
location in Fig. 3f). Enamel (E) exhibits tendencies of decussating
prisms (arrows) and immature enamel. D, dentin; R, resin. 3810.
decussation, but here the distinction between prisms
and interprism is retained due to a concomitant, distinctly increased incisal inclination of the prisms. The
formation of enamel lacking prisms is generally related
to ameloblasts lacking Tomes’ processes (Risnes, in
press). There are two possible explanations for the
convergence of prism and interprism crystals observed
in enamel without decussation in the present study: 1)
there may be a regression of Tomes’ processes, and 2)
Tomes’ processes remain intact, but the direction of
ameloblast movement, and hence orientation of prism
crystals, approaches the orientation of interprism crystals. If the second explanation is correct, it means that
there are no inherent physicochemical properties of the
matrix or the mineral that enforce a certain angle
between prism and interprism crystals.
The reason prism decussation is largely absent from
the summit of the convex aspect of the cusps and from
the deepest part of grooves/invaginations remains to be
clarified. The same phenomenon was observed in rat
molars (Risnes, 1979a). It may be due to the spatial
conditions in an ameloblastema which covers the complexly folded molar surface or to regional differences in
ameloblast characteristics.
The thickness of mouse molar enamel of about 100
µm is produced in about 15 days (Cohn, 1957), giving a
production rate of about 7 µm per day, very close to the
278
S.P. LYNGSTADAAS ET AL.
Fig. 9. Schematic representation of orientation of prism lamellae in regions with prism decussation.
Based on observations of serial transverse grindings. The left part of the figure shows right maxillary
(MAX.) and mandibular (MAN.) molars viewed from the lingual side, while the right part of the figure
shows same from the buccal side.
Fig. 10. Enamel without prism decussation. Age 35 days. a: Longitudinal plane through cusp B3 of M1 (see location in Fig. 1f). Orientation
of interprism crystals (white arrows) and prisms/prism crystals (black
arrows) changes from dentinoenamel junction (DEJ) toward surface,
diminishing the difference in crystal orientation. Occlusal direction
toward left. 31,150. b: Third transversely ground plane through cusp
B3 of M1 (see location in Fig. 3i) showing approximately same enamel
region as in Fig. 10a. Prisms are distinct in inner enamel, are hardly
distinguishable in middle enamel, and are again visible in outer
enamel. Arrows point at radial sheets of interprism. DEJ, dentinoenamel junction. 31,010.
production rate calculated for mouse incisor enamel
(Møinichen et al., 1996) but considerably slower than
the 12–13 µm per day apposition rate calculated for rat
incisor enamel (Risnes, 1979c; Smith and Nanci, 1989).
The thin enamel covering the distal aspect of the cusps
in the maxillary molars and the mesial aspect of the
cusps in the mandibular molars may represent an
enamel layer produced at a normal rate for only a short
period of time, an enamel layer produced at a slow rate
for a normal period of time, or an enamel layer pro-
279
MOUSE MOLAR ENAMEL
Fig. 11. Aprismatic enamel. a: Transverse plane. Thin aprismatic
enamel (E) on distal aspect of cusps 1/L1 of M1 (see location in Fig. 3e).
Incremental lines (arrows) with periodicity of about 1 µm are oriented
parallel with dentin surface. D, dentin. Age 35 days. 32,500. b:
Longitudinal plane. Aprismatic enamel (E) adjacent to EFA on M3 (see
location in Fig. 3f). Incremental lines (arrows) spaced about 1 µm run
parallel with dentin surface. Cracks (C) traverse whole enamel
thickness. D, dentin; R, resin. Age 35 days. 32,400. c: Transverse
plane. Aprismatic enamel (E) adjacent to EFA on cusp B3 of M1 (see
location in Fig. 3h). At transition from prismatic enamel (to the right),
occasional prisms (p) are embedded in abundant interprism (ip).
Cracks (C) may traverse whole thickness of enamel. D, dentin. Age 35
days. 31,550. d: Enamel (E) abutting on EFA between cusps L1 and L2
on M1 (see location in Fig. 1b). Dentin surface (D) exhibits globular
appearance. Enamel (E) surface adjacent to EFA shows many cracks
(C). Age 14 days. 33,540.
duced at a slow rate for a short period of time. The
presence of closely spaced lines with a periodicity of
about 1 µm, resembling incremental lines, indicates a
slow rate.
of rat (Francis and Briner, 1966) and hamster (Tsjujimoto et al., 1994).
Enamel Maturation
Areas of hypomineralized enamel are normally present in mouse molar enamel at the time of eruption
(14–17 days). Since at 35 days the areas of immature
enamel are mostly eliminated, it appears that mouse
molar enamel is subjected to posteruptive maturation.
This phenomenon has also been described in the molars
CONCLUSIONS
Based on morphological considerations, modifications in the mouse molar cusp nomenclature of Gaunt
(1955) are suggested.
The distribution of enamel on the various aspects of
the molars has been clarified together with the extent of
the enamel-free areas. Considerable variations in its
thickness and structure have been demonstrated, which
should be taken into consideration when developmen-
280
S.P. LYNGSTADAAS ET AL.
tal stages of mouse molars are studied. Such knowledge
is crucial for a correct localization and interpretation of
thin sections from the complexly folded molars. A great
advantage of scanning electron microscopy in this connection is that it is possible to combine detailed structural
information with an exact localization within the tooth.
Tooth development is regulated by molecular mechanisms common to most organs (Thesleff et al., 1995b).
The mouse molar is a convenient experimental model
for the study of the basic mechanisms of organ development, including cell differentiation, cell interaction,
morphogenesis, and production and mineralization of
extracellular matrices. The present study provides a
part of the morphological foundation which such studies should be based on and related to.
ACKNOWLEDGMENTS
This work was supported by grants from the Research Council of Norway and the Norwegian Cancer
Society. We are grateful to D. Sørensen and his staff at
The Laboratory Animal Unit, The National Hospital,
University of Oslo, for the use of the animal facilities, to
S. Stølen for his technical assistance with the SEM, and
to P. Gran for the photographic work.
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