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Response of the intermaxillary suture cartilage to alterations in masticatory function.

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THE ANATOMICAL RECORD 220:376-387 (1988)
Response of the lntermaxillary Suture Cartilage to
Alterations in Masticatory Function
Department of Anatomy, Baylor College of Dentistry, Dallas, TX 75246
Like the condylar cartilage of the mandible, the cartilage at the
intermaxillary suture in the rat is secondary in origin and persists well into adulthood. While the condylar cartilage is generally considered to be responsive to
changes in its local biomechanical environment, little is known of the response of
the intermaxillary suture cartilage to similar stimuli. In order to study the effect of
changes in occlusal loads on intermaxillary suture cartilage metabolism, male
weanling Sprague-Dawleyrats were divided into four groups: soft diet (to discourage
molar mastication), incisor clipped (to discourage incision), incisor clippedsoft diet
(both treatments), and control (untreated). At 39 days of age, the intermaxillary
suture opposite the first molars was removed and compared to control tissue by
using histological, histochemical, and biochemical analyses. Alcian blue-stained
coronal sections demonstrated moderate decreases in staining intensity and decreased chondrocyte hypertrophy in the soft diet and incisor-clipped groups. However, sutures in the incisor-clippedsoft diet group showed a considerably reduced
extent of cartilage and greatly diminished cartilage hypertrophy. [35S]-sulfateincorportion (dpdpg DNA) into acid-insoluble proteoglycans was significantly reduced
(P<.01) in all three experimental groups compared t o controls. [35S] incorporation
was further reduced in the incisor-clippedsoftdiet group relative to both of the other
experimental groups. Staining for alkaline phosphatase activity showed decreased
intensity in the experimental groups and was almost absent in the incisor-clipped
soft diet group. These data demonstrate the plasticity of the intermaxillary suture
cartilage and provide circumstantial evidence that the observed changes in cartilage
morphology and metabolism may be attributable to alterations in the local biomechanical environment of the suture.
In the craniofacial region, cartilage is formed at the
margins of membranous bone during prenatal and early
postnatal development. These sites of cartilage formation have usually been designated secondary cartilages
based largely on their apparent derivation from periosteum and their formation apart from and later than the
primary cartilaginous skeleton (Beresford,1981;Vinkka,
1982). In addition, certain secondary cartilages have
been found to differ from primary cartilages in their
structure: e.g., the nature of the cells undergoing mitosis
and the mode of ossification (cf., Petrovic, 1972; Petrovic
et al., 1975; Vinkka, 1982). In the young rat, numerous
areas of secondary cartilage have been identified
(Vinkka, 1982), but many of these are transitory in
nature, and most do not persist long into postnatal life.
An exception to this generalization is the cartilage which
develops at the midline sutures of the palate: i.e., the
intermaxillary and interpalatine sutures (Fig. 1). At the
intermaxillary suture, cartilage has been identified both
prenatally and postnatally in the rat (Pritchard et al.,
1956; Mohammed, 1957;Anderson et al., 1967; Persson,
1973; Vinkka, 1982; Ghafari, 19841, mouse (LinderAronson and Larsson, 1965; Griffiths et al., 1967), and
cat (Bloore et al., 1969). Although much less studied,
small nodules of cartilage have been noted in the inter@ 1988 ALAN
palatine suture (Youssef, 1969; Vinkka, 1982). In the
rat, cartilage is said to be present at the intermaxillary
suture by the 19th to 20th day postinsemination (Mohammed, 1957;Vinkka, 1982)and continues to be found
after birth and well into postnatal life, still persisting at
100 days of age (Ghafari, 1984).
Vinkka (1982) has suggested that in its earlier stage
of development, the intermaxillary suture cartilage may
contribute to the increase in width of the palate in
concert with the rapidly enlarging oral and nasal spaces
occurring at this time. However, since growth in width
of the rat palate is largely complete by 20 days of age
(Anderson et al., 1967; Koskinen, 1977), he speculates
that the cartilage noted at later stages of development
is more likely a response to biomechanical forces resulting from the eruption of the first molars at 15-20 days
and weaning at 21 days. In organ culture (Stutzmann
and Petrovic, 1976), the cartilage at the intermaxillary
suture exhibits the characteristics of a secondary rather
than a primary cartilage: i.e., the dividing cells are
undifferentiated mesenchymal cells rather than chondroblasts and its growth rate in vitro is similar to that
Received September 4, 1986; accepted October 5, 1987.
Fig. 1 . Schematic drawing of the rat palate showing the midline
intermaxibry (IM) and interpalatine (IP)sutures. Dashed line in&Cat% the approximate area from which tissue was removed for biochemical and histochemical analyses.
of other secondary cartilages but much less than primary cartilages. Consequently, it might be expected that
this cartilage would be particularly responsive to alterations in its local biomechanical environment, much
like the condylar cartilage of the mandible (Petrovic et
al., 1975; Carlson et al., 1980). Yet, despite the many
studies of normal growth and development of the intermaxillary suture cartilage, its possible response to
changes in local biomechanical stimuli has been little
studied. The purpose of this study is to investigate the
effect of induced alterations in occlusal forces on morphology and metabolism of the intermaxillary suture
At 22 days of age (immediately following weaning),
male Sprague-Dawley rats were divided into four groups
of roughly 15 animals each. In one group, both the maxillary and mandibular incisor teeth were clipped to the
level of the gingiva every other day. Since incision was
eliminated or greatly hindered in these animals, they
were provisioned with small hard rodent pellets available from bowls within the cages. A second group was
fed a soft diet of ground rodent chow dissolved in water
to the consistency of mush or a thick soup, supplied in
bowls. Because incision was discouraged but not prohibited by the soft diet, a third group underwent incisor
clipping in addition to the soft diet. A fourth group of
untreated animals fed normal rat pellets constituted the
control group. This resulted in four groups having
roughly dichotomous states of occlusal forces at the incisor and molar teeth: incisor clipped: occlusal forces
unaffected at molars but minimal a t incisors; soft diet:
minimal occlusal forces a t molars but possibly some
forces at incisors due to paramasticatory activities; incisor clippedsoft diet: minimal occlusal forces a t both
molars and incisors; controls: both molar and incisor
occlusion unaffected.
At 39 days of age, all animals were killed with a lethal
injection of sodium pentobarbital (50 mg/ml). Two hours
prior to death, most of the animals in each group were
injected intravenously (femoral vein) with 1pCVg body
weight of [35S] as sodium sulfate in water (New England
Nuclear, specific activity = 767 mCi/mmole). In several
animals in each group, sutures were removed for morphological examination using light microscope histology. Following fixation in neutral-buffered formalin, the
tissues were decalcified in EDTA, dehydrated through
graded ethanols, embedded in paraffin, and sectioned in
the frontal plane at 5 pm. Sections were then stained
with hematoxylin and eosin or with alcian blue 8GS (1%
in 3% acetic acid) counterstained with Kernechtrot aluminum sulfate for mucopolysaccharides (Luna, 1968).
Sutural tissue reserved for biochemical analysis was
removed from the region opposite the maxillary first
molar teeth (Fig. 11, since preliminary histological examination indicated cartilage to be always present in
this area in control animals and to vary relatively little
in its character and extent. However, this was not the
case for adjacent areas (i.e., opposite the more distal
molars or anterior to the first molars). Thus, care was
taken not to remove tissue anterior to the first molars
or posterior to the mesial edge of the second molars.
Some adjacent bone was necessarily removed along with
the sutural cartilage, but this was not considered to
constitute a significant impediment to the biochemical
analysis. Immediately following removal, the tissue
sample was frozen at -20°C for later processing. Sutures reserved for analysis of [35S]-sulfateincorporation
were homogenized in ice-cold 20% trichloroacetic acid
(TCA) containing 10 mM sodium sulfate. The homogenates were placed on ice for 30 minutes and centrifuged
a t 3,OOOg for 60 minutes. The precipitates were washed
twice in 5 ml of ice-cold 10% TCA and hydrolyzed for 20
minutes in 2 ml of 10% TCA at 90°C. The hydrolyzate
was placed on ice for 15 minutes and centrifuged a t
3,OOOg for 10 minutes a t 4°C. An aliquot (500 p1) of the
supernatant (containing approximately 3 mg of tissue
by weight) was added to 10 ml of ACS (aqueous counting
scintillant, Amersham) and assayed for radioactivity in
a Beckman LS 7500 scintillation counter. A further aliquot was utilized to estimate DNA content by using the
diphenylamine method (Burton, 1956). Calf thymus
DNA (Sigma) served as standard, and DNA content was
read a t 600 nm. The results were expressed as d p d p g
DNA in the acid-insoluble precipitate.
In several animals in each group, sutures were incubated for alkaline phosphatase histochemistry by using
the substituted naphthol method of Burstone (1961).
Upon removal, the sutural tissue was placed in cold
EDTA brought to a pH of approximately 6.0 with 0.1 N
sodium hydroxide to preserve enzyme activity (Anderson, 1984). Following decalcification, the tissue was
embedded in Tissue-Tek OCT compound (Miles Scientific, Naperville, IL 60566) and quick-frozen in isopentane cooled with dry ice; 10-pm sections were cut on a
cryostat and then bathed in a 1% MgC12 solution for 4
hours to reactivate the enzyme activity (cf., Anderson,
1984). Sections were then incubated for 30 minutes in
naphthol AS-MX phosphate in dimethylformamide and
Tris buffer, pH 8.5, with Fast Red TR as the coupling
agent, and counterstained with Mayer’s hematoxylin.
Fig. 2. Intermaxillary suture in control rat. Coronal sections, alcian
blue-Kernechtrot aluminum sulfate. Orig. magnif. x40. Darkest densities correspond to alcian blue staining. a: Oral half of suture cartilage. Note hypertrophic chondrocytes (arrows) adjacent to areas of
vascular growth, suggesting an erosion front. Beginning about one-
third of the way up from the oral surface, collagen bundles extend
across the suture, giving the impression of transverse columns of cells.
b: Nasal half of suture cartilage. Note again the transverse columns of
cells and evidence of endochondral bone formation at the sutural
Sections from the control group and each experimental
group were included in every round of staining to ensure
that any differences in staining intensity among the
groups would not be attributable to subtle variations in
staining procedures or solutions from one round of staining to the next.
In 39-day-old control animals, the intermaxillary suture opposite the first molar was a butt-ended suture
characterized by cartilage lining the adjacent bone borders, thereby fusing the right and left maxillary bones.
Although no distinct midline could be demarcated, a
progression from smaller cells near the middle of the
sutural gap to mature chondrocytes to hypertrophic
chondrocytes was apparent. In the oral half to lower
third of the cartilage, the smaller cells in the central
zone were rounded and tended to occur in nests of several cells that were often oriented in vertical columns
(Fig. 2a). In the upper (or nasal) two-thirds or half of the
suture, the central zone contained more slender, spindleshaped cells arranged in transverse rows across the sutural gap (Fig. 2b). Staining for mucopolysaccharides
was present for nearly the entire height of the suture
but was most intense in the oral half where the ground
substance tended to bridge the sutural gap completely.
In contrast, the more intensely stained areas diverged
laterally as one approached the nasal surface, lending
the areas of cartilage matrix staining a V-shaped appearance (Fig. 2a,b). In general, the oral half of the
cartilage exhibited the greatest number of hypertrophic
chondrocytes (Fig. 2a). The lateral extent of the cartilage
was sometimes characterized by a sharp border with the
maxillary bone, but other areas displayed evidence of an
erosion front (Fig. 2a,b).
The morphological appearance of the suture in the
incisor-clipped and soft diet groups was not greatly different from that in control animals (Fig. 3a). Cartilage
was still present along the entire height of the suture,
and the orientation and character of the cells in the
central zone were similar to control morphology. However, the areas of mucopolysaccharide staining did not
bridge the sutural gap in the oral half of the cartilage
as in controls and in general did not stain as intensely.
Moreover, cartilage hypertrophy in the oral half of the
cartilage was not as great as that seen in controls (Fig.
By contrast, the animals receiving both treatments
(incisor-clippedsoft diet animals) displayed a markedly
different morphology from controls. Areas of mucopolysaccharide staining were drastically decreased throughout the height of the suture (Fig. 4a). In particular, a
greatly diminished staining and reduced chondrocyte
hypertrophy were evident in the oral half of the suture
(Fig. 4b), the region of relatively greater staining and
hypertrophy in controls. What staining was present occurred primarily in the nasal half of the suture and
often in a very thin layer adjacent to the bone border or
pericellularly around individual chondrocytes. In many
sections, the areas of mucopolysaccharide staining were
separated from the sutural gap by a tissue containing
cartilagelike cells in a matrix which did not stain for
mucopolysaccharides (Fig. 4c). In fact, the overall appearance of the articulation resembled that of a typical
fibrous suture with some remnants of mucopolysaccharide staining on its lateral margins. Vascularization,
never present within the central zone of control sutures
or those of the other experimental groups, was also
Morphometric Analysis
In an attempt to quantitatively assess any differences
in cartilage thickness and area among the groups, three
measurements were taken: (1)width of the cartilage
near the oral surface of the suture; (2) width of the
cartilage near the nasal surface of the suture; and (3)
total area occupied by cartilage. The measurements were
obained by using the Bioquant System IV (R & M Biometrics, Inc.) morphometric software interfaced with a
Zenith-158microcomputer to calculate the above parameters directly from images of the alcian blue-stained
histological sections.
Analysis of variance of these data provided results
which largely confirmed visual impressions of the histology (Table 1).The only significant differenceswere noted
in the incisor-clippedsoft diet sample, which had a
smaller width at the nasal surface (P< .05) and a considerably smaller cartilage area (P<.01)than controls. Although the other two experimental groups showed
generally lower dimensions than controls, no significant
size differences in the sutural cartilage were detected,
at least at these small sample sizes.
Biochemical Analysis
Incorporation of [35S]-sulfatewas reduced (P< .01) in
all three experimental groups relative to controls (Table
2). In addition, incorporation in the incisor-clippedlsoft
diet group was decreased further relative to the incisorclipped (PG.01) and soft diet (PG.05) groups. This resulted in a three-tiered array of incorporation values,
with controls highest, incisor-clippedand soft diet groups
intermediate, and the incisor-clippedsoft diet group
Enzyme Histochemistry
In control animals, staining for sites of alkaline phosphatase activity was most pronounced in the lateralmost extent of the cartilage adjacent to bone, with fainter
staining present in the matrix surrounding the more
medially located cells. Of the four groups, staining was
most intense in control animals (Fig. 5a); incisor-clipped
animals showed considerable, but somewhat less intense, staining (Fig. 5b). No apparent difference in staining pattern or intensity was noted between oral and
nasal parts of the suture. Staining intensity in the other
two groups was more attenuated. Tissue from soft-diet
animals (Fig. 5c) exhibited considerably less enzyme
activity than in control and somewhat less than in incisor-clipped animals. Staining was almost absent in incisor-clippedsoft diet animals (Fig. 5d), with only wisps of
red coloration present at those sites stained bright red
in other groups.
Fig. 3.Intermaxillary suture in incisor clipped rat. a: Overview. N, nasal aspect; 0, oral aspect. Note
the diminished intensity of staining, especially in the oral half of the suture, in comparison to Figure 2a.
Orig. magnif. ~ 4 0b:. Oral half of suture. P, oral perichondrium. Note the reduced chondrocyte hypertrophy (arrows) in the lower third of the suture (compare to Fig. 2a). Orig. magnif. ~ 4 0 .
Fig. 3%).
TABLE 1. Widths and area of intermaxillary suture cartilage in control and experimental groups'
Incisor clipped
Soft diet
soft diet
F value
oral (pm)
nasal (pm)
Area (pm2)
218.7 f 16.6
200.7 & 39.8
201.5 & 36.4
159.5 k 16.9
296.2 & 17.6
232.1 & 45.4
241.7 f 17.3
220.3 31.0*
129,654.9 k 15,429.6
106,705.2 k 38,594.4
99,819.5 k 48,337.4
31,498.8 f 18,995.4**.t
'Data are presented as mean standard deviation; sample sizes for each group are listed in parentheses below. The value for each animal
represents the mean of the measurement from two histological sections.
*Different from control sample at P < .05.
**Different from control sample a t P 4 .01.
?Different from incisor clipped sample at P < .05.
Fig. 4. Intermaxillary suture in incisorclippedsoft diet rat. a: Overview. Orig. magnif. x40.
The suture has become largely fibrous, with small patches of mucopolysaccharide staining
confined to the periphery (arrows). Note that collagen fibers across the suture are more
undulating than the linear organization observed in the analogous region in Figures 2b and
3a. b: Oral half of suture, Orig. magnif. x40. P, oral periosteum; B, bone; S, suture. Note the
isolated areas of mucopolysaccharide staining, mostly in the pericellular regions, and the small
number of hypertrophic chondrocytes. c: Higher power of bone at sutural margin. Orig. magnif.
x 160. B, bone of palatal process of maxilla; S, suture. Note that the areas of mucopolysaccharide staining ( C ) are separated from the suture by tissue containing cartilagelike cells but with
matrix which does not stain for mucopolysaccharides (arrows). Note also evidence of vascular
channels within the suture N ) .
Fig. 4@).
hibited decreased [35S]-sulfate incorporation and staining for alkaline phosphatase relative to controls.
Although a very limited number of comparative mor,35S1-sulfate
incorporation phological studies have been performed, their findings
(dpdccgDNA) are consistent with the data reported here. Koskinen
(1977) noted a reduction in the amount of cartilage a t
89.0 16.6
the intermaxillary suture, especially on the operated
side, following unilateral resection of the masseter and
Incisor clipped
muscles in 10-day-oldrats. Similarly, the in(9)
termaxillary suture has been shown to become fibrous
Soft diet
in rats which have undergone enucleation of the maxil(10)
50.8 6.5*?
lary molars prior to eruption (Forbes and Al-Bareedi,
1986). Finally, Ghafari (1984) reported a decrease in the
soft diet
F value
amount and intensity of staining for mucopolysaccharides at the rat intermaxillary suture following inser*Less than control groups at P < .01.
tLess than soft diet group at P < .05; less than incisor-clipped group tion of a n appliance which displaced the buccal
at P < .01.
musculature away from the maxillary molar teeth.
A possible explanation for the findings of this study
may be that the lessening of occlusal loading engenDISCUSSlON
dered by the restriction of incision or by the soft diet
This study has demonstrated changes in the morphol- altered the forces transmitted from the dentition to the
ogy of the intermaxillary suture cartilage, as well as
region of the suture, thereby changing the biomechaniindications of alterations in its metabolism, subsequent cal environment in the vicinity of the cartilage. The
to induced alterations in occlusal forces at the incisor region of the hard palate is thought to have evolved in
and molar teeth. Although no obvious differences in mammals as a mechanism for strengthening the snout
size or amount of cartilage were evident in the incisor- to withstand more strenuous occlusal loading (Thomaclipped or soft diet samples, chondrocyte hypertrophy son and Russell, 1986), with the resulting flexion of the
was diminished in these groups and was greatly atten- palate (Badoux, 1966; Johnston, 1976). Moreover, comuated in the group receiving both treatments. In addi- pressive strains have been recorded in the palatal protion, the amount of cartilage was significantly reduced cesses of the maxilla during in vivo mastication in the
in this latter group. All three experimental groups ex- dog (Kakudo and Amano, 1968).Secondary cartilage has
TABLE 2. [35S]-sulfateincorporation (mean
SD) in
control and experimental groups
Fig. 5. Alkaline phosphatase histochemistry. Orig. magnif. x40. Blackened granules at periphery of
suture indicate areas of enzyme activity. Nasal aspect of suture is at top; (a)control, (b) incisor-clipped, (el
soft diet, (d) incisor-clippedisoft diet.
been shown to arise at the edges of avian membranous
bone in response to intermittent compressive forces
(Hall, 1968a,b, 1970).Moreover, growth of other secondary cartilages, most notably the condylar cartilage of
the mandible, is known to be readily influenced by alterations in the local biomechanical environment (cf. Petrovic, 1972;Petrovic et al., 1975;Copray et al., 1985a,b).
In particular, Copray et al. ( 198513)have demonstrated
that the application of intermittent compressive forces
to condylar cartilage explants in vitro increases the rate
of matrix synthesis relative to freely growing control
explants. These results are consistent with a larger body
of data for articular cartilage (Eichelberger et al., 1952;
Akeson et al., 1958; Kostenszky and Olah, 1972; Akeson
et al., 1973; Caterson and Lowther, 1978; Radin et al.,
1978; Veldhuijzen et al., 1974; Palmoski and Brandt,
1984). In most of these studies, [35S]-sulfateincorporation has been utilized as an index of the rate of matrix
synthesis. Accordingly, the decreased incorporation
noted in the experimental groups of this study may be
indicative of a reduced rate of matrix synthesis by the
chondrocytes. However, it is possible that increased
turnover of labelled proteoglycans in these groups could
produce a similar result.
The other primary finding of this study concerns the
variable reduction in alkaline phosphatase activity at
the margins of the suture cartilage in the experimental
groups. Although its exact role in the calcification process is still debatable (cf. Ali, 1983; Wuthier and Register, 1985), the involvement of alkaline phosphatase in
tissue mineralization is well established. Accordingly,
the enzyme histochemical data may be indicative of a
decrease in the rate of mineralization at the lateral
margins of the suture in certain or all of the experimental groups and perhaps is indicative of differences in the
rate of transverse growth at the suture. Other studies
(Watt and Williams, 1951; Beecher and Corruccini, 1981)
have found growing rats fed a soft diet to have significantly narrower palatal width at the level of the first
molars (i.e., the location of the intermaxillary suture
cartilage). Since mineralization characteristically accompanies chondrocyte hypertrophy in the deeper layers
of a cartilage, the diminished staining for alkaline phosphatase in experimental groups in which chondrocyte
hypertrophy is also reduced is perhaps not surprising.
Some recent studies have even suggested that the hypertrophic chondrocyte may synthesize macromolecules that
are involved in the process of mineralization (cf. Poole
et al., 1984;Cowell et al., 1987; Boden et al., 1987).Other
instances in which the maturation of chondrocytes is
retarded or absent, such as in certain forms of dwarfism
(Boden et al., 1987)or in immobilization of a joint (Carlson et al., 19801, are also associated with reduced or
impaired endochondral bone growth. In fact, the so-called
“nonhypertrophic” stage of the condylar cartilage of the
mandible is characteristic of the mature joint at which
growth has essentially ceased (Durkin et al., 1973).However, the manner by which the biomechanical stimuli
influence the mineralization process is less well understood. Perhaps the most relevant data are those of Copray and assbciates, who analyzed alkaline phosphatase
activity in condylar cartilage explants subjected to compressive forces (Copray et al., 1985~).They found that
the application of small intermittent compressive forces
increased staining for alkaline phosphatase relative to
freely growing in vitro controls. These results for secondary cartilage, which are paralleled by similar evidence
for primary cartilage (Klein-Nulend et al., 19861, suggest that the rate of mineralization in the deeper layers
of a cartilage may be responsive to alterations in biomechanical forces.
Having said this, it cannot be concluded with certainty
that the changes in the sutural cartilage observed in
this study are attributable specifically to the alterations
in occlusion. The snout region in the rat is very complex,
with the continuously erupting incisors and the various
cartilages of the nasal complex situated in proximity to
the palate. Any of these neighboring structures may
potentially influence growth at the intermaxillary suture. For example, Stutzmann and Petrovic (1976) have
demonstrated that resection of the lateral cartilaginous
portions of the ethmoid and the cartilage between the
body and greater wings of the sphenoid results in decreased incorporation of [3H]-thymidine,lessened hypertrophy, and a reduction in the amount of cartilage at
the rat intermaxillary suture. Perhaps of greater relevance to the current study, the intermaxillary suture
forms a part of the floor of the area adjacent to the roots
of the maxillary incisors. Since repeated cutting of the
incisors has been shown to accelerate their rate of eruption (Taylor and Butcher, 1951;Ness, 1956; Chiba et al.,
19761, it is posible that the alterations at the sutural
cartilage may be related to this factor rather than to the
lack of incision. However, provisioning of rats with a
soft or finely ground diet has been shown to reduce the
rate of incisal eruption (Taylor and Butcher, 1951; Kiliaridis, 1986).Moreover, these contrasting effects on rate
of eruption have been found to produce a rate little
different from controls in animals undergoing both incisor clipping and soft diet (Kiliaridis, 1986). Thus, induced variation in the rate of incisor eruption, taken
alone, would not appear to account for the changes in
suture cartilage morphology demonstrated in this study.
This study has demonstrated that changes in dietary
consistency andor incisor amputation can result in alterations of varying magnitude in the morphology and
the metabolism of the intermaxillary suture cartilage,
with the suture becoming largely fibrous in animals
receiving both treatments. Although the complexity of
the snout region makes it difficult to attribute these
changes with certainty to a single factor, these data
provide further insight into the responsiveness of the
intermaxillary suture cartilage during postnatal life.
The findings of this study also underscore the adaptive
and sometimes transient character of secondary cartilages in the craniofacial region.
This work was supported by NIH grant DE06982 from
the National Institute for Dental Research and by
Biomedical Research Support Grant funds from the Baylor College of Dentistry. Thanks are due Trina Morgan
for technical assistance.
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