Response of the intermaxillary suture cartilage to alterations in masticatory function.код для вставкиСкачать
THE ANATOMICAL RECORD 220:376-387 (1988) Response of the lntermaxillary Suture Cartilage to Alterations in Masticatory Function ROBERT J . HINTON Department of Anatomy, Baylor College of Dentistry, Dallas, TX 75246 ABSTRACT 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 R. LISS, INC. 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. INTERMAXILLARY SUTURE CARTILAGE 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 cartilage. MATERIALS AND METHODS 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. 377 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 margins. INTERMAXILLARY SUTURE CARTILAGE 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. R ESULTS Histology 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. 3b). 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 379 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 noted. 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 lowest. 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. 380 R.J. HINTON 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 . INTERMAXILLARY SUTURE CARTILAGE 381 Fig. 3%). TABLE 1. Widths and area of intermaxillary suture cartilage in control and experimental groups' Width Control Incisor clipped Soft diet Incisor-clipped soft diet F value oral (pm) nasal (pm) Area (pm2) 218.7 f 16.6 (4) 200.7 & 39.8 (5) 201.5 & 36.4 (3) 159.5 k 16.9 (3) 2.21 296.2 & 17.6 (4) 232.1 & 45.4 (5) 241.7 f 17.3 (3) 220.3 31.0* (3) 4.05 129,654.9 k 15,429.6 (4) 106,705.2 k 38,594.4 (5) 99,819.5 k 48,337.4 (3) 31,498.8 f 18,995.4**.t (3) 5.36 '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 ) . INTERMAXILLARY SUTURE CARTILAGE 383 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 Group (1977) noted a reduction in the amount of cartilage a t 89.0 16.6 Control the intermaxillary suture, especially on the operated (9) side, following unilateral resection of the masseter and Incisor clipped 69.6 temporalis muscles in 10-day-oldrats. Similarly, the in(9) 67.0 11.2* termaxillary suture has been shown to become fibrous Soft diet in rats which have undergone enucleation of the maxil(10) Incisor-clipped 50.8 6.5*? lary molars prior to eruption (Forbes and Al-Bareedi, 1986). Finally, Ghafari (1984) reported a decrease in the soft diet (10) F value 17.35 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 f 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. INTERMAXILLARY SUTURE CARTILAGE 385 386 R.J. HINTON 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. CONCLUSIONS 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. ACKNOWLEDGMENTS 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. INTERMAXILLARY SUTURE CARTILAGE LITERATURE CITED Akeson, W.H., L. Eichelberger, and M. Roma 1958 Biochemical studies of articular cartilage. 11. Values following the denervation of a n extremity. J. Bone Joint Surg., 40:153-162. Akeson, W.H., S.L.Y. Woo, D. Amiel, R.D. Coutts, and D. 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