Histological and histomorphometric study of growth-related changes of cranial sutures in the animal model

INTRODUCTION

During the early development, numerous histological and morphometric changes occur in the cranial sutures the exact knowledge of which is of fundamental significance for understanding clinically relevant cranial anomalies. In this paper a histological and histomorphometric longitudinal study of the coronal, sagittal and lambdoid sutures in the rat is reported in relation to age.

MATERIAL AND METHODS

Forty-eight male Wistar rats (Rattus norvegicus Berkenhout) were raised under standard conditions. Eight animals each were sacrificed at defined time points (10, 14, 28, 42, 70, 98 days post partum) for specimen preparation. Histological preparations of the sagittal, coronal and lambdoid sutures were produced and examined morphologically and histomorphometrically (suture width, height, and area).

RESULTS

Histologically, three phases of sutural growth with characteristic structural features were found. Histomorphometry reveals a quasi linear increase in height from the 30th to the 98th day post partum. Suture width remained relatively constant in the area of dura mater and periosteum.

CONCLUSION

The sutures of the test animals studied had a similar growth behaviour primarily consisting of an increase in height with almost constant width. The three-phases of development could be demonstrated histologically in all sutures.

A Reinvestigation of Murine Cranial Suture Biology: Microcomputed Tomography versus Histologic Technique

BACKGROUND

Histology remains the standard form to analyze cranial suture in murine models, but this technique provides only limited "snapshots" of the entire suture and requires animal euthanasia with tissue destruction. Because of the bone complex microarchitecture, better methods are required to study the behavior of the cranial suture and its surrounding environment. The authors compared microcomputed tomography and histology as techniques to evaluate murine cranial sutures.

METHODS

A total of 360 microcomputed tomography images and 160 to 170 histologic sections were processed from a mouse at postnatal days 22 and 45, respectively. After euthanasia, the posterior frontal and sagittal sutures were imaged with a microcomputed tomography system and subsequently processed for histologic analysis. Quantitative analysis of two-dimensional images was performed to determine the percentage of bone in a 1-mm sample.

RESULTS

Quantitative analysis of the percentage of bone within the sutures showed identical patterns by microcomputed tomography and histology techniques. Both methods demonstrated the posterior frontal suture to have heavier fusion patterns in the anterior and endocranial portions, with variable skip areas of complete patency on the endocranial surface, ectocranial surface, or both at day 45.

CONCLUSIONS

Cranial suture fusion in the murine model is not an "all-or-none" phenomenon. The posterior frontal suture, previously thought to be completely fused on day 45 by histological analysis, showed variable fusion along the length of the suture by both methods. Quantitative assessment of the percentage of bone within the posterior frontal and sagittal sutures and morphologic assessment of these sutures demonstrated similar findings by both methods. Whereas thorough histologic evaluation of an entire suture would be extremely labor intensive and impractical, these findings help to validate microcomputed tomography as a rapid and reliable method of examining the entire suture in murine models.

Role of the osteoclast in cranial suture waveform patterning

This study investigates the role of bone resorption in defining interdigitations characteristic of cranial suture waveform. Male mice from the CD-1 (ICR) background were analyzed at six age groups (n = 5 mice per group) in order to study the ontogenetic changes of osteoclast counts using tartrate-resistant acid phosphatase-stained histological sections of sagittal sutures. Additionally, the complexity of suture lines were measured ectocranially from the same age groups (n = 5 per group) using image capture and fractal geometry (ruler dimension method). The results suggest that osteoclast resorption is a contributor to suture patterning. Specifically, osteoclasts show the greatest activity along concave suture regions at 42 and 84 days (Kruskal-Wallis test statistic = 14.9; P < or = 0.01). This coincides with significant increases incrementally in suture complexity as measured with fractal dimension at 42 and 84 days of age (ANOVA F-statistic = 19.84; P < or = 0.001). In congruence with these data, mice given osteoclast-depleting injections of alendronate show a decrease in sagittal suture complexity. Data from this experiment indicate a positive relationship between suture complexity and osteoclast count (P < 0.01; r = 76%). Increases in suture complexity and osteoclast activity occur after peak rates of cranial width growth and coincide with weaning and the transition to a hard chow diet. These data demonstrate osteoclasts along the bone margin of the cranial suture and also indicate that sutures attain their complex shape at the same age when osteoclast number is highest along concave suture margins, underscoring the role of osteoclasts in generating the suture waveform pattern.

Restricting facial bone growth with skeletal fixation: A preliminary study

INTRODUCTION

Conventional orthodontic treatment of vertical or anterior maxillary excess by growth modification can be problematic in children because of the high levels of patient compliance required. The purpose of this preliminary study was to investigate the use of rigid skeletal fixation to modify facial bone growth without compliance.

METHODS

Three 30-day old female pigs from the same litter were included in phase I. Pediatric miniplates were rigidly fixated with monocortical screws in the experimental pig to bridge the zygomaticomaxillary suture and both the frontonasal and nasomaxillary sutures, bilaterally. In the sham experimental pig, the same surgical protocol was followed, but miniplates were omitted (ie, screw placement only). In the control pig, surgery was not performed. All 3 pigs were housed and fed a normal diet under identical conditions postoperatively for 63 days; then they were killed, their right hemi-skulls were prepared for and underwent 3-dimensional coordinate landmark analysis, and en-bloc specimens from the zygomaticomaxillary, frontonasal, and nasomaxillary sutures of the left hemi-skulls underwent histologic analysis. Two 50-day-old female pigs from the same litter were used in phase II. The same experimental protocol was followed as before for the experimental pig and the sham experimental pig. Both pigs were fed a normal diet for 105 days; then they were killed, and their skulls were prepared for and underwent 3-dimensional coordinate landmark analysis.

RESULTS

Rigid plating restricted zygomaticolacrimal suture length, maxillary bone length, nasal bone length, midfacial breadth, and frontal bone length by an average of -14% to -15% (range, -4% to -36%). No growth differences were noted between the animals in maxillary height, mid-premaxillary length, bregma-lambda length, palatal lengths, or mandibular length. Also, plating the sutures produced a clear depressed concavity in the infraorbital region, altered the alignment of the infraorbital plane lateral to the concavity, inhibited the anterior migration of the maxillary tuberosity, and resulted in raised folding on the bony surface adjacent to the zygomaticomaxillary suture.

CONCLUSIONS

Rigidly fixating frontonasomaxillary and zygomaticomaxillary sutures inhibits growth of facial bones and might provide a means of restricting excess growth without having to rely on patient compliance. In addition, these altered growth patterns in the plated pig model produced similar and potentially homologous infraorbital features shared by living humans in comparison with ancestral fossil forms.

[The role of the maxillary incisors in the development of the base of the nose. Applications in dento-facial orthopedics]

The neonatal respiratory distress observed in the event of a solitary median maxillary central incisor compels us to reconsider some of the traditional concepts relative to the transverse growth of the nasal level of the face. The "container-contents" connections associating maxillary incisor odontogenesis with the development of the premaxillary and facial envelopes draw the attention to the significant geometrical and mechanical expression of this morphogenesis. They require attributing to the maxillary incisors an important motor role in this development. They lead to granting the ontogenetic bonds, between malocclusions and disturbed nasal breathing, the place they deserve, taking into account the morphological integration combining them. They eventually open a new therapeutic prospect: the optimization of the development of the growing face, with regard to the various tissue mechanics and physiologies, becoming the best guarantor for the prevention of relapse after dentofacial orthopedics.

The extracellular matrix environment in suture morphogenesis and growth

Sutures are the major bone growth sites of the craniofacial skeleton and form in response to developmental approximation of and interaction between two opposing osteogenic fronts. Premature obliteration of these craniofacial bone growth sites or craniosynostosis results in compensatory growth at other bone growth sites, with concomitant craniofacial dysmorphology. While much is now known about the growth and transcriptional factor regulation of suture formation and maintenance, little about the nature of the extracellular environment within sutures and their surrounding bones has been described. This review elucidates the nature of the sutural extracellular matrix and its role in mediating suture maintenance and growth through the regulation of cellular and biomechanical signaling.

Intracranial Volume Changes in Craniosynostotic Rabbits: Effects of Age and Surgical Correction

BACKGROUND

The premature fusion of one or more cranial sutures, termed craniosynostosis, alters normal brain growth patterns and results in compensatory changes in the cranial vault. The authors previously reported that bilateral coronal suture fusion resulted in a reduction in intracranial volume in a rabbit model of nonsyndromic, familial coronal suture synostosis.

METHODS

The current follow-up study involved collecting cross-sectional three-dimensional computed tomographic head scans from 142 rabbits (70 normal, 44 with uncorrected synostosis, and 28 synostosed rabbits with coronal suturectomy) at 0, 10, 25, 42, 84, and 126 days of age. Intracranial contents were reconstructed, and indirect intracranial volume was calculated.

RESULTS

Results revealed a significant (p < 0.05) postsynostotic reduction of intracranial volume (23 percent) by 25 days of age in rabbits with uncorrected craniosynostosis compared with normal controls, which continued through 84 days of age. Also, rabbits with surgically released synostosis, using a simple strip suturectomy, showed significantly (p < 0.05) greater intracranial volume at 25 days of age compared with unoperated synostosed rabbits. However, no changes in intracranial volume were noted between 42 and 84 days of age in rabbits with surgically released synostosis, at which point their intracranial volume was 30 percent less than that in normal control rabbits.

CONCLUSIONS

These data suggest that in rabbits with uncorrected craniosynostosis, compensatory changes in the neurocranium were not capable of compensating for the loss of sutures as growth sites. The results also showed that that surgical release of the synostosed suture improved intracranial volume in the short term (25 to 42 days) but failed to change it in the long term (42 to 84 days), possibly because of rapid resynostosis of the suturectomy site. This study suggests that surgical release of the suture fusion site alone may not be adequate to allow for normal intracranial volume growth in synostotic rabbits. For this reason, it may be efficacious to design and develop adjunct protein and gene therapies to prevent resynostosis and improve postoperative intracranial volume in craniosynostotic individuals.

Geometry and cell density of rat craniofacial sutures during early postnatal development and upon in vivo cyclic loading

Cranial sutures are unique to skull bones and consist of multiple connective tissue cell lineages such as mesenchymal cells, fibroblast-like cells, and osteogenic cells, in addition to osteoclasts. Mechanical modulation of intramembranous bone growth in the craniofacial suture is not well understood, especially during postnatal development. This study investigated whether in vivo mechanical forces regulate sutural growth responses in postnatal rats. Cyclic compressive forces with a peak-to-peak magnitude of 300 mN and 4 Hz were applied to the maxilla in each of 17-, 23-and 32-day-old rats for 20 min/day over 5 consecutive days. Computerized histomorphometric analysis revealed that cyclic loading significantly increased the average geometric widths of the premaxillomaxillary suture (PMS) to 86 +/- 7 microm, 99 +/- 12 microm, and 149 +/- 30 microm, representing 32%, 50%, and 39% increases for P17, P23, and P32 in comparison with age-matched sham controls. For the nasofrontal suture (NFS), cyclic loading significantly increased the average sutural widths to 88 +/- 15 microm, 92 +/- 10 microm, and 100 +/- 14 microm, representing 33%, 24%, and 32% increases for P17, P23, and P32 relative to age-matched controls. The average PMS cell density upon cyclic loading was 10182 +/- 132 cells/mm(2), 9752 +/- 661 cells/mm(2), and 9521 +/- 628 cells/mm(2), representing 62%, 35%, and 30% increases for P17, P23, and P32 in comparison with age-matched controls. For the NFS, cyclic loading increased the average cell density to 9884 +/- 893 cells/mm(2), 9818 +/- 1091 cells/mm(2), 9355 +/- 661 cells/mm(2), representing 44%, 46% and 40% increases at P17, P23, and P32 respectively. Osteoblast-occupied sutural bone surface was significantly greater in cyclically loaded sutures for P17, P23, and P32 than corresponding controls for both the PMS and NFS. On the other hand, cyclic loading elicited significantly higher sutural bone surface populated by osteoclast-like cells by P17 and P23 days, but not P32 days, for the PMS. For the NFS, sutural osteoclast surface was significantly higher upon cyclic loading for P23 and P32 days, but not P17. The present data demonstrate that cyclic forces are potent stimuli for modulating postnatal sutural development, potentially by stimulating both osteogenesis and osteoclastogenesis. Cyclic loading may have clinical implications as novel mechanical stimuli for modulating craniofacial growth in patients suffering from craniofacial anomalies and dentofacial deformities.

FGF signalling in craniofacial development and developmental disorders

The Fgf signalling pathway is highly conserved in evolution and plays crucial roles in development. In the craniofacial region, it is involved in almost all structure development from early patterning to growth regulation. In craniofacial skeletogenesis, the Fgf signal pathway plays important roles in suture and synchondrosis regulation. Mutations of FGF receptors relate to syndromatic and non-syndromatic craniosynostosis. The Fgf10/Fgfr2b signal loop is critical for palatogenesis and submandibular gland formation. Perturbation of the Fgf signal is a possible mechanism of palatal cleft. Fgf10 haploinsufficiency has been identified as the cause of autosomal dominant aplasia of lacrimal and salivary glands. The Fgf signal is also a key regulator of tooth formation: in the absence of Fgfr2b tooth development is arrested at the bud stage. Fgfr4 has recently been identified as the key signal mediator in myogenesis. In this review, these aspects are discussed in detail with a focus on the most recent advances.

Effects of the LactoSorb bioabsorbable plates on the craniofacial development of rabbits: direct morphometric analysis using linear measurements

The use of plates for the treatment of fractures can affect craniofacial bone development. This study investigated the effect of bioabsorbable plates and titanium microscrews on the growth of the craniofacial skeleton of rabbits (Oryctolagus cuniculus), in the neonatal period. A LactoSorb plate and PROMM titanium microscrews were positioned across the coronal suture in animals in the study group. In the control group, only PROMM titanium microscrews were attached to the cranium. Anteroposterior linear measurements were obtained using 3 different gauging devices: digital precision caliper, EKG caliper and nylon string. Frontal-nasal (FN) distances were statistically different between the left and right side when the digital caliper (P<or=0.01), EKG caliper (P<or=0.01) and nylon string (P<or=0.05) were used. Mean right-side FN distances in the control and study groups were significantly different (P<or=0.05) when measured with the EKG caliper. Normal neonatal craniofacial growth of rabbits was affected by the use of bioabsorbable plates in this study. Further studies should be conducted to investigate whether these plates affect growth in human beings.

Functional influence on sutural bone apposition in the growing rat

INTRODUCTION

The aim of this study was to quantify the influence of reduced masticatory muscle function on sutural bone apposition in the growing rat.

METHODS

Twenty-six growing male albino rats were randomly divided into 2 equal groups; 1 (hard-diet group) received the ordinary diet of hard pellets, and the other (soft-diet group) received the powdered form of the ordinary diet mixed with water. The experimental period started when the rats were 4 weeks old and lasted 42 days. At days 0, 14, and 28, calcein was injected into all animals. At the end of the experiment, the animals were killed, and the heads were taken for preparation of undecalcified frontal sections, 120 microm thick. Three representative homologous sections for each animal in both groups were selected and studied under a fluorescence microscope. The level of bone apposition at the time of calcein injection was marked with separate fluorescing lines. Because the lines from the first injection could not be seen in all areas, bone apposition in the internasal, naso-premaxillary, and inter-premaxillary sutures was quantified from day 14 to the end of the experimental period, by using an image analysis software.

RESULTS

In both groups, greater bone apposition was found between days 14 and 28 than between days 28 and 42 of the experimental period. Less bone apposition was found in the soft-diet group than in the hard-diet group in all sutures studied.

CONCLUSIONS

The findings suggest that bone apposition in the studied facial sutures in the anterior facial skeleton of the growing rat is significantly affected by reduced masticatory function.

[Growth of the skull and bony kinetics interfering with facial morphogenesis. Conceptual bases of success in orthopedic treatments before the age of 6]

In order to perform an orthopedic treatment without relapse, one needs to identify properly the cranial patterns responsible for the malocclusion and act on them before the age of 6.

Premature fusion and excessive calcification of coronal sutures in patients with Klinefelter syndrome

OBJECTIVE

The purpose of this paper is to report on radiographic features of the cranial vault abnormalities frequently seen but not yet described in detail in patients with Klinefelter syndrome.

SUBJECTS

Our studies comprised 72 patients with Klinefelter syndrome and 47,XXY karyotype. The majority of the patients were young between the ages of 16 to 28 years.

METHODS

Plain skull radiographs were taken in the frontal and lateral projections.

RESULTS

Several abnormalities were observed on skull radiographs of the Klinefelter patients. The most frequent was a premature fusion of the coronal sutures that occurred in 54 of 62 younger patients. In 42 cases extensive calcifications of these sutures were observed. The calcified sutures were dense and widened. In 24 patients the inner table in the anterior part or the parietal bone displayed a marked disruption with thinning of the calvaria at this place. In the posterior part of the parietal bone in 21 patients the inner table was thickened and excessively dense. On frontal radiographs, in 30 patients, there was a flattening of the squamous part of the temporal bones.

CONCLUSIONS

Skull radiographs in Klinefelter patients frequently display abnormalities: premature fusion and excessive calcifications of the coronal sutures, irregularities of the inner table of the parietal bone, and flattening of the temporal bones. These radiographic features, when present, may draw attention to the XXY syndrome as the underlying cause of the abnormalities.

Twist1 dimer selection regulates cranial suture patterning and fusion

Saethre-Chotzen syndrome is associated with haploinsufficiency of the basic-helix-loop-helix (bHLH) transcription factor TWIST1 and is characterized by premature closure of the cranial sutures, termed craniosynostosis; however, the mechanisms underlying this defect are unclear. Twist1 has been shown to play both positive and negative roles in mesenchymal specification and differentiation, and here we show that the activity of Twist1 is dependent on its dimer partner. Twist1 forms both homodimers (T/T) and heterodimers with E2A E proteins (T/E) and the relative level of Twist1 to the HLH inhibitor Id proteins determines which dimer forms. On the basis of the expression patterns of Twist1 and Id1 within the cranial sutures, we hypothesized that Twist1 forms homodimers in the osteogenic fronts and T/E heterodimers in the mid-sutures. In support of this hypothesis, we have found that genes regulated by T/T homodimers, such as FGFR2 and periostin, are expressed in the osteogenic fronts, whereas genes regulated by T/E heterodimers, such as thrombospondin-1, are expressed in the mid-sutures. The ratio between these dimers is altered in the sutures of Twist1+/- mice, favoring an increase in homodimers and an expansion of the osteogenic fronts. Of interest, the T/T to T/E ratio is greater in the coronal versus the sagittal suture, and this finding may contribute to making the coronal suture more susceptible to fusion due to TWIST haploinsufficiency. Importantly, we were able to inhibit suture fusion in Twist1+/- mice by modulating the balance between these dimers toward T/E formation, by either increasing the expression of E2A E12 or by decreasing Id expression. Therefore, we have identified dimer partner selection as an important mediator of Twist1 function and provide a mechanistic understanding of craniosynostosis due to TWIST haploinsufficiency.

Apoptosis in membranous bone formation: role of fibroblast growth factor and bone morphogenetic protein signaling

Membranous ossification occurs by the condensation of mesenchymal cells followed by their progressive differentiation into osteoblasts that form a mineralized matrix in ossification centers. The balance between proliferating and differentiated osteogenic cells at the suture areas between calvarial bones is essential for the control of suture maintenance and membranous bone formation. The mechanisms of regulation of na apoptosis in suture areas begin to be understood. Fibroblast growth factor (FGF) and bone morphogenetic protein (BMP) are important regulators of mesenchymal, preosteoblast, and osteoblast apoptosis in suture areas. Perturbations in FGF or BMP signaling lead to alter the number of apoptotic osteogenic cells, resulting in premature or delayed suture closure. Recent data indicate that FGF signaling downregulates preosteoblast apoptosis, thereby preventing premature fusion of adjacent mineralizing extremities. In contrast, continuous FGF signaling or constitutive FGF receptor activation, as well as BMP signaling, upregulate osteoblast apoptosis. Additionally, multiple signaling mechanisms, including PI3K and PKC, appear to be involved in the control of calvarial osteoblast apoptosis by FGF and BMP. These mechanisms allow a fine control of the number of functional bone-forming cells and, thereby, the normal progression of membranous bone formation.

Expression of matrix metalloproteinase genes in the rat intramembranous bone during postnatal growth and upon mechanical stresses

A cranial suture consists of neural-crest derived cells and matrices between mineralized skull bones. Little is known regarding the involvement of matrix metalloproteinases (MMPs) in the degradation of extracellular matrix of cranial sutures. In the postnatal rat model, the posterior frontal suture (PFS) undergoes complete ossification between P12-P22, whereas the sagittal suture (SS) remains patent. The present study utilized reverse transcriptase-polymerase chain reaction (RT-PCR) to explore the expression of MMP-1 and MMP-2 genes in the PFS and SS in P8 and P32 rats, and also to determine whether these MMP genes are modulated by exogenous mechanical forces. RNA was isolated from P8 and P32 normal PFS and SS each by pooling sutural specimens from 14 to 20 rats. RT-PCR analysis and semi-quantitative luminosity demonstrated the expression of MMP-1 and MMP-2 genes in the patent P8 PFS, P8 SS, and P32 SS, but no apparent MMP-2 expression in the physiologically ossified P32 PFS. Exogenous cyclic forces applied to the maxilla at 1000 mN and 4 Hz elicited corresponding cyclic bone strain waveforms with peak strain of 134.14+/-38.15 muepsilon (mean+/-S.D.) for the PFS, and 28.35+/-10.86 muepsilon for the SS in P32 rats. These cyclic forces delivered for 20 min/d over 2 consecutive days induced the expression of MMP-2 gene in the physiologically fused P32 PFS that was not expressed without mechanical stresses. Taken together, these data suggest potentially important roles of MMP genes in the postnatal development of cranial sutures, and their susceptibility to mechanical stresses.

Gene Expression Profiling in the Rat Cranial Suture

Although many theories have attempted to explain the etiopathogenesis of premature cranial suture fusion, which results in craniosynostosis, recent studies have focused on the role of growth factors and receptors. Using a well-established model of cranial suture biology, the authors developed a novel approach to quantitatively analyze the gene expression profiles of candidate cranial suture growth factors and their receptors. We collected suture mesenchyme and adjacent osteogenic fronts from Sprague-Dawley rats at postnatal days 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, and 35. RNA was extracted from posterior frontal (PF) and sagittal (SAG) sutures, and reverse transcription-polymerase chain reaction (RT-PCR) was performed for cranial suture candidate cytokines BMP2, BMP3, BMP4, FGF-2, FGFR1, FGFR2, FGFR4, TGF-betaRI, TGF-betaRII, and TGF-betaRIII. The authors confirmed quantitative RT-PCR results with Southern and dot blot analyses. Suture growth factor and receptor expression levels changed significantly with time. Expression levels decreased toward baseline in the SAG suture by day 35. There was a marked difference in FGFR1, FGF-2, TGF-betaRI, and TGF-betaRII expression levels when comparing the fusing PF and nonfusing SAG sutures. Although FGF-2 ligand expression was low, FGF receptor 1 (FGFR1) levels were markedly elevated with a bimodal expression pattern in both PF and SAG similar to that of BMP2, BMP3, and BMP4. Although there were statistically significant differences in TGF-betaRI and TGF-betaRII expression in the PF and SAG sutures, TGF-betaRIII levels were unchanged. The authors report a novel approach to cranial suture growth factor/receptor profiling and confirm their results with standard analytic tools. The data confirm, quantify, and extend the results of previously published studies. By quantifying the gene expression profiles of normal cranial suture biology, we may begin to understand the aberrant growth factor cascades of craniosynostosis and devise targeted therapeutic interventions that can alter the course of this malady.

The persistence of an open anterior fontanel in a 4-year-old girl

CASE REPORT

A 3-year-old girl was transferred to our hospital with a history of persistent open anterior fontanel. The patient was conscious and had no neurological deficits. Upon arrival, the patient appeared normal for her age and had no defects or anomalies other than the aforementioned lesion. The initial skull X-ray and CT were significant for a 20-mm open anterior fontanel. All other findings were normal.

OUTCOME

After a follow-up period of 1.5 years, the anterior fontanel was still open, with a slight decrease in size to 15 mm. Delayed closure of the anterior fontanel without intracranial hypertension is associated with various disorders. The pathogenesis of the current patient's condition is unclear. Due to the patient's normal appearance and stable neurological status, we will follow her conservatively for any changes in condition.

The Zebrafish (Danio rerio): A Model System for Cranial Suture Patterning

The zebrafish (Danio rerio) is an alluring model system currently used to study early embryonic development, organogenesis and gene functional analysis. However, few studies have been devoted to post-embryonic development. We have explored the possibility of using this organism to analyze how cranial suture patterning occurs. This study reports on the establishment of the zebrafish skull vault anatomy, calvarial osteogenesis, and cranial suture morphology. Our results demonstrate that the anatomy of the zebrafish cranial vault and cranial sutures is very similar to that of mammalian organisms. Indeed, the zebrafish represents a versatile and valuable model system for the study of the biogenesis of cranial sutures.

Cranial sutures and bones: growth and fusion in relation to masticatory strain

Cranial bones and sutures are mechanically loaded during mastication. Their response to masticatory strain, however, is largely unknown, especially in the context of age change. Using strain gages, this study investigated masticatory strain in the posterior interfrontal and the anterior interparietal sutures and their adjacent bones in 3- and 7-month-old miniature swine (Sus scrofa). Double-fluorochrome labeling of these animals and an additional 5-month group was used to reveal suture and bone growth as well as features of suture morphology and fusion. With increasing age, the posterior interfrontal suture strain decreased in magnitude and changed in pattern from pure compression to both compression and tension, whereas the interparietal suture remained in tension and the magnitude increased unless the suture was fused. Morphologically, the posterior interfrontal suture was highly interdigitated at 3 months and then lost interdigitation ectocranially in older pigs, whereas the anterior interparietal suture remained butt-ended. Mineralization apposition rate (MAR) decreased with age in both sutures and was unrelated to strain. Bone mineralization was most vigorous on the ectocranial surface of the frontal and the parietal bones. Unlike the sutures, with age bone strain remained constant while bone MARs significantly increased and were correlated with bone thickness. Fusion had occurred in the interparietal suture of some pigs. In all cases fusion was ectocranial rather than endocranial. Fusion appeared to be associated with increased suture strain and enhanced bone growth on the ectocranial surface. Collectively, these results indicate that age is an important factor for strain and growth of the cranium. .

Regulation of cranial suture morphogenesis

The cranial sutures are the primary sites of bone formation during skull growth. Morphogenesis and phenotypic maintenance of the cranial sutures are regulated by tissue interactions, especially those with the underlying dura mater. Removal of the dura mater in fetuses causes abnormal suture development and premature suture obliteration. The dura mater interacts with overlying tissues of the cranial vault by providing: (1) intercellular signals, (2) mechanical signals and (3) cells, which undergo transformation and migrate to the suture. The intercellular signaling governing suture development employs the fibroblast growth factors (FGFs). In rats during formation of the sutures in the fetus, FGF-1 is localized mainly in the dura mater, while other FGFs are expressed in the overlying tissues. By birth, FGF-2 largely replaces FGF-1 in the dura mater. FGFs present in the calvaria bind either the IIIb or IIIc mRNA splice variants of the FGF receptors (FGFRs) 1, 2, or 3. Monoclonal antibodies to the b variant of FGFR2 were used to determine the distribution of FGFR2IIIb during suture development and its extracellular localization. FGFR2IIIb is present in association with mature osteoblasts and osteogenic precursor cells of the suture in the fetus. Ectodomains of FGFR2IIIb, the products of proteolytic cleavage of the receptors, were present throughout the extracellular matrix of sutures resisting obliteration (coronal and sagittal), but absent from the core of sutures undergoing normal fusion (posterior intrafrontal). This observation is consistent with a possible mechanism, in which truncated receptors bind FGFs, thus regulating free FGF available to nearby cells. Mechanical signaling in the calvaria results from tensional forces in the dura mater generated during rapid expansion of the neurocranium. Posterior intrafrontal sutures of rats, which fuse between days 16 and 24, were subjected to cyclical tensional forces in vitro. Significant delay in the timing of suture fusion and increases in the expression domains of FGFR1 and 2 were observed, demonstrating the sensitivity of suture patency to mechanical signals and a possible role of the FGF system in mediating such stimuli. Finally, cells of the dura mater beneath the intrafrontal and sagittal sutures were observed to undergo a morphological transformation to a dendritic morphology and migrate into the suture mesenchyme between days 10 and 16 of development. This process may participate in suture and bone morphogenesis and influence the patency of the sutures along the anterior-posterior axis.

[Effects of the directions of an extraoral retractive force on the displacement of and stress distribution in palate]

OBJECTIVE

The aim of this study was to assess the effects of varying retractive force directions on the displacement of and stress distributions in palate.

METHODS

A three-dimensional finite element model of the nasomaxillary complex was created, consisting of 1638 isoparametric elements and 1827 nodes. An extraoral retractive force of 9.8N was applied to the upper canines in 3 different directions, i.e. parallel, 30 degrees upwards and downwards to the functional occlusal plane. Then we investigated the nature of stress distributions, the directions and amounts of displacement in palate by finite element analysis.

RESULTS

In the midpalatal suture and transpalatal suture, all displacement directed to bone suture and increased gradually when the retractive force direction ranged from +30 degrees to -30 degrees. In the midmost part of palate, correlation between stress distributions and retractive force directions was not significant. Relatively compressive stresses were induced in the front and middle of the midpalatal suture, whereas nearly no stress was induced in the back. In the lateral part of palate, compressive stresses were nearly equal to tensile stresses in the front, while tensile stresses were induced in the middle and back. Meanwhile, there was no significant correlation between stress distributions and force directions.

CONCLUSION

This study showed that the directions of retractive force had obvious effects on displacement but but little effect on stress distributions.

Nanomechanical properties of facial sutures and sutural mineralization front

The mechanical properties of craniofacial sutures have rarely been investigated. Three facial sutures-the pre-maxillomaxillary (PMS), the nasofrontal (NFS), and the zygomaticotemporal (ZTS)-and their corresponding sutural mineralization fronts in 8 young New Zealand White rabbits were subjected to nano-indentation with atomic force microscopy as a test of the hypothesis that they have different mechanical properties. The average elastic modulus of the PMS was 1.46 +/- 0.24 MPa (mean +/- SD), significantly higher than both the ZTS (1.20 +/- 0.20) and NFS (1.16 +/- 0.18). The average elastic moduli of sutural mineralization fronts 30 micro m away were significantly higher than their corresponding sutures and had the same distribution pattern: the PMS (2.07 +/- 0.24 MPa) significantly higher than both the ZTS (1.56 +/- 0.29) and NFS (1.71 +/- 0.22). Analysis of these data suggests that facial sutures and their immediately adjacent sutural mineralization fronts have different capacities for mechanical deformation. The elastic properties of sutures and sutural mineralization fronts are potentially useful for improving our understanding of their roles in development.

Growth and development: hereditary and mechanical modulations

Growth and development is the net result of environmental modulation of genetic inheritance. Mesenchymal cells differentiate into chondrogenic, osteogenic, and fibrogenic cells: the first 2 are chiefly responsible for endochondral ossification, and the last 2 for sutural growth. Cells are influenced by genes and environmental cues to migrate, proliferate, differentiate, and synthesize extracellular matrix in specific directions and magnitudes, ultimately resulting in macroscopic shapes such as the nose and the chin. Mechanical forces, the most studied environmental cues, readily modulate bone and cartilage growth. Recent experimental evidence demonstrates that cyclic forces evoke greater anabolic responses of not only craniofacial sutures, but also cranial base cartilage. Mechanical forces are transmitted as tissue-borne and cell-borne mechanical strain that in turn regulates gene expression, cell proliferation, differentiation, maturation, and matrix synthesis, the totality of which is growth and development. Thus, hereditary and mechanical modulations of growth and development share a common pathway via genes. Combined approaches using genetics, bioengineering, and quantitative biology are expected to bring new insight into growth and development, and might lead to innovative therapies for craniofacial skeletal dysplasia including malocclusion, dentofacial deformities, and craniofacial anomalies such as cleft palate and craniosynostosis, as well as disorders associated with the temporomandibular joint.