Light and electron microscopy of the new born sagittal suture

Sensitivity and specificity of magnetic resonance imaging and computed tomography for the determination of the developmental state of cranial sutures and synchondroses in the dog

BACKGROUND

During skull ontogenesis, growth centers in the skull base and calvarial bones allow gradual expansion of the cranial vault. Premature growth termination of cranial base synchondroses and/or calvarial sutures can result in devastating skull dysmorphologies. There is evidence to believe that a premature closure in one or more cranial growth centers contribute to the brachycephalic skull morphology in dogs. To provide a proof of concept for the non-invasive investigation of ontogenetic changes in cranial sutures and synchondroses in living dogs, we compared magnet resonance imaging (MRI) and computed tomography (CT) with histologic findings. Our aim was to determine the in vitro sensitivity and specificity for conventional clinical imaging methods in the assessment of cranial suture closure and synchondroses ossification in dogs.

RESULTS

The evaluation of cranial base synchondroses in MRI had a sensitivity of up to 93.1% and a specificity of 72.7% dependent on the observer. The evaluation of cranial base synchondroses in CT had a sensitivity of 92.2% and a specificity of 86.4%. Suture assessment on MRI suture assessment had a sensitivity of 82.1% dependent on the observer and a specificity of 19.3%. CT suture assessment had a sensitivity of 85.1% and a specificity of 40.4% in dependence of the observer.

CONCLUSION

Conventional cross-sectional imaging techniques (MRI and CT) allow reliable assessment of the open or closed state of synchondroses within the cranial base. In contrast CT and MRI are not suitable for a reliable assessment of the cranial sutures in dogs.

Unilateral Coronal Synostosis: A Histomorphometric Study

Objective

This histomorphometric study compared the open and prematurely fused side of the coronal suture in subjects with unilateral coronal synostosis (UCS).

Methods

Sutures and parasutural bone were obtained from seven subjects with nonsyndromic UCS during operative correction at 3 to 24 months of age. Histological and cellular analyses were performed for the affected and open sutures. Specimens were examined by light and polarizing microscopy. Sutural patterns, osseous morphology, calvarial thickness, tartrate-resistant acid phosphatase (TRAP)-positive cells, and marrow spaces were evaluated histomorphologically, qualitatively, and semiquantitatively. Histomorphometry was performed to determine total projected area of marrow space as a percentage of unit area, total number of TRAP-positive cells per specimen, and perisutural cranial thickness.

Results

Polarizing microscopy showed that affected sutures were composed of more lamellar bone than the normal sutures. By light microscopy, the clinically fused sutures were 1.7-fold thicker (p < .02), had twofold larger marrow spaces (p < .0006), and contained sixfold more TRAP-positive osteoclasts in marrow spaces near the suture (p < .04) than the normal sutures. Quantitative analysis of the normal sutures revealed that calvarial thickness was greater with age and that there was an inverse correlation between medullary area and age. For the affected sutures, there was also an age-related increase in calvarial thickness. There were also trends for age-related declines in numbers of osteoclasts in both open and affected sides.

Conclusions

These results question the hypothesis that defective osteoclastic activity is pivotal in the pathogenesis of UCS and support the hypothesis that this condition results from abnormally active bony remodeling.

Cell Mechanics of Craniosynostosis

Craniosynostosis is the premature fusion of the calvarial sutures that is associated with a number of physical and intellectual disabilities spanning from pediatric to adult years. Over the past two decades, techniques in molecular genetics and more recently, advances in high-throughput DNA sequencing have been used to examine the underlying pathogenesis of this disease. To date, mutations in 57 genes have been identified as causing craniosynostosis and the number of newly discovered genes is growing rapidly as a result of the advances in genomic technologies. While contributions from both genetic and environmental factors in this disease are increasingly apparent, there remains a gap in knowledge that bridges the clinical characteristics and genetic markers of craniosynostosis with their signaling pathways and mechanotransduction processes. By linking genotype to phenotype, outlining the role of cell mechanics may further uncover the specific mechanotransduction pathways underlying craniosynostosis. Here, we present a brief overview of the recent findings in craniofacial genetics and cell mechanics, discussing how this information together with animal models is advancing our understanding of craniofacial development.

Pathology in metopic synostosis

Premature closure and subsequent ossification of the metopic suture results in triangular head shape called trigonocephaly and is characterized by a midline metopic ridge, frontotemporal narrowing, and an increased biparietal diameter. Trigonocephaly is the second most frequent type of craniosynostosis. It can be isolated and associated with other congenital anomalies without any known syndrome, or occurs as part of a multiple malformation syndrome. Improvement in treatment is directed by a thorough understanding of the basic pathology of this condition. This review aims to provide an overview of metopic synostosis by correlating what is known about pathogenesis and pathology of this entity.

Cranial suture biology and dental development: genetic and clinical perspectives

Premature fusion of the calvarial bones at the sutures, or craniosynostosis (CS), is a relatively common birth defect (1:2000-3000) frequently associated with limb deformity. Patients with CS may present oral defects, such as cleft soft palate, hypodontia, hyperdontia, and delayed tooth eruption, but also unusual associations of major dental anomalies such as taurodontism, microdontia, multiple dens invaginatus, and dentin dysplasia. The list of genes that are involved in CS includes those coding for the different fibroblast growth factor receptors and a ligand of ephrin receptors, but also genes encoding transcription factors, such as MSX2 and TWIST. Most of these genes are equally involved in odontogenesis, providing a pausible explanation for clinical associations of CS with dental agenesis or tooth malformations. On the basis of the present knowledge on genes and transcription factors that are involved in craniofacial morphogenesis, and from dental clinics of CS syndromes, the molecular mechanisms that control suture formation and suture closure are expected to play key roles in patterning events and development of teeth. The purpose of this article is to review and merge the recent advances in the field of suture research at the genetic and cellular levels with those of tooth development, and to apply them to the dental clinics of CS syndromes. These new perspectives and future challenges in the field of both dental clinics and molecular genetics, more in particular the identification of possible candidate genes involved in both CS and dental defects, are discussed.

Bone Regeneration in Athymic Calvarial Defects With Accell DBM100

Bioimplants containing bone morphogenetic proteins (BMP) such as demineralized bone matrix (DBM) are used clinically to repair bone defects because of their ability to stimulate bone regeneration. Because of handling issues, DBM granules are often combined with an inert carrier, which reduces the DBM content to 40% or less by volume. Recently, Accell DBM100 (Accell, IsoTis OrthoBiologics, Irvine, CA) has been developed, which uses processed DBM as the carrier, resulting in a DBM content of 100%. The purpose of this investigation was to evaluate the use of Accell for bone defect healing.Forty-two athymic male rats were divided into three groups. Bilateral 5 mm calvarial defects were created in each animal. In group 1, one defect was filled with Accell and the other defect was left unfilled (control). In group 2, one defect was filled with OP-1 putty (recombinant human BMP-7 and type I collagen), and the other was left unfilled. In group 3, one defect was filled with Accell and the other with OP-1. Animals were sacrificed at 4 and 8 weeks, postoperatively. Specimens were analyzed by histomorphometry to evaluate bone regeneration quantitatively. Accell and OP-1 both induced significantly more bone at 4 and 8 weeks compared with the unfilled contralateral defects. OP-1-filled defects produced significantly more total reparative tissue (bone + marrow) compared with Accell (P < 0.01); however, the increase in new bone did not reach significance at either time (P = 0.06 at 4 wk; P = 0.10 at 8 wk). In conclusion, these results suggest that Accell DBM100 will be useful in repairing craniofacial bone defects clinically.

FGF-2 Acts through an ERK1/2 Intracellular Pathway to Affect Osteoblast Differentiation

An abundance of genetic and experimental data have suggested that fibroblast growth factor (FGF) signaling plays a central role in physiological and pathological cranial suture fusion. Although alterations in the differentiation and proliferation of sutural osteoblasts may be a key mediator of this process, the mechanisms by which FGF signaling regulates osteoblast differentiation remain incompletely understood. In the current study, the authors show that recombinant human FGF-2 alters osteoblastic expression of bone morphogenetic protein-2 and Msx-2 in vitro to favor cellular differentiation and osteoinduction. The ERK1/2 intracellular signaling cascade was shown to be necessary for recombinant human FGF-2-mediated bone morphogenetic protein-2 transcriptional changes. Furthermore, the cellular production of an intermediate transcriptional modifier was found to be necessary for the recombinant human FGF-2-mediated gene expression changes in bone morphogenetic protein-2 and Msx-2. Together, these findings offer new insight into the mechanisms by which FGF-2 modulates osteoblast biology.

Expression of stromelysin-1 (MMP-3), gelatinase B (MMP-9), and plasminogen activator system during fetal calvarial development

AIMS

To investigate whether degrading proteases can be found in patent calvarial sutures. Sutural growth and fusion means replacement of the sutural connective tissue, rich in fibronectin and collagen type V, by expanding calvarial bone. Proliferation of one tissue into the border area of another implies the presence of enzymes able to degrade extracellular matrix (ECM). An important family of proteases is the matrix metalloproteinases (MMPs), as is the plasminogen/plasmin system.

METHODS AND RESULTS

Expression of two MMPs with substrate specifity for fibronectin and collagen type V and of the plasminogen activator system was studied by immunohistochemistry in samples of human fetal calvariae (age range weeks 19-35 of gestation). In all cases, intense staining for MMPs, urokinase, and urokinase receptor was found in the sutural connective tissue and along the outer and inner borders of calvarial bone.

CONCLUSIONS

Our findings suggest that degradation of sutural connective tissue takes place during sutural growth. This might facilitate proliferation of calvarial bone. Recently, it was shown that an important regulatory mechanism of sutural growth is apoptosis of osteoblasts in the osteogenic front. Intact fibronectin is known to prevent apoptosis of proliferating osteoblasts while fibronectin degradation induces their apoptosis.

BMP signals regulate Dlx5 during early avian skull development

The vertebrate skull vault forms almost entirely by the direct mineralisation of mesenchyme, without the formation of a cartilaginous template, a mechanism called membranous ossification. Dlx5 gene mutation leads to cranial dismorphogenesis which differs from the previously studied craniosynostosis syndromes [Development 126 (1999), 3795; Development 126 (1999), 3831]. In avians, little is known about the genetic regulation of cranial vault development. In this study, we analyze Dlx5 expression and regulation during skull formation in the chick embryo. We compare Dlx5 expression pattern with that of several genes involved in mouse cranial suture regulation. This provides an initial description of the expression in the developing skull of the genes encoding the secreted molecules BMP 2, BMP 4, BMP 7, the transmembrane FGF receptors FGFR 1, FGFR 2, FGFR 4, the transcription factors Msx1, Msx2, and Twist, as well as Goosecoid and the early membranous bone differentiation marker osteopontin. We show that Dlx5 is activated in proliferating osteoblast precursors, before osteoblast differentiation. High levels of Dlx5 transcripts are observed at the osteogenic fronts (OFs) and at the edges of the suture mesenchyme, but not in the suture itself. Dlx5 expression is initiated in areas where Bmp4 and Bmp7 genes become coexpressed. In a calvarial explant culture system, Dlx5 transcription is upregulated by BMPs and inhibited by the BMP-antagonist Noggin. In addition, FGF4 activates Bmp4 but not Bmp7 gene transcription and is not sufficient to induce ectopic Dlx5 expression in the immature calvarial mesenchyme. From these data, we propose a model for the regulatory network implicated in early steps of chick calvarial development.

Malformations of the craniofacial region: evolutionary, embryonic, genetic, and clinical perspectives

Malformations of the craniofacial region are reviewed with respect to evolutionary, embryonic, genetic, and clinical perspectives under the following headings: How Old Is Our Head?, Head Organization Genes, Genetics of Craniofacial Anomalies, Craniofacial Derivatives, Anencephaly, Cephalocele, Holoprosencephaly, Craniosynostosis, Hypertelorism, Branchial Arch Anomalies, and Orofacial Clefting.

Differential expression patterns of Runx2 isoforms in cranial suture morphogenesis

Runx2 (previously known as Cbfal/Pebp2alphaA/AML3), a key transcription factor in osteoblast differentiation, has at least two different isoforms using alternative promoters, which suggests that the isoforms might be expressed differentially. Haploinsufficiency of the Runx2 gene is associated with cleidocranial dysplasia (CCD), the main phenotype of which is inadequate development of calvaria. In spite of the biological relevance, Runx2 gene expression patterns in developing calvaria has not been explored previously, and toward this aim we developed three probes: pRunx2, which comprises the common coding sequence of Runx2 and hybridizes with all isoforms; pPebp2alphaA, which specifically hybridizes with the isoform transcribed with the proximal promoter; and pOsf2, which hybridizes with the isoform transcribed with the distal promoter. These probes were hybridized with tissue sections of mouse calvaria taken at various time points in development. Runx2 expression was localized to the critical area of cranial suture closure, being found in parietal bones, osteogenic fronts, and sutural mesenchyme. Pebp2alphaA and Osf2 showed tissue-specific expression patterns. The sites of Pebp2alphaA expression were almost identical to that of pRunx2 hybridization but expression was most intense in the sutural mesenchyme, where undifferentiated mesenchymal cells reside. The Osf2 isoform was strongly expressed in the osteogenic fronts, as well as in developing parietal bones, where osteopontin (OP) and osteocalcin (OC) also were expressed. However, in contrast to Pebp2alphaA, Osf2 expression did not occur in sutural mesenchyme. Pebp2alphaA also was expressed prominently in primordial cartilage that is found under the sutural mesenchyme and is not destined to be mineralized. Thus, Osf2 isoforms contribute to events later in osteoblast differentiation whereas the Pebp2alphaA isoform participates in a wide variety of cellular activities ranging from early stages of osteoblast differentiation to the final differentiation of osteoblasts.

Individual osteoblasts in the developing calvaria express different gene repertoires

Several studies in vitro and a few in vivo have suggested that mature osteoblasts heterogeneously express osteoblast markers. In one recent study of the osteoblasts associated with bone nodules formed in vitro in rat calvaria cell populations, extensive diversity was documented in the overall gene repertoires expressed. To address whether comparable heterogeneity is evident in vivo, we investigated the expression of nine osteoblast lineage markers by both in situ hybridization and immunohistochemistry. At 21 days of fetal rat development, the calvaria is a rapidly growing bone with distinct maturational zones that are readily observed in coronal sections; that is, an osteogenic front emerging at sagittal and coronal sutures is adjacent to areas of growing trabeculae of bone, followed by more mature areas of remodeling bone. Based on expression patterns, markers can be divided into two categories. One category comprises markers that are globally expressed by all osteoblasts irrespective of their position in the calvaria. Of those tested, only two, alkaline phosphatase and the pth/pthrp receptor, fit into this category. All other markers analyzed, including transcription factors (c-fos and msx-2), matrix molecules (bone sialoprotein, osteopontin, and osteocalcin), and a hormone (pthrp), were differentially expressed only in subpopulations of osteoblasts, based on cell maturational status, environment (ectocranial vs. endocranial surfaces), and microenvironment (adjacent osteoblasts). Preosteoblasts and osteocytes in different regions of the calvaria also expressed different subsets of the lineage markers. Mechanisms responsible for generating differential gene expression profiles appear to be both transcriptional and posttranscriptional. These results indicate that postproliferative, morphologically indistinguishable osteoblasts are not a homogeneous class of cells, but instead are molecularly diverse. The present results also raise the possibility that lineage progression and/or maintenance of the differentiated state may be adaptable in the calvaria.

Immunolocalization of keratan sulfate proteoglycan in rat calvaria

We investigate, by the immunogold method, the localization of keratan sulfate (KS) proteoglycan in rat calvaria in order to clarify the detailed process of intramembranous ossification. KS was localized in bone nodules corresponding to calcified nodules, close to the saggital suture of calvaria. The immunoreactivity decreased in fully calcified regions distant from the suture. Electron microscopic observation revealed that KS was distributed in and around matrix vesicles, among collagen fibrils at the initial crystal deposition stage, and then concentrated in bone nodules. According to the progress of mineralization, KS tended to be localized in the peripheral region of the nodules. In addition, these nodules came in contact with collagen fibrils which also showed KS-positive reactivity. In cell organelles of osteoblasts, KS was detected in the Golgi apparatus. These findings suggest that osteoblasts in intramembranous ossification sites actively synthesize KS. KS in the calcified nodules, as well as other glycosaminoglycans in osteoid, may play an important role in additional and/or collagenous calcification by trapping calcium ions through its negative charge.

The Ser252Trp fibroblast growth factor receptor-2 (FGFR-2) mutation induces PKC-independent downregulation of FGFR-2 associated with premature calvaria osteoblast differentiation

We recently showed that the Apert Ser252Trp fibroblast growth factor receptor-2 (FGFR-2) mutation causes premature osteoblast differentiation and increased subperiosteal calvaria bone matrix formation. To gain further insight into the cellular mechanisms involved in these effects, we examined the effects of the mutation on the expression of FGFRs in relation to cell proliferation and differentiation markers in vivo and in vitro, and we analyzed the underlying signaling pathways in mutant cells. Immunohistochemical analysis of the Apert calvaria suture showed that the Ser252Trp FGFR-2 mutation increased type 1 collagen, osteocalcin, and osteopontin expression in preosteoblasts compared to normal, whereas cell growth was not affected. The premature osteoblast differentiation induced by the mutation was associated with lower than normal FGFR-2 immunolabeling, whereas FGFR-1 and FGFR-3 levels were not decreased. Immunocytochemical analysis in osteoblasts isolated from Apert coronal suture showed that the Ser252Trp mutation induced constitutive downregulation of FGFR-2 in mutant cells. Western blot analysis of FGFRs in immortalized mutant osteoblastic cells confirmed that the mutation induced FGFR-2 downregulation. FGFR-2 mRNA levels were not altered in mutant cells, indicating that FGFR-2 downregulation resulted from receptor internalization rather than from changes in receptor mRNA. The signaling pathway involved in FGFR-2 downregulation was studied using specific inhibitors of FGF signaling molecules. The selective PKC inhibitor calphostin C markedly reduced FGFR-2 protein levels in mutant cells, in contrast to the p38 MAP kinase inhibitor SB 203580 or the Erk 1,2 MAP kinase inhibitor PD-98059, showing that PKC is involved in FGFR-2 regulation, but not in FGFR-2 downregulation in mutant cells. The results indicate that the premature osteoblast differentiation induced by the FGFR-2 Ser252Trp mutation is associated with a PKC-independent downregulation of FGFR-2 in human calvaria cells.

Morphological characterization of skeletal cells in Cbfa1-deficient mice

To clarify the mechanisms by which core-binding factor-alpha1 (Cbfa1), an essential transcription factor in osteogenesis, functions in osteoblast matrix formation, as well as in chondrocyte differentiation and osteoclastic bone resorption, Cbfa1-deficient embryonic mice were investigated ultrastructurally and histocytochemically at 18.5 days postcoitum. In homozygotic mice, both endochondral and intramembranous ossification were arrested, although bone tissue had already formed at this stage in the wild type. The tibiae of homozygotic mice were characterized by calcified cartilage and alkaline phosphatase (ALP)-positive perichondrium, whereas membranous structures indicating the presence of ALP activity in the lateral portion were observed in the calvariae, rather than the bone tissue. Most of the ALP-positive perichondrial cells in homozygotic tibiae possessed a spindle-shaped cell contour and small cytoplasm, the extracellular matrix of which contained neither type I collagen nor calcifying matrix vesicles. In contrast, some perichondrial cells at the very middle part of tibiae became flattened. In the vicinity of these cells, a thin layer of type I collagen-based calcified matrix, containing osteopontin, bone sialoprotein, or osteocalcin, was observed. In the cartilage of mutant mice, we observed a hypoplasic zone of proliferative chondrocytes, the flattening of hypertrophic chondrocyte-like cells, and calcified chondrocytes which, while not degraded, did display a high level of cell function. Mononuclear osteoclastic cells were found in the perichondrium, near calcified chondrocytes, in mutant mice. Multinuclear osteoclasts possessing H+-ATPase and ruffled borders were also present, although only in limited numbers. Neither the development of ruffled borders nor intracellular polarization was complete. Because the majority of osteogenic cells in Cbfa1-deficient mice can neither form nor calcify the bone matrix, Cbfa1 principally plays essential roles in osteoblastic differentiation and bone matrix formation. Cbfa1 also affects both the proliferation and the differentiation of chondrocytes, whereas its absence prevents normal osteoclast formation and related functions.

Msx2 gene dosage influences the number of proliferative osteogenic cells in growth centers of the developing murine skull: a possible mechanism for MSX2-mediated craniosynostosis in humans

Throughout its complex morphogenesis, the vertebrate skull must at once protect the brain and expand to accommodate its growth. A key structural adaptation that allows this dual role is the separation of the bony plates of the skull with sutures, fibrous joints that serve as growth centers and allow the calvarial bones to expand as the brain enlarges. Craniosynostosis, the premature fusion of one or more calvarial bones with consequent abnormalities in skull shape, is a common developmental anomaly that disrupts this process. We found previously that a single amino acid substitution in the homeodomain of the human MSX2 gene is associated with the autosomal dominant disorder craniosynostosis, Boston type. This mutation enhances the affinity of Msx2 for its target sequence, suggesting that the mutation acts by a dominant positive mechanism. Consistent with this prediction, we showed that general overexpression of Msx2 under the control of the broadly expressed CMV promoter causes the calvarial bones to invade the sagittal suture. Here we use tissue-specific overexpression of Msx2 within the calvarial sutures to address the developmental mechanisms of craniosynostosis and skull morphogenesis. We demonstrate that a segment of the Msx2 promoter directs reporter gene expression to subsets of cells within the sutures. In late embryonic and neonatal stages, this promoter is expressed in undifferentiated mesenchymal cells medial to the growing bone. By P4, promoter activity is reduced in the suture, exhibiting a punctate pattern in undifferentiated osteoblastic cells in the outer margin of the osteogenic front. Overexpression of Msx2 under the control of this promoter is sufficient to enhance parietal bone growth into the sagittal suture by P6. This phenotype is preceded by an increase in both the number and the BrdU labeling of osteoblastic cells in the osteogenic fronts of the calvarial bones. These findings suggest that an important early event in MSX2-mediated craniosynostosis in humans is a transient retardation of osteogenic cell differentiation in the suture and a consequent increase in the pool of osteogenic cells.

Investigation of the influences of biomechanical force on the ultrastructure of human sagittal craniosynostosis

This study presents comparisons of the ultrastructure of synostotic and open portions of synostotic sagittal sutures using histomorphometry, scanning electron microscopy, and microcomputed tomography. By using stereologic and histomorphometric analysis, this study proposes to demonstrate evidence of the influence of biomechanical force on the suture during the process of sagittal craniosynostosis. Finally, we propose to link the pathologic changes transforming normal suture fusion to craniosynostosis with concurrent changes in the polarity of suture fusion initiation. Seven infants (four boys and three girls) with sagittal craniosynostosis, ranging in age from 1.4 to 4.8 months (mean = 3.0 months), underwent sagittal synostectomies. The synostotic bone specimens were sectioned into three regions: an open suture, partial synostosis, and complete synostosis. Microcomputed tomographic and scanning electron microscopic scanning as well as histomorphometry was performed on all specimens to obtain detailed qualitative and quantitative information regarding the trabecular microarchitecture of the synostosed suture. Microcomputed tomographic analysis determined the bone volume fraction, trabecular thickness, trabecular separation, bone surface to bone volume ratio, and anisotropy for all specimens. Our results showed significant differences in all of these quantitative measurements when comparing the complete synostotic suture with the open portion of the synostotic sutures (p < 0.05). Microcomputed tomographic stereologic analysis showed evidence of the influence of biomechanical force on the synostotic and open portions of the synostotic sutures. Results of scanning electron microscopy show a definite qualitative difference in the trabecular pattern of the partial and complete synostotic suture when compared with the open portion of the synostotic sagittal suture. In this study, we performed both qualitative and quantitative comparisons of the ultrastructure of the complete synostotic and nonsynostotic sagittal sutures using stereologic and histomorphometric techniques. We also demonstrated evidence of the influence of biomechanical force on the synostotic sagittal suture. Finally, we established a link between the pathologic changes transforming normal suture fusion to craniosynostosis and concurrent changes in both the vector and direction of suture fusion initiation.

FGF-, BMP- and Shh-mediated signalling pathways in the regulation of cranial suture morphogenesis and calvarial bone development

The development of calvarial bones is tightly co-ordinated with the growth of the brain and needs harmonious interactions between different tissues within the calvarial sutures. Premature fusion of cranial sutures, known as craniosynostosis, presumably involves disturbance of these interactions. Mutations in the homeobox gene Msx2 as well as the FGF receptors cause human craniosynostosis syndromes. Our histological analysis of mouse calvarial development demonstrated morphological differences in the sagittal suture between embryonic and postnatal stages. In vitro culture of mouse calvaria showed that embryonic, but not postnatal, dura mater regulated suture patency. We next analysed by in situ hybridisation the expression of several genes, which are known to act in conserved signalling pathways, in the sagittal suture during embryonic (E15-E18) and postnatal stages (P1-P6). Msx1 and Msx2 were expressed in the sutural mesenchyme and the dura mater. FGFR2(BEK), as well as Bmp2 and Bmp4, were intensely expressed in the osteogenic fronts and Bmp4 also in the mesenchyme of the sagittal suture and in the dura mater. Fgf9 was expressed throughout the calvarial mesenchyme, the dura mater, the developing bones and the overlying skin, but Fgf4 was not detected in these tissues. Interestingly, Shh and Ptc started to be expressed in patched pattern along the osteogenic fronts at the end of embryonic development and, at this time, the expression of Bmp4 and sequentially those of Msx2 and Bmp2 were reduced, and they also acquired patched expression patterns. The expression of Msx2 in the dura mater disappeared after birth. &lt;P&gt; FGF and BMP signalling pathways were further examined in vitro, in E15 mouse calvarial explants. Interestingly, beads soaked in FGF4 accelerated sutural closure when placed on the osteogenic fronts, but had no such effect when placed on the mid-sutural mesenchyme. BMP4 beads caused an increase in tissue volume both when placed on the osteogenic fronts and on the mid-sutural area, but did not effect suture closure. BMP4 induced the expression of both Msx1 and Msx2 genes in sutural tissue, while FGF4 induced only Msx1. We suggest that the local application of FGF on the osteogenic fronts accelerating suture closure in vitro, mimics the pathogenesis of human craniosynostosis syndromes in which mutations in the FGF receptor genes apparently cause constitutive activation of the receptors. Taken together, our data suggest that conserved signalling pathways regulate tissue interactions during suture morphogenesis and intramembranous bone formation of the calvaria and that morphogenesis of mouse sagittal suture is controlled by different molecular mechanisms during the embryonic and postnatal stages. Signals from the dura mater may regulate the maintenance of sutural patency prenatally, whereas signals in the osteogenic fronts dominate after birth.

Studies in cranial suture biology: IV. Temporal sequence of posterior frontal cranial suture fusion in the mouse

The biology underlying normal and premature cranial suture fusion remains unknown. To develop a model for normal cranial suture fusion, the temporal sequence of the posterior frontal cranial suture fusion in the mouse was determined. To do this, all the cranial sutures of three distinct strains of mice (CD-1, CF-1, and C57bl-6) were studied histologically for fusion at sequential time points. Two studies were set up using group A mice (n = 72, all sutures studied) and group B mice (n = 78, only the posterior frontal suture studied, but more precisely along its anatomic length). In the group A cranial suture study, mice were sacrificed starting at newborn age and then every 5 days until age 50 days. In addition, two mature mice (250 days old) from each strain were sacrificed. In all three mouse strains, histologic examinations showed that the anterior frontal, sagittal, coronal, lambdoid, and occipitointerparietal sutures remained patent at up to 50 days of age and were patent in the 250-day mature mice. However, examination of the midpoint of the posterior frontal suture showed patency at 30 days, partial fusion at 35 days, and complete fusion by 40 days. These data prompted the posterior frontal suture fusion study. In the group B posterior frontal suture fusion study, mice were sacrificed at age 23 days and then every 2 days until 47 days of age. The anterior, midpoint, and posterior aspects of the posterior frontal suture were examined: The anterior aspect fused between 25 and 29 days; the midpoint fused between 31 and 37 days; and the posterior aspect fused between 39 and 45 days. These data indicate that fusion of the posterior frontal cranial suture in the mouse proceeds in a defined temporal sequence from an anterior to posterior direction in three distinct strains of mice, while in the same mice all other cranial sutures remain patent. By describing and understanding the fusion of the normal posterior frontal suture, a biologic basis of normal suture development and fusion can be established and used as a comparison for murine cranial sutures altered surgically, biochemically (with growth factors), or genetically (with craniosynostotic phenotypes).

A mutation in the homeodomain of the human MSX2 gene in a family affected with autosomal dominant craniosynostosis

Craniosynostosis, the premature fusion of calvarial sutures, is a common developmental anomaly that causes abnormal skull shape. The locus for one autosomal dominant form of craniosynostosis has been mapped to chromosome 5qter. The human MSX2 gene localizes to chromosome 5, and a polymorphic marker in the MSX2 intron segregates in a kindred with the disorder with no recombination. Moreover, a histidine substitutes for a highly conserved proline at position 7 of the MSX2 homeodomain exclusively in affected members. In the mouse, transcripts of the Msx2 gene are localized to calvarial sutures. These results provide compelling evidence that the mutation causes this craniosynostosis syndrome.

Sutural biology and the correlates of craniosynostosis

The purpose of this paper is to provide a new perspective on craniosynostosis by correlating what is known about sutural biology with the events of craniosynostosis per se. A number of key points emerge from this analysis: 1) Sutural initiation may take place by overlapping, which results in beveled sutures, or by end-to-end approximation, which produces nonbeveled, end-to-end sutures. All end-to-end sutures occur in the midline (e.g., sagittal and metopic) probably because embryonic biomechanical forces on either side of the initiating suture tend to be equal in magnitude. A correlate appears to be that only synostosed sutures of the midline have pronounced bony ridging. 2) Long-term histologic observations of the sutural life cycle call into question the number of layers within sutures. The structure varies not only in different sutures, but also within the same suture over time. 3) Few, if any, of the many elegant experimental research studies in the field of sutural biology have increased our understanding of craniosynostosis per se. An understanding of the pathogenesis of craniosynostosis requires a genetic animal model with primary craniosynostosis and molecular techniques to understand the gene defect. This may allow insight into pathogenetic mechanisms involved in primary craniosynostosis. It may prove to be quite heterogeneous at the basic level. 4) The relationship between suture closure, cessation of growth, and functional demands across sutures poses questions about various biological relationships. Two conclusions are provocative. First, cessation of growth does not necessarily, or always lead to fusion of sutures. Second, although patent sutures aid in the growth process, some growth can take place after suture closure. 5) In an affected suture, craniosynostosis usually begins at a single point and then spreads along the suture. This has been shown by serial sectioning and calls into question results of studies in which the affected sutures are only histologically sampled. 6) Craniosynostosis is etiologically and pathogenetically heterogeneous. Known human causes are reviewed. Is craniosynostosis simply normal suture closure commencing too early?(ABSTRACT TRUNCATED AT 400 WORDS)

Adrenergic innervation of the calvarium of the neonatal rat. Its relationship to the sagittal suture and developing parietal bones

The presence and distribution of adrenergic nerves in the developing calvarium of the newborn rat documented by means of the formaldehyde-induced fluorescence technique in rats aged 2 or 7 days. Nerve fibres exhibiting catecholamine-specific fluorescence were seen within the developing calvarium of all animals. In coronal sections, these fibres could be seen in the developing bone, especially in the lamina interna, while in sagittal sections, they were seen to traverse the tissue to reach the central of the diploë. These fibres originate from a denser plexus within the dura mater. Especially in the younger age group, the fluorescent fibres often exhibited an immature appearance, being coarse and devoid of varicosities. In the older animals the fibres were often varicose. The sutural tissue proper was always found to be devoid of adrenergic innervation. The possible origin and functional significance of the adrenergic innervation in the developing bone in relation to skull growth and sutural closure are discussed.

Calcified tissues involved in the ontogenesis of the human cranial vault

The cranial vault of fifteen human subjects varying in age from 20th week of gestational life to 9th month post-matum were submitted to microradiographic and histological analysis. Different phenomena such as cortical drift, bone cavitation and progressive substitution of different calcified tissues by lamellar bone are illustrated. Moreover, this study reveals in several areas the presence of chondroid tissue; it constitutes the edges of the sutures and is responsible for their growth till the post-natal period. Therefore, it can be supported that the role of chondroid tissue is essential for the harmonious development of the cranial vault.

Sutures and forces: a review

This review gives a description of the biologic significance of craniofacial sutures with respect to growth and to growth corrections. Sutural growth and its regulation are discussed briefly. Morphogenesis of sutures, sutural morphology, both microscopic and macroscopic, the structure and function of the sutural periosteum and secondary cartilages, and the biochemical composition of sutures are described. Furthermore, in vivo and in vitro experiments, including transplantation experiments, are discussed. The relationship between extrinsic mechanical forces and the resulting tissue responses in sutures is given special attention. The present article describes the state of our knowledge on the interaction between sutures and forces, and indicates problems that need to be investigated.

Craniosynostosis update 1987

Information on craniosynostosis in this paper updates "Craniosynostosis: Diagnosis, Evaluation, and Management" (Cohen MM Jr: New York: Raven Press, 1986). It also discusses recent developments that were included in the book but need further explanation or emphasis. Subjects discussed are: epidemiology, etiology, sutural biology, growth and development, neurological and psychosocial aspects, surgery, cloverleaf skulls, craniosynostosis syndromes, and prenatal diagnosis. Under the subject of etiology, fetal head constraint, maternal thyroid disease, calcified cephalohematoma, teratogens, and delayed suture closure and Wormian bones are considered. An updating of 15 cloverleaf skull conditions includes four monogenic disorders, two chromosomal disorders, one disruption, one iatrogenic condition, and seven syndromes of unknown cause. Newly recognized disorders with cloverleaf skull include Beare-Stevenson cutis gyratum syndrome and Say-Poznanski syndrome. Craniosynostosis syndromes and associations discussed include acrocraniofacial dysostosis, Apert syndrome, Beare-Stevenson cutis gyratum syndrome, Calabro syndrome, calvarial hyperostosis, chromosomal craniostenosis, Cole-Carpenter type osteogenesis imperfecta, Crouzon syndrome, Curry-Jones syndrome, Curry variant of Carpenter syndrome, cutis aplasia and cranial stenosis, Fontaine-Farriaux syndrome, Gomex-López-Hernández syndrome, Hersh syndrome, hyper-IgE syndrome and craniostenosis, hypomandibular faciocranial dysostosis, Marfanoid features and craniostenosis, Pfeiffer-type cardiocranial syndrome, Pfeiffer-type dolichocephalosyndactyly, and Say-Barber syndrome.