Expression and mechanical modulation of matrix metalloproteinase-1 and -2 genes in facial and cranial sutures

Suture Cells in a Mechanical Stretching Niche: Critical Contributors to Trans-sutural Distraction Osteogenesis

Trans-sutural distraction osteogenesis has been proposed as an alternative technique of craniofacial remodelling surgery for craniosynostosis correction. Many studies have defined the contribution of a series of biological events to distraction osteogenesis, such as changes in gene expression, changes in suture cell behaviour and changes in suture collagen fibre characteristics. However, few studies have elucidated the systematic molecular and cellular mechanisms of trans-sutural distraction osteogenesis, and no study has highlighted the contribution of cell-cell or cell-matrix interactions with respect to the whole expansion process to date. Therefore, it is difficult to translate largely primary mechanistic insights into clinical applications and optimize the clinical outcome of trans-sutural distraction osteogenesis. In this review, we carefully summarize in detail the literature related to the effects of mechanical stretching on osteoblasts, endothelial cells, fibroblasts, immune cells (macrophages and T cells), mesenchymal stem cells and collagen fibres in sutures during the distraction osteogenesis process. We also briefly review the contribution of cell-cell or cell-matrix interactions to bone regeneration at the osteogenic suture front from a comprehensive viewpoint.

FACEts of mechanical regulation in the morphogenesis of craniofacial structures

During embryonic development, organs undergo distinct and programmed morphological changes as they develop into their functional forms. While genetics and biochemical signals are well recognized regulators of morphogenesis, mechanical forces and the physical properties of tissues are now emerging as integral parts of this process as well. These physical factors drive coordinated cell movements and reorganizations, shape and size changes, proliferation and differentiation, as well as gene expression changes, and ultimately sculpt any developing structure by guiding correct cellular architectures and compositions. In this review we focus on several craniofacial structures, including the tooth, the mandible, the palate, and the cranium. We discuss the spatiotemporal regulation of different mechanical cues at both the cellular and tissue scales during craniofacial development and examine how tissue mechanics control various aspects of cell biology and signaling to shape a developing craniofacial organ.

Expression of MMP-2 and TIMP-1 during rapid maxillary expansion in rats

OBJECTIVE

The aim of this study was to investigate the expression of MMP-2 and TIMP-1 during midpalatal suture expansion in rats.

DESIGN

72 male Wistar rats were randomly divided into 2 groups: the experimental group and the control group. In the experimental group, opening loops were applied across the midpalatal suture with an initial force of 50g, whereas in the control group, rats were subjected to sham installation of opening loops without activation. On day 1, 4, 7 and 14, nine rats from each group were sacrificed, and the maxillae were dissected and prepared for Immunohistochemistry (IHC) and RT- PCR examination of MMP-2 and TIMP-1 expression.

RESULTS

The results of IHC and Real Time PCR revealed that both protein and mRNA expression of MMP-2 and TIMP-1 were significantly increased after midpalatal expansion, and the ratio of MMP-2/TIMP-1 was also significantly enhanced.

CONCLUSIONS

The data suggested that MMP-2 and TIMP-1 might play an important role during the mid-palatal suture remodeling process of maxillary expansion.

Timing of ectocranial suture activity in Gorilla gorilla as related to cranial volume and dental eruption

Research has shown that Pan and Homo have similar ectocranial suture synostosis patterns and a similar suture ontogeny (relative timing of suture fusion during the species ontogeny). This ontogeny includes patency during and after neurocranial expansion with a delayed bony response associated with adaptation to biomechanical forces generated by mastication. Here we investigate these relationships for Gorilla by examining the association among ectocranial suture morphology, cranial volume (as a proxy for neurocranial expansion) and dental development (as a proxy for the length of time that it has been masticating hard foods and exerting such strains on the cranial vault) in a large sample of Gorilla gorilla skulls. Two-hundred and fifty-five Gorilla gorilla skulls were examined for ectocranial suture closure status, cranial volume and dental eruption. Regression models were calculated for cranial volumes by suture activity, and Kendall's tau (a non-parametric measure of association) was calculated for dental eruption status by suture activity. Results suggest that, as reported for Pan and Homo, neurocranial expansion precedes suture synostosis activity. Here, Gorilla was shown to have a strong relationship between dental development and suture activity (synostosis). These data are suggestive of suture fusion extending further into ontogeny than brain expansion, similar to Homo and Pan. This finding allows for the possibility that masticatory forces influence ectocranial suture morphology.

Mechanical influences on suture development and patency

In addition to their role in skull growth, sutures are sites of flexibility between the more rigid bones. Depending on the suture, predominant loading during life may be either tensile or compressive. Loads are transmitted across sutures via collagenous fibers and a fluid-rich extracellular matrix and can be quasi-static (growth of neighboring tissues) or intermittent (mastication). The mechanical properties of sutures, while always viscoelastic, are therefore quite different for tensile versus compressive loading. The morphology of individual sutures reflects the nature of local loading, evidently by a process of developmental adaptation. In vivo or ex vivo, sutural cells respond to tensile or cyclic loading by expressing markers of proliferation and differentiation, whereas compressive loading appears to favor osteogenesis. Braincase and facial sutures exhibit similar mechanical behavior and reactions despite their different natural environments.

Zygomaticotemporal synostosis: a rare cause of progressive facial asymmetry

Cranial synostosis is relatively common; however, premature closure of a facial suture is remarkably rare. We describe an adolescent male with zygomaticotemporal synostosis that manifested as progressive midfacial and orbital asymmetry, angulation of the cranial base, and nasal deviation. To our knowledge, this is the first description of this anomaly in the literature.

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.

Matrix and gene expression in the rat cranial base growth plate

Recent data have shown that the proliferation and differentiation of the cranial base growth plate (CBGP) chondrocytes are modulated by mechanical stresses. However, little is known about the expression of genes and matrix molecules in the CBGP during development or under mechanical stresses. The objective of the present study was to determine whether several cartilage- and bone-related molecules are expressed in the CBGP and whether their expression is modulated by cyclic loading. The CBGP of normal 8-day-old rats (n=8) were isolated immediately after death, followed by extraction of total RNA and reverse transcription/polymerase chain reaction (RT-PCR) analysis. All studied genes, including type II and X collagens, biglycan, versican, osteocalcin, osteopontin, and fetal liver kinase 1, were expressed in the CBGP with a reproducible absence of decorin mRNA. In age- and sex-matched rats (n=10), exogenous cyclic forces were applied to the maxilla at 500 mN and 4 Hz for 20 min/day over 2 days, followed by RNA isolation and RT-PCR analysis. This exogenous cyclic loading consistently induced the expression of the decorin gene, which was non-detectable, by the current RT-PCR approach, in control neonatal CBGPs without loading. Immunolocalization of several of the above-studied gene products demonstrated their remarkable site-specific expression. Decorin proteoglycan was primarily expressed in the perichondrium instead of various cartilage growth zones, especially upon mechanical loading. These findings serve as baseline data for the expression of several genes and gene products in the neonatal CBGP. Mechanical modulation of decorin expression is consistent with recent reports of its susceptibility to mechanical loading in several connective tissues.