Cranial osteogenesis and suture morphology in Xenopus laevis: a unique model system for studying craniofacial development

Background

The tremendous diversity in vertebrate skull formation illustrates the range of forms and functions generated by varying genetic programs. Understanding the molecular basis for this variety may provide us with insights into mechanisms underlying human craniofacial anomalies. In this study, we provide evidence that the anuran Xenopus laevis can be developed as a simplified model system for the study of cranial ossification and suture patterning. The head structures of Xenopus undergo dramatic remodelling during metamorphosis; as a result, tadpole morphology differs greatly from the adult bony skull. Because of the extended larval period in Xenopus, the molecular basis of these alterations has not been well studied.

Methodology/Principal Findings

We examined late larval, metamorphosing, and post-metamorphosis froglet stages in intact and sectioned animals. Using micro-computed tomography (μCT) and tissue staining of the frontoparietal bone and surrounding cartilage, we observed that bone formation initiates from lateral ossification centers, proceeding from posterior-to-anterior. Histological analyses revealed midline abutting and posterior overlapping sutures. To determine the mechanisms underlying the large-scale cranial changes, we examined proliferation, apoptosis, and proteinase activity during remodelling of the skull roof. We found that tissue turnover during metamorphosis could be accounted for by abundant matrix metalloproteinase (MMP) activity, at least in part by MMP-1 and -13.

Conclusion

A better understanding of the dramatic transformation from cartilaginous head structures to bony skull during Xenopus metamorphosis may provide insights into tissue remodelling and regeneration in other systems. Our studies provide some new molecular insights into this process.

The Bmp pathway in skull vault development

The Bmp pathway is of critical importance in the development of the skull vault. Analysis of gain and loss of function phenotypes of Bmp pathway effectors, particularly Msx genes, has shown that the Bmp pathway functions in the growth of both mesodermal and neural crest-derived calvarial bones. It is required for the development of the frontal and parietal bones during the interval between the initial osteogenic mesenchymal condensations at E12.5 to the apposition of the paired frontal and parietal bones at E18.5. During postnatal development, forced expression of the Bmp inhibitor, noggin, maintains the patency of sutures, consistent with a role for the Bmp pathway in regulating suture development. The availability of conditional mutants of Bmp ligands, receptors and downstream effectors will make possible an increasingly high resolution analysis of precisely how the Bmp functions in these processes and how aberrations in its activity can contribute to pathological conditions such as familial parietal foramina and craniosynostosis.

Wnt/beta-catenin signaling regulates cranial base development and growth

Wnt proteins and beta-catenin signaling regulate major processes during embryonic development, and we hypothesized that they regulate cranial base synchondrosis development and growth. To address this issue, we analyzed cartilage-specific beta-catenin-deficient mice. Mutant synchondroses lacked typical growth plate zones, and endochondral ossification was delayed. In reciprocal transgenic experiments, cartilage overexpression of a constitutive active Lef1, a transcriptional mediator of Wnt/beta-catenin signaling, caused precocious chondrocyte hypertrophy and intermingling of immature and mature chondrocytes. The developmental changes seen in beta-catenin-deficient synchondroses were accompanied by marked reductions in Ihh and PTHrP as well as sFRP-1, an endogenous Wnt signaling antagonist and a potential Ihh signaling target. Thus, Wnt/beta-catenin signaling is essential for cranial base development and synchondrosis growth plate function. This pathway promotes chondrocyte maturation and ossification events, and may exert this important role by dampening the effects of Ihh-PTHrP together with sFRP-1.

Perioperative management of pediatric patients with craniosynostosis

Craniosynostosis, premature closures of the skull sutures, results in dysmorphic features if left untreated. Brain growth and cognitive development may also be impacted. Craniosynostosis repair is usually performed in young infants and has its perioperative challenges. This article provides background information about the different forms of craniosynostosis, with an overview of associated anomalies, genetic influences, and their connection with cognitive function. It also discusses the anesthetic considerations for perioperative management, including blood-loss management and strategies to reduce homologous blood transfusions.

[Genetics of craniofacial development]

Congenital craniofacial malformations vary widely in both expression and gravity. To understand congenital craniofacial malformations, knowledge of embryonic development is of essential importance. Craniosynostosis has its origin in the failure of suture development between 2 bone centres or in early closure of the suture by bone centre tissue fusion. Hereditary craniosynostosis phenotypes predominantly arise by autosomal dominant inheritance. So far, the majority of mutations have been found in fibroblast growth-factor receptor genes (FGFR-genes). Different phenotypes are not primarily created by disparities of the receptors, but particularly by tissue-specific expressions.

On the effect of cranial deformation in determining age from ectocranial suture closure

Available techniques for determining age from human cranial remains are limited. This study examines the efficacy of Meindl and Lovejoy's (1985) method of determining age based on ectocranial suture closure patterns as compared to a baseline of ages developed from a multifactorial approach employing various age determining factors from across the skull. What makes this study different is that the sample upon which this comparison is performed contains a large number of artificially deformed crania. Our hypothesis is that aging techniques that rely on suture closure patterns as markers are complicated by the results of artificial modification of the cranial vault. The study is conducted on adult, human crania from prehispanic archaeological sites in South America. Results demonstrate a significant difference between the two aging methods, more particularly when applied to deformed skulls. We conclude that when a skull is deformed age should be estimated utilizing multiple factors that exclude Meindl and Lovejoy's ectocranial suture aging technique.

Tgf-beta regulation of suture morphogenesis and growth

Premature suture obliteration results in an inability of cranial and facial bones to grow, with resulting craniofacial dysmorphology requiring surgical correction. Understanding the biological signaling associated with suture morphogenesis will enable less invasive treatment of patients with fused sutures, combined with therapy using biological molecules. While a number of advances have been made in identifying the genetic etiologies of various craniosynostotic syndromes, the pathogenesis of this condition is still not completely understood. Recently, it has been shown that differential expression of various transforming growth factor-beta (Tgf-beta) isoforms plays a crucial role in regulating suture patency once the sutures have formed. It has also been shown that differential expression of Tgf-beta isoforms may also play a role in craniosynostosis by altering proliferation, differentiation, and apoptosis within the suture. This chapter focuses on the role of Tgf-beta in suture morphogenesis and growth, exploring Tgf-beta biology, receptors, signaling pathways, animal models, and expression in both normal and pathological sutures.

Fibroblast growth factor signaling in cranial suture development and pathogenesis

Apert, Pfeiffer and Crouzon syndromes are congenital craniosynostosis syndromes caused by mutations that perturb the level of fibroblast growth factor receptor (FGFR) signaling. The cellular and molecular impact of these mutations have been studied in vitro and in animal models in vivo. Here, I highlight the complexity of the FGF/FGFR signaling system and review the candidate modifiers responsible for regulating the levels of FGF/FGFR signaling in tissues. I also review what we have learned from the phenotypic analysis of mice that model these craniosynostosis syndromes and discuss some in vivo strategies for further understanding as well as alleviating the associated craniofacial defects.

The size of anterior fontanel in neonates and infants in Addis Ababa

INTRODUCTION

Assessment of the size of the anterior fontanel is a part of the routine examination in neonatology and pediatrics. Knowledge of the normal variations in AF size may be helpful in the early recognition of disorders. The mean size of the AF in neonates and infants in Addis Ababa is not known and the current looks in to this issue.

METHODS

A cross sectional study was carried out on 687 apparently healthy neonates and infants who were seen consecutively from January 2003 to December 2003.

RESULTS

There were 363 males and 324 females at the ages of 3, 46, 76, 106 and 270 days. The mean of anterior-posterior and lateral dimensions was considered to be the size of anterior fontanel. The mean AF size at the age of 3 days was 3.35 +/- 0.94 cm and showed a decline in the respective postnatal ages. 0.8% of infants at the age of 76 days had closed AF and the percentage of closure rose to 2.3 and 39.6 at the ages of 106 and 270 days respectively.

CONCLUSION

This study supported previous reports indicating racial difference in the size and time of closure of AF. The size and time of closure of AF are used to monitor the development of brain and general state of health of the individual. Further study is recommended indifferent parts of Ethiopia to establish age-related standard of AF size.

Cell proliferation and osteogenic differentiation of growing pig cranial sutures

Bone growth at the cranial sutures relies on proliferation of osteogenic progenitor cells and/or differentiation of osteoblasts. The current study was undertaken to assess these events in relation to suture growth and fusion. A total of 21 pigs, divided into three age groups (0.5-1.5 months, 3-4 months and 5-7 months), were used for immunohistochemical evaluation of cell proliferation (BrdU) and osteogenic differentiation (Cbfa1/Runx2) in the interfrontal and interparietal sutures. Proliferation and osteogenic differentiation were both more prominent near the bone fronts than in the central zone. With age, both proliferation and osteogenic differentiation diminished. Proliferation ceased on the endocranial (dura mater) side by the age of 3-4 months. Proliferation on the pericranial side was accompanied by active bone formation and initiation of suture fusion from this side. In conclusion, (1) decreased suture bone growth with age reflects decreased cell proliferation and probably also osteogenic differentiation, and (2) suture fusion occurs from the pericranial side where activity remains relatively high.

RAB23 mutations in Carpenter syndrome imply an unexpected role for hedgehog signaling in cranial-suture development and obesity

Carpenter syndrome is a pleiotropic disorder with autosomal recessive inheritance, the cardinal features of which include craniosynostosis, polysyndactyly, obesity, and cardiac defects. Using homozygosity mapping, we found linkage to chromosome 6p12.1-q12 and, in 15 independent families, identified five different mutations (four truncating and one missense) in RAB23, which encodes a member of the RAB guanosine triphosphatase (GTPase) family of vesicle transport proteins and acts as a negative regulator of hedgehog (HH) signaling. In 10 patients, the disease was caused by homozygosity for the same nonsense mutation, L145X, that resides on a common haplotype, indicative of a founder effect in patients of northern European descent. Surprisingly, nonsense mutations of Rab23 in open brain mice cause recessive embryonic lethality with neural-tube defects, suggesting a species difference in the requirement for RAB23 during early development. The discovery of RAB23 mutations in patients with Carpenter syndrome implicates HH signaling in cranial-suture biogenesis--an unexpected finding, given that craniosynostosis is not usually associated with mutations of other HH-pathway components--and provides a new molecular target for studies of obesity.

Does the mendosal suture exist in the adult?

There is a paucity of information in the literature regarding the mendosal suture. Furthermore, reports of the closure of this presumed suture of childhood are variable. This study seeks to establish the presence or absence of this structure in the adult. Fifty adult skulls were evaluated for the presence or absence of the mendosal suture. Sixteen percent of specimens were found to have a mendosal suture. Six specimens were found to have these sutures bilaterally, and two had sutures on the right side only. Most sutures were linear in nature. The mendosal suture approximated the superior nuchal line in all specimens and more or less traveled medial and perpendicular to the lambdoidal suture. The length of these sutures ranged from 0.8 to 1.4 cm (1.1 cm). Our hopes are that these data will prove useful to both the anatomist and clinicians, so that, when present, misinterpretation of the mendosal suture will be avoided.

Anti-TGF-??2 Antibody Therapy Inhibits Postoperative Resynostosis in Craniosynostotic Rabbits

BACKGROUND

Postoperative resynostosis is a common clinical finding. It has been suggested that an overexpression of transforming growth factor (TGF)-beta2 may be related to craniosynostosis and may contribute to postoperative resynostosis. Interference with TGF-beta2 function with the use of neutralizing antibodies may inhibit resynostosis. The present study was designed to test this hypothesis.

METHODS

New Zealand White rabbits with bilateral coronal suture synostosis were used as suturectomy controls (group 1, n = 9) or given suturectomy with nonspecific, control immunoglobulin G antibody (group 2, n = 9) or suturectomy with anti-TGF-beta2 antibody (group 3, n = 11). At 10 days of age, a 3 x 15-mm coronal suturectomy was performed. The sites in groups 2 and 3 were immediately filled with 0.1 cc of a slowly resorbing collagen gel mixed with either immunoglobulin G (100 mug per suture) or anti-TGF-beta2 (100 mug per suture). Three-dimensional computed tomography scan reconstructions of the defects were obtained at 10, 25, 42, and 84 days of age, and the sutures were harvested for histomorphometric analysis.

RESULTS

Computed tomography scan data revealed that the suturectomy sites treated with anti-TGF-beta2 showed significantly (p < 0.05) greater areas through 84 days of age compared with controls. Histomorphometry also showed that suturectomy sites treated with anti-TGF-beta2 had patent suturectomy sites and more fibrous tissue in the defects compared with sites in control rabbits and had significantly (p < 0.001) less new bone area (by approximately 215 percent) in the suturectomy site.

CONCLUSIONS

These data support the initial hypothesis that interference with TGF-beta2 function inhibited postoperative resynostosis in this rabbit model. They also suggest that this biologically based therapy may be a potential surgical adjunct to retard postoperative resynostosis in infants with craniosynostosis.

Impaired posterior frontal sutural fusion in the biglycan/decorin double deficient mice

Biglycan (Bgn) and decorin (Dcn) are highly expressed in numerous tissues in the craniofacial complex. However, their expression and function in the cranial sutures are unknown. In order to study this, we first examined the expression of biglycan and decorin in the posterior frontal suture (PFS), which predictably fuses between 21 and 45 days post-natal and in the non-fusing sagittal (S) suture from wild-type (Wt) mice. Our data showed that Bgn and Dcn were expressed in both cranial sutures. We then characterized the cranial suture phenotype in Bgn deficient, Dcn deficient, Bgn/Dcn double deficient, and Wt mice. At embryonic day 18.5, alizarin red/alcian blue staining showed that the Bgn/Dcn double deficient mice had hypomineralization of the frontal and parietal craniofacial bones. Histological analysis of adult mice (45-60 days post-natal) showed that the Bgn or Dcn deficient mice had no cranial suture abnormalities and immunohistochemistry staining showed increased production of Dcn in the PFS from Bgn deficient mice. To test possible compensation of Dcn in the Bgn deficient sutures, we examined the Bgn/Dcn double deficient mice and found that they had impaired fusion of the PFS. Semi-quantitative RT-PCR analysis of RNA from 35 day-old mice revealed increased expression of Bmp-4 and Dlx-5 in the PFS compared to their non-fusing S suture in Wt tissues and decreased expression of Dlx-5 in both PF and S sutures in the Bgn/Dcn double deficient mice compared to the Wt mice. Failure of PFS fusion and hypomineralization of the calvaria in the Bgn/Dcn double deficient mice demonstrates that these extracellular matrix proteoglycans could have a role in controlling the formation and growth of the cranial vault.

Testing causal mechanisms of nonsyndromic craniosynostosis using path analysis of cranial contents in rabbits with uncorrected craniosynostosis

OBJECTIVE

Various causal mechanisms of familial nonsyndromic craniosynostosis have been presented. One hypothesis suggests that overproduction of bone at the suture is the primary origin of craniosynostosis, which affects brain and cranial growth secondarily through altered intracranial pressure (Primary Suture Fusion Model). Other hypotheses suggest that decreased cranial base growth or abnormal brain growth are the primary cause of craniosynostosis (Cranial Base, Brain Parenchyma Models, respectively). This study was designed to investigate which model best describes neurocranial changes associated with craniosynostosis in a rabbit model through multivariate path analysis.

DESIGN

Serial magnetic resonance imaging scans and intracranial pressure measurements were obtained at 10, 25, and 42 days of age from 18 rabbits: six controls, six with delayed-onset synostosis, and six with early-onset synostosis. Five variables were collected from each rabbit: calvarial thickness at the affected suture, cranial base length, brain volume, cerebrospinal fluid volume, and intracranial pressure. This data set was used to test causal pathway relationships generated by the proposed models. Goodness of fit was measured by experimental group for each model.

RESULTS

Primary Suture Fusion Model best explained the variables in both delayed-onset and early-onset synostotic rabbits (Goodness of fit = 93%, 97%, respectively). Cranial Base Model (Goodness of fit = 94%) best explained the data in control rabbits.

CONCLUSION

Results suggest that the primary site of craniosynostosis in craniosynostotic rabbits is most likely the synostosed suture. Other cranial vault anomalies are most likely secondary compensatory changes. Results of the present study may provide insight regarding the causal pathway of craniosynostosis.

Age dependence of fusion of primary occipital sutures: a radiographic study

PURPOSE

When linear lucency is present in the occipital bone on radiographs throughout childhood, differential diagnosis becomes important because some primary sutures are similar to fractures. The authors here chronicled the normal development of ossification centers, sutures, and synchondroses in the chondrocranium by radiographic examination.

METHODS

One hundred and twenty-seven children, aged from newborns to 6 years and without any skull base deformities, were referred to for radiographs of Towne's projection.

RESULTS

In the occipital bone at birth, three primary sutures could be identified. At the age of 0-3 years, occipital and innominate sutures started to fuse, this being complete by 4 years, whereas mendosal sutures persisted until 6 years of age, after which no primary sutures could be seen.

CONCLUSION

The complex process of skull base development features a step-wise process sutural closure for which radiographic standards allow differential diagnosis from fractures with judgment of the timing.

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.