Endogenous parathyroid hormone-related peptide enhances proliferation and inhibits differentiation in the osteoblast-like cell line ROS 17/2.8

The role of peptides in bone healing and regeneration: a systematic review

BACKGROUND

Bone tissue engineering and the research surrounding peptides has expanded significantly over the last few decades. Several peptides have been shown to support and stimulate the bone healing response and have been proposed as therapeutic vehicles for clinical use. The aim of this comprehensive review is to present the clinical and experimental studies analysing the potential role of peptides for bone healing and bone regeneration.

METHODS

A systematic review according to PRISMA guidelines was conducted. Articles presenting peptides capable of exerting an upregulatory effect on osteoprogenitor cells and bone healing were included in the study.

RESULTS

Based on the available literature, a significant amount of experimental in vitro and in vivo evidence exists. Several peptides were found to upregulate the bone healing response in experimental models and could act as potential candidates for future clinical applications. However, from the available peptides that reached the level of clinical trials, the presented results are limited.

CONCLUSION

Further research is desirable to shed more light into the processes governing the osteoprogenitor cellular responses. With further advances in the field of biomimetic materials and scaffolds, new treatment modalities for bone repair will emerge.

Validation of a simple and fast method to quantify in vitro mineralization with fluorescent probes used in molecular imaging of bone

Alizarin Red S staining is the standard method to indicate and quantify matrix mineralization during differentiation of osteoblast cultures. KS483 cells are multipotent mouse mesenchymal progenitor cells that can differentiate into chondrocytes, adipocytes and osteoblasts and are a well-characterized model for the study of bone formation. Matrix mineralization is the last step of differentiation of bone cells and is therefore a very important outcome measure in bone research. Fluorescently labelled calcium chelating agents, e.g. BoneTag and OsteoSense, are currently used for in vivo imaging of bone. The aim of the present study was to validate these probes for fast and simple detection and quantification of in vitro matrix mineralization by KS483 cells and thus enabling high-throughput screening experiments. KS483 cells were cultured under osteogenic conditions in the presence of compounds that either stimulate or inhibit osteoblast differentiation and thereby matrix mineralization. After 21 days of differentiation, fluorescence of stained cultures was quantified with a near-infrared imager and compared to Alizarin Red S quantification. Fluorescence of both probes closely correlated to Alizarin Red S staining in both inhibiting and stimulating conditions. In addition, both compounds displayed specificity for mineralized nodules. We therefore conclude that this method of quantification of bone mineralization using fluorescent compounds is a good alternative for the Alizarin Red S staining.

BMP-2 regulation of PTHrP and osteoclastogenic factors during osteoblast differentiation of C2C12 cells

Bone morphogenetic protein-2 (BMP-2) is strongly involved in the induction of osteoblast differentiation from mesenchymal cell precursors, as well as in enhancing bone matrix production by osteoblastic cells. Likewise, the osteoporotic phenotype of PTHrP deficient mice makes clear the importance of this paracrine regulator in bone physiology. Here, we report that BMP-2 rapidly down-regulated PTHrP gene expression through a transcriptional mechanism in pluripotent mesenchymal C2C12 cells, whereas BMP-2 increased expression of PTHrP receptor. PTHrP did not significantly alter the BMP-dependent Smad transcriptional pathway. Similarly, PTHrP did not significantly modify the BMP-regulated expression of RANKL or OPG, cytokines involved in osteoclastogenesis. More importantly, addition of PTHrP, through the PKA signaling pathway, partially prevented the BMP-dependent induction of some osteogenic markers such as Runx2 and Osterix in C2C12 cells. Our data suggest that BMP-2 down-regulation of PTHrP could facilitate terminal differentiation of osteoblasts.

Cyclin D1 as a target for the proliferative effects of PTH and PTHrP in early osteoblastic cells

PTHrP induced a proliferative cyclin D1 activation in low-density osteoblastic cells. The process was PKA and MAPK dependent and involved both AP-1 and CRE sites. In ectopic ossicles generated from implanted bone marrow stromal cells, PTH upregulated cyclin D1 after acute or intermittent anabolic treatment. These data suggest a positive role of PTH and PTHrP in the cell cycle of early osteoblasts.

INTRODUCTION

The mechanisms underlying the actions of PTH and its related protein (PTHrP) in osteoblast proliferation, differentiation, and bone remodeling remain unclear. The action of PTH or PTHrP on the cell cycle during osteoblast proliferation was studied.

MATERIALS AND METHODS

Mouse calvarial MC3T3-E1 clone 4 cells were synchronized by serum starvation and induced with 100 nM PTHrP for 2-24 h under defined low serum conditions. Western blot, real-time PCR, EMSAs, and promoter/luciferase assays were performed to evaluate cyclin D1 expression. Pharmacological inhibitors were used to determine the relevant signaling pathways. Ectopic ossicles generated from implanted bone marrow stromal cells were treated with acute (a single 8- or 12-h injection) or intermittent anabolic PTH treatment for 7 days, and RNA and histologic analysis were performed.

RESULTS

PTHrP upregulated cyclin D1 and CDK1 and decreased p27 expression. Cyclin D1 promoter/luciferase assays showed that the PTHrP regulation involved both activator protein-1 (AP-1) and cyclic AMP response element binding protein (CRE) sites. AP-1 and CRE double mutants completely abolished the PTHrP effect of cyclin D1 transcription. Upregulation of cyclin D1 was found to be protein kinase A (PKA) and mitogen-activated protein kinase (MAPK) dependent in proliferating MC3T3-E1 cells. In vivo expression of cyclin D1 in ectopic ossicles was upregulated after a single 12-h PTH injection or intermittent anabolic PTH treatment for 7 days in early developing ossicles.

CONCLUSIONS

These data indicate that PTH and PTHrP induce cyclin D1 expression in early osteoblastic cells and their action is developmental stage specific.

Vascular Calcifications in Uremia: Old Concepts and New Insights

The annual mortality rate in uremic patients, corrected for age, sex, and race, is significantly higher than in the general population. This is primarily due to cardiovascular events. Vascular calcifications play a vital role in the development of cardiovascular morbidity and subsequent increased mortality. Vascular calcification affects both vascular intima and media layers and its mechanism remains poorly understood. Over the last few years it has been shown that, in addition to traditional cardiovascular risk factors, disturbances in mineral metabolism in the uremic milieu, calcium-containing phosphate binders, and vitamin D treatment of secondary hyperparathyroidism may contribute to the pathogenesis of vascular calcifications. Other uremia-related risk factors (e.g.increased oxidized low-density lipoprotein cholesterol, uremic toxins, increased oxidative stress, dialysis and dialysate-related factors, hemodynamic overload, hyperhomocysteinemia) may also play a role. In uremic patients, apart from these facilitating factors, decreased levels of endogenous calcification inhibitors such as fetuin-Amatrix Gla protein, osteoprotegerin, and osteopontin have also been associated with increased calcium-phosphate precipitation in extraskeletal tissues. Finally, vascular calcification is the outcome of the active and dynamic balance of procalcifying and anticalcifying influences. For the prevention and treatment of vascular calcifications, it is essential to avoid treatment modalities that lead to calcium overload, achieve good metabolic control, and optimize dialysis.

Multiple mechanisms are involved in inhibition of osteoblast differentiation by PTHrP and PTH in KS483 Cells

We examined the mechanism by which PTHrP and PTH inhibit KS483 osteoblastic differentiation. We show that PTHrP and PTH inhibit differentiation downstream of early BMP signaling and downregulated components of the hedgehog (Hh) signaling cascade. In addition, PTHrP and PTH repressed RunX2 and osx expression. Overexpression of either gene, however, could not relieve PTHrP and PTH's inhibitory actions. Our data suggest that multiple parallel mechanisms are involved in the inhibition of osteoblast differentiation and matrix mineralization by PTHrP and PTH.

INTRODUCTION

PTH-related peptide (PTHrP) and PTH are potent inhibitors of osteoblast differentiation in vitro by as yet unexplained mechanisms.

MATERIALS AND METHODS

We treated murine bone marrow stromal cells and the mesenchymal progenitor cell line KS483 with PTHrP and PTH in combination with either BMPs or hedgehog (Hh) and measured early and late markers of osteoblast differentiation and studied the expression of RunX2 and Osterix (osx). In addition, we examined the PTHrP and PTH response in stable KS483 cells overexpressing either RunX2 or osx.

RESULTS

PTHrP and PTH inhibited BMP- and Hh-induced osteogenesis downstream of early BMP signaling and by downregulation of components of the Hh signaling cascade. PTHrP and PTH prevented the upregulation of RunX2 expression associated with osteoblast differentiation in an indirect response. However, PTHrP and PTH could still inhibit differentiation, and particularly matrix mineralization, of cells expressing RunX2. In addition, PTHrP and PTH potently downregulated osx expression only in mature osteoblasts in an intermediate early response, but osx overexpression could not relieve the inhibitory effects of PTHrP and PTH on matrix mineralization.

CONCLUSIONS

Our data suggest that, besides transcriptional repression of RunX2 and osx, other mechanisms in parallel with or downstream of RunX2 and osx are involved in the inhibition of osteoblast differentiation and matrix mineralization by PTHrP and PTH in vitro.

Exploring the mechanisms regulating regeneration of deer antlers

Deer antlers are the only mammalian appendages capable of repeated rounds of regeneration; every year they are shed and regrow from a blastema into large branched structures of cartilage and bone that are used for fighting and display. Longitudinal growth is by a process of modified endochondral ossification and in some species this can exceed 2 cm per day, representing the fastest rate of organ growth in the animal kingdom. However, despite their value as a unique model of mammalian regeneration the underlying mechanisms remain poorly understood. We review what is currently known about the local and systemic regulation of antler regeneration and some of the many unsolved questions of antler physiology are discussed. Molecules that we have identified as having potentially important local roles in antlers include parathyroid hormone-related peptide and retinoic acid (RA). Both are present in the blastema and in the rapidly growing antler where they regulate the differentiation of chondrocytes, osteoblasts and osteoclasts in vitro. Recent studies have shown that blockade of RA signalling can alter cellular differentiation in the blastema in vivo. The trigger that regulates the expression of these local signals is likely to be changing levels of sex steroids because the process of antler regeneration is linked to the reproductive cycle. The natural assumption has been that the most important hormone is testosterone, however, at a cellular level oestrogen may be a more significant regulator. Our data suggest that exogenous oestrogen acts as a 'brake', inhibiting the proliferation of progenitor cells in the antler tip while stimulating their differentiation, thus inhibiting continued growth. Deciphering the mechanism(s) by which sex steroids regulate cell-cycle progression and cellular differentiation in antlers may help to address why regeneration is limited in other mammalian tissues.

Modeling the interactions between osteoblast and osteoclast activities in bone remodeling

We propose a mathematical model explaining the interactions between osteoblasts and osteoclasts, two cell types specialized in the maintenance of the bone integrity. Bone is a dynamic, living tissue whose structure and shape continuously evolves during life. It has the ability to change architecture by removal of old bone and replacement with newly formed bone in a localized process called remodeling. The model described here is based on the idea that the relative proportions of immature and mature osteoblasts control the degree of osteoclastic activity. In addition, osteoclasts control osteoblasts differentially depending on their stage of differentiation. Despite the tremendous complexity of the bone regulatory system and its fragmentary understanding, we obtain surprisingly good correlations between the model simulations and the experimental observations extracted from the literature. The model results corroborate all behaviors of the bone remodeling system that we have simulated, including the tight coupling between osteoblasts and osteoclasts, the catabolic effect induced by continuous administration of PTH, the catabolic action of RANKL, as well as its reversal by soluble antagonist OPG. The model is also able to simulate metabolic bone diseases such as estrogen deficiency, vitamin D deficiency, senescence and glucocorticoid excess. Conversely, possible routes for therapeutic interventions are tested and evaluated. Our model confirms that anti-resorptive therapies are unable to partially restore bone loss, whereas bone formation therapies yield better results. The model enables us to determine and evaluate potential therapies based on their efficacy. In particular, the model predicts that combinations of anti-resorptive and anabolic therapies provide significant benefits compared with monotherapy, especially for certain type of skeletal disease. Finally, the model clearly indicates that increasing the size of the pool of preosteoblasts is an essential ingredient for the therapeutic manipulation of bone formation. This model was conceived as the first step in a bone turnover modeling platform. These initial modeling results are extremely encouraging and lead us to proceed with additional explorations into bone turnover and skeletal remodeling.

B-type natriuretic peptide exerts broad functional opposition to transforming growth factor-beta in primary human cardiac fibroblasts: fibrosis, myofibroblast conversion, proliferation, and inflammation

The natriuretic peptides, including human B-type natriuretic peptide (BNP), have been implicated in the regulation of cardiac remodeling. Because transforming growth factor-beta (TGF-beta) is associated with profibrotic processes in heart failure, we tested whether BNP could inhibit TGF-beta-induced effects on primary human cardiac fibroblasts. BNP inhibited TGF-beta-induced cell proliferation as well as the production of collagen 1 and fibronectin proteins as measured by Western blot analysis. cDNA microarray analysis was performed on RNA from cardiac fibroblasts incubated in the presence or absence of TGF-beta and BNP for 24 and 48 hours. TGF-beta, but not BNP, treatment resulted in a significant change in the RNA profile. BNP treatment resulted in a remarkable reduction in TGF-beta effects; 88% and 85% of all TGF-beta-regulated mRNAs were affected at 24 and 48 hours, respectively. BNP opposed TGF-beta-regulated genes related to fibrosis (collagen 1, fibronectin, CTGF, PAI-1, and TIMP3), myofibroblast conversion (alpha-smooth muscle actin 2 and nonmuscle myosin heavy chain), proliferation (PDGFA, IGF1, FGF18, and IGFBP10), and inflammation (COX2, IL6, TNFalpha-induced protein 6, and TNF superfamily, member 4). Lastly, BNP stimulated the extracellular signal-related kinase pathway via cyclic guanosine monophosphate-dependent protein kinase signaling, and two mitogen-activated protein kinase kinase inhibitors, U0126 and PD98059, reversed BNP inhibition of TGF-beta-induced collagen-1 expression. These findings demonstrate that BNP has a direct effect on cardiac fibroblasts to inhibit fibrotic responses via extracellular signal-related kinase signaling, suggesting that BNP functions as an antifibrotic factor in the heart to prevent cardiac remodeling in pathological conditions.

Recapitulation of the parathyroid hormone-related peptide-Indian hedgehog pathway in the regenerating deer antler

Parathyroid hormone (PTH)-related peptide (PTHrP) and the PTH/PTHrP receptor (PPR) play an essential role in controlling growth plate development. The aim of the present study was to use the deer antler as a model to determine whether PTHrP and PPR may also have a function in regulating cartilage and bone regeneration in an adult mammal. Antlers are the only mammalian appendages that are able to undergo repeated cycles of regeneration, and their growth from a blastema involves a modified endochondral process. Immunohistochemistry was used to establish sites of localization of PTHrP and PPR in antlers at different stages of development. The pattern of Indian Hedgehog (IHH) and transforming growth factor-beta1 (TGF beta1) distribution was also investigated, because PTHrP expression in the developing limb is regulated by IHH and during embryonic growth plate formation TGF beta1 acts upstream of PTHrP to regulate the rate of chondrocyte differentiation. In the antler blastema (<10 days of development), PTHrP, PPR, and TGF beta1 were localized in epidermis, dermis, regenerating epithelium, and in mesenchymal cells but IHH expression was not detected. In the rapidly growing antler (weeks 4-8 of development), PTHrP, PPR, and TGF beta1 were localized in skin, perichondrium, undifferentiated mesenchyme, recently differentiated chondrocytes, and in perivascular cells in cartilage but not in fully differentiated hyperytrophic chondrocytes. IHH was restricted to recently differentiated chondrocytes and to perivascular cells in cartilage. In mineralized cartilage and bone, PTHrP, PPR, IHH, and TGF beta1 were immunolocalized in perivascular cells and differentiated osteoblasts. PTHrP and PPR were also present in the periosteum. TGF beta1 in vitro stimulated PTHrP synthesis by cells from blastema, perichondrium, and cartilage. The findings of this study suggest that molecules which regulate embryonic skeletal development and postnatal epiphyseal growth may also control blastema formation, chondrogenesis, and bone formation in the regenerating deer antler. This finding is further evidence that developmental signaling pathways are recapitulated during adult mammalian bone regeneration.

Effect of overexpressing fibroblast growth factor 2 protein isoforms in osteoblastic ROS 17/2.8 cells

Fibroblast growth factor-2 (FGF-2) is made by osteoblasts and modulates their function. There are high molecular weight (HMW) protein isoforms of FGF-2 that have nuclear localization sequences and a low molecular weight (LMW) 18 kDa FGF-2 protein that is exported from cells. Since FGF-2 is a trophic factor and potent mitogen for osteoblasts, the goal of this study was to utilize targeted overexpression of FGF-2 as a novel means of assessing different FGF-2 isoforms on osteoblastic cell viability and proliferation. Either LMW or HMW human Fgf2 cDNAs were cloned downstream of 3.6 kb alpha1(I)-collagen 5' regulatory elements (Col 3.6). A set of expression vectors, called Col3.6-Fgf2 isoforms-IRES-GFPsaph, capable of concurrently overexpressing either LMW or HMW FGF-2 isoforms concomitant with GFPsaph from a single bicistronic mRNA were built. Viable cell number in ROS 17/2.8 cells stably transfected with Vector (Col3.6-IRES-GFPsaph) versus each of the Col3.6-Fgf2-IRES-GFPsaph constructs were compared. In the presence of 1 or 10% serum, DNA synthesis was increased in cells expressing any isoform of FGF-2 compared with vector. However, cells transfected with HMW isoform had augmented DNA synthesis in 1 or 10% serum compared with cells expressing either ALL or LMW FGF-2 isoforms. A neutralizing FGF-2 antibody significantly reduced the mitogenic response in cells harboring ALL or the LMW FGF-2 isoforms but did not block the mitogenic effect of cells harboring the HMW isoforms. In summary, overexpression of any isoform of FGF-2 protein increased viable cell number and OB proliferation in the presence of low or high concentrations of serum. However, the HMW/nuclear isoforms preferentially mediate augmented OB proliferation. We conclude that differential expression of FGF-2 proteins isoforms is important in modulating OB function.

Effect of parathyroid hormone-related protein on fibroblast proliferation and collagen metabolism in human skin

The parathyroid hormone-related protein (PTHrp), structurally similar to the parathyroid hormone (PTH) in its NH(2)-terminal part, was first identified as a tumour-derived peptide responsible for a paraneoplastic syndrome known as humoral hypercalcemia of malignancy. The PTHrp gene is expressed not only in cancer but also in normal tissues during adult and/or fetal life, where it plays predominantly paracrine and/or autocrine roles. In the skin PTHrp produced by keratinocytes acts on fibroblasts by complex cooperative circuits involving cytokines and growth factors. In this report, we studied the direct effects of synthetic PTHrp 1-40 on proliferation and collagen synthesis and matrix metalloproteinase-2 (MMP-2) activity in cultures of fibroblasts isolated from normal human skin. Fibroblasts exposure to varying doses of PTHrp for 48 h, significantly and dose-dependently inhibited proliferation evaluated by [(3)H]-thymidine incorporation into DNA. A dose-dependent stimulation of cAMP released into the medium was concomitantly observed. In contrast, PTHrp had no effect on collagen synthesis evaluated either by [(3)H]-proline incorporation or by radioimmunoassay (RIA) of the carboxyterminal fragment of type I procollagen (PICP). MMP-2 activity, evaluated by quantitative zymographic analysis, was significantly increased by PTHrp treatment at doses of 160 and 320 nM. These findings indicate that PTHrp may play a role in normal dermal physiology by controlling both fibroblast proliferation and extracellular matrix degradation.

Age-related bone loss: old bone, new facts

The human skeleton serves several functions for the body: support, locomotion, protection of vital organs, and housing of bone marrow. Bone remodeling is the result of the interactions of multiple elements, including osteoblasts, osteoclasts, hormones, growth factors, and cytokines, the end result being the maintenance of the bone architecture and to maintain systemic calcium homeostasis. In early life, a careful balance exists between bone formation by osteoblasts and bone resorption by osteoclasts. With aging, the process of coupled bone formation is affected by the reduction of osteoblast differentiation, activity, and life span which is further potentiated in the perimenopausal years with hormone deprivation and increased osteoclast activity. Age-related bone loss is thus not only a consequence of hormone deprivation, but also the result of changes in bone formation and cell-cell interactions with a unique pathophysiology. In this review, we describe the cellular and metabolic changes associated with aging bone and present recent evidence regarding cell differentiation within the bone marrow. We also consider the mechanism of programmed cell death, apoptosis, as being an important determinant of aging in bone as well as describe possible future interventions to prolong the life span of osteoblasts.

Parathyroid hormone-related protein overexpression in the human colon cancer cell line HT-29 enhances adhesion of the cells to collagen type I

The present experiments examined the potential ability of parathyroid hormone-related protein (PTHrP) to influence growth of the human colon cancer cell HT-29 and the ability of the cell to adhere to several extracellular matrix (ECM) proteins found in normal tissues. Addition of PTHrP analogs, PTHrP (1-34), PTHrP (67-86), or PTHrP (107-139), to HT-29 cells in culture did not influence cell growth or the adhesion of the cells to wells coated with fibronectin, laminin, or collagen type I. Likewise, in HT-29 cells induced to overexpress PTHrP by stable transfection with PTHrP cDNA, compared to vector-transfected control HT-29 cells, no effect on cell growth occurred. However, in the transfected cells, the increased production of PTHrP significantly enhanced cell adhesion to type I collagen but not to fibronectin or laminin. The results raise the possibility that PTHrP might play a role in colon tumor invasion and metastasis by influencing cell adhesion to specific extracellular matrix proteins.

Pathophysiological mechanisms of vascular calcification in end-stage renal disease

Vascular calcification has been clearly defined as a risk factor for cardiovascular mortality in the general population and is highly prevalent in end-stage renal disease (ESRD), where it is associated with a number of markers of increased mortality such as left ventricular hypertrophy. The pattern of calcification in ESRD is characterized by mineral deposition in the tunica media, in contrast to non-ESRD populations, where calcification of atheromatous plaque predominates. This difference may have important clinical implications. The pathophysiological mechanisms underlying both types of vascular calcification remain to be clarified; however, current evidence suggests that they are active processes rather than passive mineral precipitation, and the presence in the vasculature of cells expressing an osteoblastic phenotype may be of central importance. In ESRD, the presence of secondary and tertiary hyperparathyroidism, disordered calcium and phosphate homeostasis, and the use of vitamin D- and calcium-based treatments in its therapy may all contribute to vascular calcification. These issues and the impact on other current and future therapies have great importance for clinical nephrology, and a better understanding of vascular calcification through a focused research effort is essential.

Overexpression of parathyroid hormone-related protein promotes cell growth in the rat intestinal cell line IEC-6

The rat intestinal cell line, IEC-6, was used as a model to study effects of parathyroid hormone-related protein (PTHrP) on crypt cell growth. Studies showed that addition of PTHrP analogs (1-34), (67-86), or (107-139) to growth medium did not affect proliferation of cells grown in either high (10% Nu-Serum) or low serum (1% Nu-Serum). However, studies on clonal lines of IEC-6 cells stably transfected with PTHrP cDNA and overexpressing PTHrP showed that increased PTHrP production enhanced cell growth and 3H-thymidine incorporation in high, but not low, serum. Additional studies examined the role of the nuclear localization sequence (NLS) of PTHrP in mediating the growth effect. In three clonal IEC-6 lines transfected with PTHrP cDNA bearing a mutated NLS, the ability of PTHrP to stimulate 3H-thymidine incorporation and cell growth was lost. The results suggest that endogenously produced PTHrP can promote proliferation of IEC-6 cells and that the integrity of the NLS of PTHrP is required for its growth effects.

Cranial sutures as intramembranous bone growth sites

Intramembranous bone growth is achieved through bone formation within a periosteum or by bone formation at sutures. Sutures are formed during embryonic development at the sites of approximation of the membranous bones of the craniofacial skeleton. They serve as the major sites of bone expansion during postnatal craniofacial growth. For sutures to function as intramembranous bone growth sites, they need to remain in an unossified state, yet allow new bone to be formed at the edges of the overlapping bone fronts. This process relies on the production of sufficient new bone cells to be recruited into the bone fronts, while ensuring that the cells within the suture remain undifferentiated. Unlike endochondral growth plates, which expand through chondrocyte hypertrophy, sutures do not have intrinsic growth potential. Rather, they produce new bone at the sutural edges of the bone fronts in response to external stimuli, such as signals arising from the expanding neurocranium. This process allows growth of the cranial vault to be coordinated with growth of the neurocranium. Too little or delayed bone growth will result in wide-open fontanels and suture agenesis, whereas too much or accelerated bone growth will result in osseous obliteration of the sutures or craniosynostosis. Craniosynostosis in humans, suture fusion in animals, and induced suture obliteration in vitro has been associated with mutations or alterations in expression of several transcription factors, growth factors, and their receptors. Much of the data concerning signaling within sutures has been garnered from research on cranial sutures; hence, only the cranial sutures will be discussed in detail in this review. This review synthesizes classic descriptions of suture growth and pathology with modern molecular analysis of genetics and cell function in normal and abnormal suture morphogenesis and growth in a unifying hypothesis. At the same time, the reader is reminded of the importance of the suture as an intramembranous bone growth site.