systemic sclerosis ( ssc , scleroderma ) is an autoimmune disease characterized early in the process by vasculopathy and subsequently by varying degrees of fibrosis in skin , lungs , and other tissues . the presence of vasculopathy is the hallmark of this condition , represented clinically as raynaud 's syndrome which occurs almost universally in both the limited and diffuse cutaneous subsets of this disease . vasculopathy possibly results from abnormal vasoreactivity , hypoxia , and/or direct damage of vascular and perivascular cells . perivascular inflammatory infiltrates and neoangiogenesis ensues resulting in varying degrees of fibrosis in the skin and internal organs . this paper describes why details of the vascular microenvironment might determine the degree of end - organ damage that occurs in ssc , with a focus on vascular cell senescence , endothelial progenitor cells ( epc ) including mesenchymal stem cells ( msc ) , pericytes , and angiogenic monocytes . an explanation of the role of epc , pericytes , and angiogenic monocytes is important to understanding ssc pathogenesis . ssc is thought to be a genetically complex disease , influenced by multiple genes , with a substantial environmental component . nonetheless , ssc occurs significantly more frequently in families with scleroderma ( 1.6% ) than in the general population ( 0.026% ) . genome - wide association studies have found a strong association with the hla ii region on chromosome 6 , and non - hla candidate genes that regulate interferon production , such as interferon regulatory factor 5 ( irf 5 ) as well as genes that regulate immunological responses , such as signal transducer and activator 4 ( stat 4 ) [ 5 , 6 ] . as such , systemic sclerosis is an autoimmune disease ; however the inherited effects of vasculopathy and fibrosis remain to be determined . our previous work showed that vasculopathy imparts a greater relative risk to family members than does autoimmune inflammatory conditions or fibrotic lung disease . irrespective of the subset of ssc , perivesicular inflammatory infiltrates result in endothelial derangement in lesioned as well as perilesional tissue [ 10 , 11 ] . these perivascular changes precede the excessive accumulation of extracellular matrix components , and fibrosis may represent a default pathway from vascular failure [ 12 , 13 ] . the histopathological hallmark in ssc is a result of endothelium activation with cell adhesion molecule expression , inflammatory cell recruitment , intimal proliferation , and adventitial fibrosis , which results in apoptosis of endothelial cells [ 13 , 14 ] .    despite the ensuing severe tissue hypoxia , proper adapted angiogenesis vascular cells normally have a finite lifespan which is determined in part by telomere length and/or telomerase activity . telomerase is a reverse transcriptase which helps maintain telomere length , thereby preventing cell senescence and protecting chromosomes from aberration . although telomerase activity is increased in many connective tissue diseases , it is decreased in systemic sclerosis ( ssc ) , perhaps due to gene polymorphism [ 16 , 17 ] . artlett and colleagues reported a decrease telomere length in a combined cohort of limited ssc ( lssc ) and diffuse ssc ( dssc ) whereas macintyre and colleagues reported increased telomere length and lack of age - related telomere erosion in lssc [ 18 , 19 ] .   in a pilot study , we used a monochrome multiplex quantitative pcr ( mmqpcr ) method to evaluate the relative telomere lengths ( t / s ratios ) in dna samples of 6 lssc ( 1 male ; 5 females ) and 6 dssc ( 3 males ; 3 females ) aging 4060 , and 50 healthy controls ( hc ) aging 3760 . two factors were statistically associated ( p value < .001 ) to t / s : age and diagnosis ( figure 1 ) . not correcting for age , the average length measure was 1.2 for normals , 1.15 for dssc and 0.96 for lssc patients ( figure 2 ) . telomere length , which is shorter in ssc patients than in normal hc , is possibly a risk factor for vasculopathy . while the appearance of vasculopathy does not vary per subtype of ssc , the effect of telomere length on the fibrocyte or myofibroblast may be different in lssc and dssc , possibly contributing to differences in disease manifestations .   the reduced telomere length in the endothelial cell likely results in chronic underperfusion and ischemia in the skin and internal organs in both lssc and dssc . however , if fibrosis is the default pathway of insufficient angiogenic response , the subsequent reduced lifespan of the fibrocyte ( determined by telomere length ) may be protective in the lssc subtype . the vascular network is a dynamic organ with an estimated surface area of > 1000  m . neovascularization is a complex process that requires both the mobilization of cells derived from the bone marrow , named endothelial progenitor cells ( epcs ) , and proliferation and differentiation of resident cells , known as pericytes , to migrate to the correct location and assemble into vascular structures . new vessels are produced by a combination of angiogenesis and vasculogenesis .   in angiogenesis , fully differentiated endothelial cells arise from pre - existing vessels whereas vasculogenesis describes the formation of new vessels by circulating epc which act to replenish damaged or senescent blood vessels . this process requires dynamic and temporally regulated interactions between   endothelial cells , soluble proangiogenic and antiangiogenic growth factors , and extracellular matrix molecules . primary contact between endothelial cells and mural cells ( pericyte and vascular smooth muscle cells ) is central to the regulation of vascular formation in angiogenesis . recently formed endothelial tubes are initially unstable and become stabilized through the formation of a peri - vascular matrix and the connection with pericytes . pericytes are embedded within the endothelial basement membrane and are found primarily around blood capillaries , precapillary arterioles , postcapillary venules , and collecting venules . they are arranged to facilitate and assimilate cell communication .   with particular interest to ssc , pericytes may play a role in ectopic calcification and are able to transdifferentiate into fibroblast - like cells if they escape from the capillary basement membrane . the pericyte role as a perivasicular mesenchymal stem cell with macrophage - like properties has not been welldefined in ssc , but is intriguing . its ability to perform this function is correlated with marker expression and the microenvironment of the endothelial - pericyte contact . most likely , specific intercellular signals mediated by ligand - receptor systems are required for endothelial and pericyte vascular stability . numerous studies demonstrate the critical importance of transforming growth factor- ( tgf)-beta signaling for vascular development and function . tgf - beta has context - dependent effects on endothelial cells ; proliferation is mediated by signaling through alk / smad1/5 and differentiation is mediated by alk smad2/3 . tgf - beta / smad signaling has been suggested to play a key role in the pathogenesis of ssc .    in postnatal vasculogenesis identification and quantification of epc population in ssc has been challenging and has resulted in consensus recommendations to help unify epc research . research by avouac and colleagues , using an accurate , reliable , and reproducible method of epc quantification , supports that ssc is associated with epc mobilization , but in active or severe stages , epc may be recruited to injured sites and thus decrease in the circulation . multipotential mesenchymal stem cells ( mscs ) might be a source of epc in vasculogenesis . mscs show normal functional properties and a normal pattern of biological markers , but the angiogenic potential of these endothelial - like mscs is reduced . cipriani and colleagues showed that when msc from ssc patients are seeded on matrigel , they have a reduced ability to form capillary - like structures and give rise to incomplete endothelial networks , even after vascular endothelial growth factor ( vegf ) and stromal - derived factor ( sdf-1 ) stimulation [ 23 , 32 ] . vegf is an important angiogenic peptide with specific proliferative , differentiation , and mobilization effects on epcs , and is known to be upregulated in ssc , especially in advanced disease . vegf gene expression is also regulated by growth factors ( such as tgf - beta ) and other proinflammatory cytokines . the platelet - derived growth factor ( pdgf ) family is essential to vascular remodeling and maturation .   in a study of 62 ssc patients , epcs were significantly increased in patients with early - stage ssc disease , but not in those with late disease irrespective of diagnosis subtype , and there was no correlation between the number of circulating epcs and vegf . bone marrow biopsy samples from 14 of these ssc patients ( 3 early limited ssc , 4 with late limited ssc , 4 with early diffuse ssc , and 3 with late ssc ) showed fewer and functionally impaired epcs in all patients . another study showed that the subset of ssc patients with digital lesions and high severity scores had low epc counts . it is possible that bone marrow from ssc patients can not satisfy the continuous and prolonged demand for epcs , despite the target organ increase in vegf . the role of target organ microvascular environment ( pericytes and endothelial cells ) , which is producing tgf - beta , vegf , and pdgf , on ssc pathogenesis is less clear . the elevated total number and activated state of circulating endothelial cells ( cecs ) , suggest vascular damage and endothelial activation in ssc patients regardless of subtype correlates to disease activity . it is also known that tgf - beta and pdgf from this microvasculature cooperate in inducing the activation of fibroblasts and their differentiation into myofibroblasts in ssc patients . thus , understanding the microvascular environment of target organs in ssc is of primary importance . it is suggested that the major contribution of the bone marrow to angiogenic processes may come from progenitors of the periendothelial vascular mural cells . endothelial differentiation of monocyte - derived multipotential cells ( momcs ) can occur with angiogenic stimuli and result in the formation of mature endothelial cell tubules in matrigel cultures . pericytes establish morphologic interactions with transmigrating leukocytes , mainly monocytes ( macrophages ) .   during angiogenesis , macrophages contribute to the dissociation and detachment of pericytes from the endothelial cell . pericytes can act as antigen - presenting cells and can behave as macrophages ; they also can show plasticity with potential to become myofibroblasts . thus , understanding the role of the interaction of circulating angiogenic monocyte and resident pericyte in ssc microvasculature has important implications . it is possible that this interaction is of primary importance for linking the inflammatory aspect of the disease to the vascular abnormalities . stromal cell - derived factor-1 ( sdf-1 ) and its receptor ( cxcr4 ) system is a component of the microvascular environment which is extremely important for new vessel formation . sdf-1 released by endothelial cells creates a gradient dictating directional response of endothelial cells expressing cxcr4 . skin biopsies in early disease of both ssc subtypes show a strong positive pattern of sdf-1 and its receptor cxcr4 in the endothelial cells and pericytes of microvessels , attesting to an attempted reparative process . of interest , in diffuse ssc , these skin biopsies also showed dense mononuclear cells in the perivascular infiltrate , possibly suggesting a role of the monocytes in a more fibrotic phenotype . the staining for cxcr4 was weak in the late ( sclerotic or atrophic ) phases in both ssc subsets . another study of 40 ssc patients demonstrated higher serum levels of vegf , pdgf , and increased concentration of sdf-1 , particularly in the diffuse subset .   in this same study population circulating cxcr4 + circulating progenitor cells coexpressing monocytic and endothelial cells positively correlated to the severity score , modified rodnan skin score , and pulmonary involvement . taken in sum , these results suggest that overall disease activity correlates to the markers of activity in the microvascular environment . it has recently been suggested that the actual angiogenic cell type recruited to the site of tissue injury and incorporated into a newly formed vessel is a monocyte .   activated circulating monocytes have also been reported in ssc patients , supporting a potential role of these cells in disease pathogenesis . gene expression profiling of peripheral blood monocytes from ssc patients suggest that type i interferon may play a key role in the activation of monocytes in this disease . if during the course of the disease , the mechanism of angiogenesis is impaired , the proangiogenic factors in the microvascular environment may serve to recruit proangiogenic monocytes which , with pericytes , result in overactivity of a myofibroblast phenotype .   in a preliminary study , there were no significant differences in the expression of circulating monocyte surface molecules involved with cell transformation , function , or migration presumed to give rise to fibrocytes , in 8 patients with limited ssc . it is possible that the role of the angiogenic monocyte may be greater in the diffuse subset of ssc and have prognostic implications . therapeutics that modulate the phenotype of reparative cells can offer new opportunities for ssc treatment . in particular , multipotential mscs have attracted interest because of low acute toxicity and their availability . the potential of human momcs which can proliferate and differentiate along the endothelial lineage in a specific permissive environment also may represent an autologous transplantable cell source for therapeutic neovasculogenesis . in early ssc , prevention of vascular senescence may be most important . n - acetyl - cysteine ( nac ) , a chemopreventive antiangiogenic and antiapoptotic drug has been suggested to modulate   parameters associated with endothelial cell aging . pilot data suggests that the statin class of medications may be beneficial in treating vascular manifestations of ssc , through an increase in angiogenic factors and reduction of vascular endothelial activation / injury markers ( p < .01 for all comparisons ) . cyclophosphamide , which remains the current gold standard for treatment of interstitial lung disease , is known to mobilize epc . nutraceutical - based mobilization of epc is an area of interest in the biomedical field , and has not yet been reported in ssc .    for the fibrotic aspect of ssc , the small molecule tyrosine kinase inhibitor imatinib and related drugs , such as dasatinib and nilotinib , which simultaneously target two of the major profibrotic pathways , tgf - beta- and pdgf - signaling are being studied . the effect of these drugs on the microvascular environment , and their efficacy and tolerability in ssc patients are not yet known . other anti - tgf - beta therapies are also in development and may have a major impact in systemic sclerosis . however , considerable concern regarding safety is needed given its pro- and antiangiogenic effects at different concentrations . ifn inhibitors are also under investigation for treatment of ssc , though modulation of interferon may be most effective in the diffuse subset , in which there is a higher perivascular monocyte infiltrate .   specifically , therapies that inhibit transdifferentiation of other cell types , such as pericytes and angiogenic monocytes   into fibroblasts and myofibroblasts hold promise . a predisposition to vascular senescence is probable in ssc and the pathogenesis may arise from a subsequent defect in vasculogenesis ( possibly due to abnormal bone marrow function ) and/or angiogenesis ( perhaps due to pericyte and angiogenic monocytes ) followed by overactivity of activated fibroblasts and myofibroblasts . . early ssc may be most amenable to treatments that decrease vascular senescence and increase epc mobilization . surprisingly , diffuse cutaneous ssc may be more responsive to therapeutics , which modulate pericyte and angiogenic monocyte differentiation into activated fibroblasts and myofibroblasts . the difficulty with therapeutics which modulate growth factor and chemokines is that locally varying levels of these substances are necessary for regulation of migration , proliferation , cell - cell interactions , differentiation , and extracellular matrix deposition . nonetheless , an improved understanding of the principle regulatory mechanisms of angiogenesis in ssc has profound potential therapeutic value . it is exciting to think that through understanding of the microvascular environment in ssc , that subsequent restoration of proper angiogenesis in ssc could limit fibrotic damage .
the role of the vascular microenvironment in the pathogenesis systemic sclerosis ( ssc ) is appreciated clinically as raynaud 's syndrome with capillary nail bed change . 
 this manifestation of vasculopathy is used diagnostically in both limited and diffuse cutaneous subsets of ssc , and is thought to precede fibrosis . 
 the degree of subsequent fibrosis may also be determined by the vascular microenvironment . 
 this paper describes why the vascular microenvironment might determine the degree of end - organ damage that occurs in ssc , with a focus on vascular cell senescence ,   endothelial   progenitor cells ( epc ) including   multipotential mesenchymal stem cells ( msc ) , pericytes , and angiogenic monocytes . 
 an explanation of the role of epc , pericytes , and angiogenic monocytes is important to an understanding of ssc pathogenesis . 
 an evolving understanding of the vascular microenvironment in ssc may allow directed treatment .