Bone formation never occurs without vascular interactions.1–3 This obvious and simple statement of biological fact does not do justice to the physiological and pharmacological implications of the relationship. The vasculature is, of course, the conduit for calcium phosphate and nutrient exchange between bone and the rest of the body; this is relevant not only to the rapid mobilization of skeletal calcium when urgently needed4 but also to the delivery of metabolic substrate to the basic multicellular unit (BMU) for bone-forming osteoblast functions.5 However, the vasculature also provides the sustentacular niche for osteoblast progenitors,6 and is the conduit for egress of bone marrow-derived formed elements from the osteoblast-regulated hematopoietic niche.7 Indeed, Ignarro and colleagues8 have coupled parathyroid hormone (PTH)-mediated bone anabolism with granulocyte colony stimulating factor (G-CSF) treatment as a strategy to mobilize endothelial progenitor cells (ePCs) and enhance ischemic limb recovery in mice. Surprisingly, the inextricable interdependence of vascular physiology, skeletogenesis, bone remodeling, and mineral metabolism has escaped widespread appreciation until very recently. Studies by Clemens et al.9 established that the constitutive elaboration of vascular endothelial growth factor A (VEGF) by osteoblasts increases bone formation via mechanisms that require expansion of skeletal vascularity. They went on to show that pharmacologic augmentation of endogenous skeletal VEGF production accelerates skeletal repair during distraction osteogenesis.10 Exogenous VEGF implants exhibit similar properties in critical bone defects.11 However, the regulation of bone vascular biology and anatomy in response to metabolic, morphogenetic, mechanical, inflammatory, and endocrine cues is very poorly understood. A fundamental understanding of bone–vascular interactions will be necessary to develop new strategies that safely and efficaciously enhance bone formation in the settings of malignancy, fracture nonunion, the open epiphyses of childhood, and the arteriosclerotic vasculopathy of diabetes, uremia, rheumatoid arthritis, and advanced age.

In this issue of the Journal,12 Prisby, Lafage-Proust, and colleagues12 address this important unmet scientific need by examining the regulation of long bone vasculature by PTH, the prototypic osteoanabolic hormone, in growing rats. PTH dosing transiently upregulates VEGF expression in bone followed by subsequent increases in bone formation and bone mass. The skeletal anabolic response to PTH is fully dependent upon intact VEGF signaling; PTH-induced increases in the key histomorphometric indices of osteoblast synthetic function and numbers—and accompanying improvements in trabecular bone mass and architecture—were all inhibited by administration of a VEGF-neutralizing antibody. Anti-VEGF treatment also inhibited PTH induction of Neuropilin1 and Neuropilin2, two key VEGF coreceptors that enhance endothelial sprouting and migration in response to matrix-bound VEGF isoforms liberated with matrix turnover.13 Intriguingly, bone vessel density was not increased by PTH administration;12 in fact, bone vessel density was decreased as assessed by synchrotron radiation microCT analysis, paralleling changes in trabecular osteoclast numbers.12 Of note, this response is remarkably different from the bone anabolic responses elicited by genetically induced, sustained osteoblast VEGF secretion;9 bone anabolic responses arising from unrestrained paracrine VEGF signaling markedly increases vessel density.9 The investigators' painstakingly detailed histomorphometric analyses demonstrated that PTH treatment reduced the distance between capillaries or small arterioles (<29-micron diameter) and the osteoid surface of the BMU.12 However, PTH did not change the spatial relationships for larger vessels (>100-micron diameter). Additionally, no PTH-induced differences were noted in the proximity of vessels of any diameter to bone trabeculae in general, highlighting the specific reorientation of microvessels toward sites of new bone formation.12 Thus, the authors conclude that intermittent PTH exerts bone anabolism via VEGF-dependent mechanisms that do not involve changes in skeletal vascular mass.12 Rather, intermittent PTH dosing regulates the spatial relationships between the anabolic BMU and the microvasculature, localizing capillaries near sites of new bone formation as necessary for bone mass accrual.

Why is this manuscript so significant? Firstly, the ingenious combination of barium sulfate perfusion with Goldner trichrome staining and digital image analysis of tibial plastic sections is a technical advance that should not go unheralded. As such, the authors' approach transforms and elevates our expectations for state-of-the-art histomorphometric phenotyping of bone anabolic responses. This unparalleled, quantitatively robust analysis of bone vascular anatomy and its regulation by PTH has provided truly novel insights into how anabolic signals control and coordinate bone mass accrual via VEGF signaling in the bone–vascular axis.14 As a transient osteoblast VEGF “secretagogue,” the endocrine actions of circulating PTH pulses12 may now be seen to help maintain BMU–microvascular proximity via VEGF paracrine signals that sustain osteoblast synthetic functions. However, although pharmacological proof-of-concept is in hand,12 the impact of endogenous endocrine PTH activity on osteoblast VEGF expression and BMU–bone microvascular alignment has yet to be examined. Nevertheless, this new biology is important to consider in the clinic—particularly so in the setting of end-stage renal disease where uremia-induced PTH resistance, aggressive parathyroidectomy, and/or calcitriol dosing can interact to iatrogenically induce a devastating low turnover mineral and bone disorder.15 Moreover, because inhibitors of VEGF signaling are increasingly utilized for treatment of malignancy,16 significant potential exists for inducing low turnover bone disease in this setting as well via perturbation of the bone–vascular axis relationship. Of note, aminobisphosphonates potently inhibit angiogenesis and endothelial cell (EC) migration;17 thus, alterations in the bone–vascular axis may explain in part the blunting of trabecular bone anabolic responses to PTH treatment when aminobisphosphonates are coadministered.18

Secondly, Prisby et al.12 highlight that it is the proximity of the BMU to the microvasculature—not the mass of the skeletal microvasculature—that is critical in bone anabolism. To be sure, perfusion- and diffusion-dependent nutrient supplies must be rate-limiting contributors.19 Indeed, VEGF is a “vascular permeability factor” that enhances trans-endothelial fluid flow, organizes conduit function, and promotes bone canalicular flow.20 However, it has become increasingly evident that the vasculature itself provides paracrine and juxtacrine cues that regulate osteoblast function.21, 22 For example, BMP2 is expressed by microvascular ECs23 and is upregulated by VEGF, hypoxia,23 and mechanical loading.24 Pro-osteogenic Wnt7 family members are also elaborated by ECs and vascular sinusoids in bone.25 In addition, ephrin B2, an important stimulus for anabolic EphB4 signaling in osteoblasts,26 is expressed by ECs27 and enhances EC motility.28 VEGF upregulates ephrin B2 expression during EC differentiation, and subsequent VEGF-induced EC migration to the BMU may provide coordinated juxtacrine and paracrine signals necessary for bone anabolism.

Finally, and most importantly, when taken against the backdrop of the recent studies of Maes et al.,29 the data of Prisby et al.12 demonstrate that the pharmacokinetic–pharmacodynamic (PK-PD) relationship for angiogenic signals in the bone–vascular axis is of vital interest to skeletal pharmacology. Constitutive VEGF expression by osteoblasts not only increases bone mass and skeletal microvascular density, but also increases osteoclast numbers and induces bone marrow fibrosis.29 All three responses—bone mass accrual, skeletal microvascular density, and marrow fibrosis—arise via constitutive downstream activation of skeletal β-catenin.29 The transient kinetics of VEGF induction by intermittent PTH dosing may go far to explain the differences in bone histology of (a) VEGF-dependent bone anabolism arising from intermittent PTH administration12 versus (b) unrestrained osteoblast VEGF expression.9 This notion is supported by the data of Jilka and colleagues30 and Schipani et al.31 Continuous rather than intermittent PTH signaling increases marrow vessel density, osteoblast, and osteoclast number—and responses were abrogated by inhibition of RANKL-mediated osteoclast recruitment.31 Importantly, the recruitment of osteoclasts indirectly enhances bone angiogenesis and fibrosis.32 Of note, an underappreciated aspect of the histological response to intermittent PTH is the complex time course with respect to osteoclast recruitment vs. trabecular and endocortical resorption—modestly increased early on but reduced with time with intermittent dosing in rats,33, 34 mice,35 and humans.36 PTH-induced reductions in osteoclast density were once again noted by Prisby et al.32 in their pulsatile paradigm at 30 days;12 this PK-PD relationship for PTH-regulated trabecular and endocortical osteoclast activity may mechanistically contribute to the “uncoupling” of osteoanabolism and osteoangiogenesis observed with intermittent dosing but not with continuous PTH administration.30 Of note, in the macrovasculature, viz, aortic vascular smooth muscle—cell-autonomous PTH/PTHrP receptor (PTH1R) activation directly inhibits profibrotic and procalcific β-catenin signaling.37 This macrovascular response is immediately germane to the biology of arteriosclerotic calcification and conduit vessel fibrosis37—but, in principle, might also partially mitigate fibrotic responses in marrow microvasculature. The expression and consequences of PTH1R activation in the microvasculature have not yet been studied in sufficient detail. Nevertheless, PTHrP, a selective PTH1R agonist, induces endothelium-dependent arterial vasorelaxation38, 39 and directly inhibits EC migration and proliferation.40 Thus, VEGF-dependent skeletal anabolism arising from approximation of the bone microvasculature to the BMU after PTH administration may reflect the net actions of (1) PTH-induced osteoblast VEGF secretion and augmented nutrient exchange, (2) microvascular EC-derived osteoanabolic signals, and (3) cell-autonomous, PTH1R-modulated EC responses.

As noted above, recruitment of osteoclasts enhances angiogenic responses and marrow vascularity but also engenders marrow fibrosis. Should inhibition of RANKL signaling reduce osteoclast numbers and activity without altering EC migration and proliferation, various combinations of PTH1R agonists, RANKL antagonists, and VEGF ligands may afford finely tuned bone anabolic responses. This might be particularly useful when crafting personalized bone health management strategies along the low turnover—high turnover—marrow fibrosis continuum that confronts physicians and their patients with end-stage renal disease and/or fracture nonunion.41 Without a doubt, the improved understanding of the bone–vascular axis and its regulation will shape future clinical strategies that address these and other unmet needs in musculoskeletal health and healthcare.


  1. Top of page
  2. Disclosures
  3. Acknowledgements
  4. References

The author states that he has no conflicts of interest.


  1. Top of page
  2. Disclosures
  3. Acknowledgements
  4. References

DAT is supported by grants HL69229, HL81138, and HL88651 from the National Institutes of Health, and by the Barnes-Jewish Hospital Foundation.

Author's role: DAT reviewed the literature and wrote the manuscript.


  1. Top of page
  2. Disclosures
  3. Acknowledgements
  4. References
  • 1
    Marotti G, Zallone AZ. Changes in the vascular network during the formation of Haversian systems. Acta Anat (Basel). 1980; 106(1): 84100.
  • 2
    Maes C, Kobayashi T, Selig MK, Torrekens S, Roth SI, Mackem S, Carmeliet G, Kronenberg HM. Osteoblast precursors, but not mature osteoblasts, move into developing and fractured bones along with invading blood vessels. Dev Cell. 2010; 19(2): 32944.
  • 3
    Zelzer E, McLean W, Ng YS, Fukai N, Reginato AM, Lovejoy S, D'Amore PA, Olsen BR. Skeletal defects in VEGF(120/120) mice reveal multiple roles for VEGF in skeletogenesis. Development. 2002; 129(8): 189304.
  • 4
    Qing H, Pajevic PD, Barry K, Dusevich V, Wysolmerski JJ, Bonewald L. PTHR1 in osteocytes plays a major role in perilacunar remodeling through the activation of “osteoclastic” genes in osteocytes. J Bone Miner Res. 2010; 25 (Suppl 1). Available at
  • 5
    Qin L, Mak AT, Cheng CW, Hung LK, Chan KM. Histomorphological study on pattern of fluid movement in cortical bone in goats. Anat Rec. 1999; 255(4): 3807.
  • 6
    Bianco P. Bone and the hematopoietic niche: a tale of two stem cells. Blood. 2011; 117(20): 52818.
  • 7
    Eash KJ, Greenbaum AM, Gopalan PK, Link DC. CXCR2 and CXCR4 antagonistically regulate neutrophil trafficking from murine bone marrow. J Clin Invest. 2010; 120(7): 242331.
  • 8
    Napoli C, William-Ignarro S, Byrns R, Balestrieri ML, Crimi E, Farzati B, Mancini FP, de Nigris F, Matarazzo A, D'Amora M, Abbondanza C, Fiorito C, Giovane A, Florio A, Varricchio E, Palagiano A, Minucci PB, Tecce MF, Giordano A, Pavan A, Ignarro LJ. Therapeutic targeting of the stem cell niche in experimental hindlimb ischemia. Nat Clin Pract Cardiovasc Med. 2008; 5(9): 5719.
  • 9
    Wang Y, Wan C, Deng L, Liu X, Cao X, Gilbert SR, Bouxsein ML, Faugere MC, Guldberg RE, Gerstenfeld LC, Haase VH, Johnson RS, Schipani E, Clemens TL. The hypoxia-inducible factor alpha pathway couples angiogenesis to osteogenesis during skeletal development. J Clin Invest. 2007; 117(6): 161626.
  • 10
    Wan C, Gilbert SR, Wang Y, Cao X, Shen X, Ramaswamy G, Jacobsen KA, Alaql ZS, Eberhardt AW, Gerstenfeld LC, Einhorn TA, Deng L, Clemens TL. Activation of the hypoxia-inducible factor-1alpha pathway accelerates bone regeneration. Proc Natl Acad Sci USA. 2008; 105(2): 68691.
  • 11
    Kaigler D, Wang Z, Horger K, Mooney DJ, Krebsbach PH. VEGF scaffolds enhance angiogenesis and bone regeneration in irradiated osseous defects. J Bone Miner Res. 2006; 21(5): 73544.
  • 12
    Prisby R, Guignandon A, Vanden-Bossche A, Mac-Way F, Linossier MT, Thomas M, Laroche N, Malaval L, Langer M, Peter ZA, Peyrin F, Vico L, Lafage-Proust MH. Intermittent PTH 1-84 is osteoanabolic but not osteoangiogenic and relocates bone marrow blood vessels closer to bone forming sites. J Bone Miner Res. 2011; 26: 25832596.
  • 13
    Grunewald FS, Prota AE, Giese A, Ballmer-Hofer K. Structure–function analysis of VEGF receptor activation and the role of coreceptors in angiogenic signaling. Biochim Biophys Acta. 2010; 1804(3): 56780.
  • 14
    Demer L, Tintut Y. The bone–vascular axis in chronic kidney disease. Curr Opin Nephrol Hypertens. 2010; 19(4): 34953.
  • 15
    London GM, Marty C, Marchais SJ, Guerin AP, Metivier F, de Vernejoul MC. Arterial calcifications and bone histomorphometry in end-stage renal disease. J Am Soc Nephrol. 2004; 15(7): 194351.
  • 16
    Wahl O, Oswald M, Tretzel L, Herres E, Arend J, Efferth T. Inhibition of tumor angiogenesis by antibodies, synthetic small molecules and natural products. Curr Med Chem. 2011; 18(21): 313655.
  • 17
    Wood J, Bonjean K, Ruetz S, Bellahcene A, Devy L, Foidart JM, Castronovo V, Green JR. Novel antiangiogenic effects of the bisphosphonate compound zoledronic acid. J Pharmacol Exp Ther. 2002; 302(3): 105561.
  • 18
    Black DM, Greenspan SL, Ensrud KE, Palermo L, McGowan JA, Lang TF, Garnero P, Bouxsein ML, Bilezikian JP, Rosen CJ. The effects of parathyroid hormone and alendronate alone or in combination in postmenopausal osteoporosis. N Engl J Med. 2003; 349(13): 120715.
  • 19
    Sim FH, Kelly PJ. Relationship of bone remodeling, oxygen consumption, and blood flow in bone. J Bone Joint Surg Am. 1970; 52(7): 137789.
  • 20
    Weinstein RS, Wan C, Liu Q, Wang Y, Almeida M, O'Brien CA, Thostenson J, Roberson PK, Boskey AL, Clemens TL, Manolagas SC. Endogenous glucocorticoids decrease skeletal angiogenesis, vascularity, hydration, and strength in aged mice. Aging Cell. 2010; 9(2): 14761.
  • 21
    Wang DS, Miura M, Demura H, Sato K. Anabolic effects of 1,25-dihydroxyvitamin D3 on osteoblasts are enhanced by vascular endothelial growth factor produced by osteoblasts and by growth factors produced by endothelial cells. Endocrinology. 1997; 138(7): 295362.
  • 22
    Stahl A, Wenger A, Weber H, Stark GB, Augustin HG, Finkenzeller G. Bi-directional cell contact-dependent regulation of gene expression between endothelial cells and osteoblasts in a three-dimensional spheroidal coculture model. Biochem Biophys Res Commun. 2004; 322(2): 68492.
  • 23
    Bouletreau PJ, Warren SM, Spector JA, Peled ZM, Gerrets RP, Greenwald JA, Longaker MT. Hypoxia and VEGF up-regulate BMP-2 mRNA and protein expression in microvascular endothelial cells: implications for fracture healing. Plast Reconstr Surg. 2002; 109(7): 238497.
  • 24
    Wohl GR, Towler DA, Silva MJ. Stress fracture healing: fatigue loading of the rat ulna induces upregulation in expression of osteogenic and angiogenic genes that mimic the intramembranous portion of fracture repair. Bone. 2009; 44(2): 32030.
  • 25
    Cheng SL, Shao JS, Cai J, Sierra OL, Towler DA. Msx2 exerts bone anabolism via canonical Wnt signaling. J Biol Chem. 2008; 283(29): 2050522.
  • 26
    Zhao C, Irie N, Takada Y, Shimoda K, Miyamoto T, Nishiwaki T, Suda T, Matsuo K. Bidirectional ephrinB2-EphB4 signaling controls bone homeostasis. Cell Metab. 2006; 4(2): 11121.
  • 27
    Korff T, Dandekar G, Pfaff D, Fuller T, Goettsch W, Morawietz H, Schaffner F, Augustin HG. Endothelial ephrinB2 is controlled by microenvironmental determinants and associates context-dependently with CD31. Arterioscler Thromb Vasc Biol. 2006; 26(3): 46874.
  • 28
    Bochenek ML, Dickinson S, Astin JW, Adams RH, Nobes CD. Ephrin-B2 regulates endothelial cell morphology and motility independently of Eph-receptor binding. J Cell Sci. 2010; 123(Pt 8): 123546.
  • 29
    Maes C, Goossens S, Bartunkova S, Drogat B, Coenegrachts L, Stockmans I, Moermans K, Nyabi O, Haigh K, Naessens M, Haenebalcke L, Tuckermann JP, Tjwa M, Carmeliet P, Mandic V, David JP, Behrens A, Nagy A, Carmeliet G, Haigh JJ. Increased skeletal VEGF enhances beta-catenin activity and results in excessively ossified bones. EMBO J. 2010; 29(2): 42441.
  • 30
    Jilka RL, O'Brien CA, Bartell SM, Weinstein RS, Manolagas SC. Continuous elevation of PTH increases the number of osteoblasts via both osteoclast-dependent and -independent mechanisms. J Bone Miner Res. 2010; 25(11): 242737.
  • 31
    Ohishi M, Chiusaroli R, Ominsky M, Asuncion F, Thomas C, Khatri R, Kostenuik P, Schipani E. Osteoprotegerin abrogated cortical porosity and bone marrow fibrosis in a mouse model of constitutive activation of the PTH/PTHrP receptor. Am J Pathol. 2009; 174(6): 216071.
  • 32
    Cackowski FC, Anderson JL, Patrene KD, Choksi RJ, Shapiro SD, Windle JJ, Blair HC, Roodman GD. Osteoclasts are important for bone angiogenesis. Blood. 2010; 115(1): 1409.
  • 33
    Meng XW, Liang XG, Birchman R, Wu DD, Dempster DW, Lindsay R, Shen V. Temporal expression of the anabolic action of PTH in cancellous bone of ovariectomized rats. J Bone Miner Res. 1996; 11(4): 4219.
  • 34
    Narita Y. [Effect of intermittent administration of human PTH on experimental osteopenia in adult rat: a histomorphometric study of both trabecular and cortical bone of the vertebrae]. Nihon Seikeigeka Gakkai Zasshi. 1995; 69(10): 102736.
  • 35
    Huang MS, Lu J, Ivanov Y, Sage AP, Tseng W, Demer LL, Tintut Y. Hyperlipidemia impairs osteoanabolic effects of PTH. J Bone Miner Res. 2008; 23(10): 16729.
  • 36
    Dempster DW, Cosman F, Kurland ES, Zhou H, Nieves J, Woelfert L, Shane E, Plavetic K, Muller R, Bilezikian J, Lindsay R. Effects of daily treatment with parathyroid hormone on bone microarchitecture and turnover in patients with osteoporosis: a paired biopsy study. J Bone Miner Res. 2001; 16(10): 184653.
  • 37
    Cheng SL, Shao JS, Halstead LR, Distelhorst K, Sierra O, Towler DA. Activation of vascular smooth muscle parathyroid hormone receptor inhibits Wnt/beta-catenin signaling and aortic fibrosis in diabetic arteriosclerosis. Circ Res. 2010; 107(2): 27182.
  • 38
    Sutliff RL, Weber CS, Qian J, Miller ML, Clemens TL, Paul RJ. Vasorelaxant properties of parathyroid hormone-related protein in the mouse: evidence for endothelium involvement independent of nitric oxide formation. Endocrinology. 1999; 140(5): 207783.
  • 39
    Abdallah Y, Ross G, Dolf A, Heinemann MP, Schluter KD. N-terminal parathyroid hormone-related peptide hyperpolarizes endothelial cells and causes a reduction of the coronary resistance of the rat heart via endothelial hyperpolarization. Peptides. 2006; 27(11): 292734.
  • 40
    Bakre MM, Zhu Y, Yin H, Burton DW, Terkeltaub R, Deftos LJ, Varner JA. Parathyroid hormone-related peptide is a naturally occurring, protein kinase A-dependent angiogenesis inhibitor. Nat Med. 2002; 8(9): 9951003.
  • 41
    Jamal SA, Hodsman AB. Reducing the risk of re-fracture in the dialysis population: is it time to consider therapy with PTH analogues?. Semin Dial. 2011; 24(1): 125.