The existence in the blood stream of MSPCs with osteo
genic potential, or circulating osteoprogenitors or other osteoblast lineage populations — here collectively termed circulating osteogenic precursor cells (COPs) — and their actual function and contribution to new bone formation is still a topic of controversy and debate.
On the one hand, COPs have been isolated based on plastic adherence and marker analysis from human and experimental animal peripheral blood, although their fre
quency was extremely low, especially in humans [60, 61].
Nevertheless, the isolated adherent fibroblast‐like cells displayed osteogenic differentiation potential in vitro and in vivo upon ectopic (subcutaneous) transplantation [60]. Circulating MSPC‐like cells have also been isolated from umbilical cord blood [62]. A number of experimen
tal studies have further provided evidence in favor of the existence of COPs, and of their ability to access bone formation sites from the circulation. Experimental approaches typically used in such studies include para
biosis experiments, in which a conjoint pair of mice is created that shares a common circulatory system (see later), and transplantation experiments using BM or MSPC populations. For instance, Otsuru and colleagues showed the existence of COPs by transplanting BM from GFP‐expressing transgenic mice into recipients undergo
ing BMP2‐induced bone formation in muscle‐implanted collagen pellets, because GFP+;Ocn+ osteoblastic cells were found to contribute to the newly formed ectopic bone [63]. The COPs were characterized to be CD45− CD44+ CXCR4+, thus expressing receptors for osteopon
tin and CXCL12/SDF‐1 and capable of differentiating into osteoblasts in vitro and in vivo [64].
On the other hand, plastic‐adherent MSPCs generally exhibit poor homing to noninjured skeletal tissues in sys
temic infusion studies, suggesting that MSPCs may gen
erally not be physiologically circulating [65]. During pubertal growth in adolescent boys and upon fracture in adult patients, however, significant increases in the num
ber of COPs have been reported [66, 67]. Still, it could be argued that (pathology‐associated) COPs could be the result of disruption of the integrity of the bone tissue, thereby releasing BMSCs or related cell populations into the circulation, or that cells with osteogenic potential retrieved from the peripheral blood may in fact represent hematopoietic‐ or EC‐derived populations, given that several such cell types have been shown to be able to revert to an osteoblastic fate under certain conditions [65, 68].
Overall, the prevailing notion appears to be that COPs may participate particularly in bone formation in cir
cumstances of highly active osteoanabolic responses, fracture healing, and/or ectopic/heterotopic ossification.
Several experimental animal models support this idea (for review, see [65]). For instance, Kumagai and colleagues created a parabiosis model by joining a wild‐type mouse
and a syngeneic mouse constitutively expressing GFP [69]. After the induction of a fibular fracture in the wild‐type partner, GFP+ cells also expressing alkaline phosphatase (ALP) were found in the fracture callus, albeit still at relatively low frequency, suggesting that osteogenic progenitor cells had been recruited through the circulation, homed to the fracture site, and contrib
uted to skeletal repair [69].
Despite the likely relatively small contribution of COPs to normal physiology and even to bone formation in circumstances of pathology and repair, the therapeutic prospects towards the enhancement of failing fracture healing are extensive. For instance, the administration of exogenous MSPCs that are primed to home to the site of the defect through molecular or genetic modifications, or stimulation of the mobilization and recruitment to the repair tissue of endogenous osteoprogenitors by means of pharmacological interventions, could be efficient and patient‐friendly ways to enhance compromised bone healing or treat nonunions. With regard to the former strategy, interesting work indicated that overexpression of integrin α4 or administration of a peptidomimetic ligand of α4β1 coupled to a bone‐seeking agent (alen
dronate) increased the homing of BMSCs to bone, leading to encouraging results in osteopenic mouse models [70, 71]. Concerning the latter approach, endogenous progenitor recruitment to the site of the defect may be enhanced by exploiting or amplifying mechanisms that mediate normal tissue regeneration. The CXCL12/
SDF‐1–CXCR4 axis has been implicated in homing mechanisms of a variety of stem cell populations, and may be the final common pathway for mobilization of BM‐derived progenitor cells by injury, inflammation, or relative hypoxia [64, 65]. In a mouse model of segmental bone defects, a combination of the CXCR4 antagonist AMD3100 and IGF‐1 provided significant augmentation of bone repair, likely by combined effects on MSPC mobilization and proliferation [72]. In another study, administration of AMD3100 significantly increased the number of endothelial and osteogenic progenitors in the circulation and improved repair of femoral fractures [73].
The potential of endogenous stem/progenitor mobiliza
tion to enhance bone regeneration may be magnified by such strategies that stimulate both osteogenesis and angiogenesis at the fracture site, because both processes are vital for bone healing [15]. The discovery of CD34+ progenitor cells in the circulation could as such be highly promising for cell‐based therapies of fracture healing and tissue engineering, because they show multilineage dif
ferentiation potential into ECs and osteoblasts [74].
ACKNOWLEDGMENTS
Research in the Laboratory for Skeletal Cell Biology and Physiology (SCEBP) is supported by grants to CM, from the European Research Council (ERC Starting Grant 282131 under the European Union’s Seventh Framework Programme, FP7), the Fund for Scientific Research of
Flanders (FWO) and the University of Leuven (KU Leuven). ND is supported by a doctoral fellowship of the Flemish government agency for Innovation by Science and Technology (IWT).
REFERENCES
1. Dirckx N, Van Hul M, Maes C. Osteoblast recruitment to sites of bone formation in skeletal development, homeostasis, and regeneration. Birth Defects Res C Embryo Today. 2013;99:170–91.
2. Caplan AI, Bruder SP. Mesenchymal stem cells: building blocks for molecular medicine in the 21st century.
Trends Mol Med. 2001;7:259–64.
3. Bianco P. “Mesenchymal” stem cells. Annu Rev Cell Dev Biol. 2014;30:677–704.
4. Bianco P, Robey PG. Skeletal stem cells. Development.
2015;142:1023–7.
5. Dominici M, Le Blanc K, Mueller I, et al. Minimal crite
ria for defining multipotent mesenchymal stromal cells.
The International Society for Cellular Therapy position statement. Cytotherapy. 2006;8:315–17.
6. Tavassoli M, Crosby WH. Transplantation of marrow to extramedullary sites. Science. 1968;161:54–6.
7. Bianco P, Robey PG, Simmons PJ. Mesenchymal stem cells: revisiting history, concepts, and assays. Cell Stem Cell. 2008;2:313–19.
8. Gronthos S, Graves SE, Ohta S, et al. The STRO‐1+
fraction of adult human bone marrow contains the oste
ogenic precursors. Blood. 1994;84:4164–73.
9. Gronthos S, Zannettino A, Hay S, et al. Molecular and cellular characterization of highly purified stromal stem cells derived from human bone marrow. J Cell Sci.
2003;116:1827–35.
10. Sacchetti B, Funari A, Michienzi S, et al. Self‐renewing osteoprogenitors in bone marrow sinusoids can organize a hematopoietic microenvironment. Cell. 2007;131:324–36.
11. Short BJ, Brouard N, Simmons PJ. Prospective isolation of mesenchymal stem cells from mouse compact bone.
Methods Mol Biol. 2009;482:259–68.
12. Chan CK, Seo EY, Chen JY, et al. Identification and spec
ification of the mouse skeletal stem cell. Cell.
2015;160:285–98.
13. Worthley DL, Churchill M, Compton JT, et al. Gremlin 1 identifies a skeletal stem cell with bone, cartilage, and reticular stromal potential. Cell. 2015;160:269–84.
14. Long F. Building strong bones: molecular regulation of the osteoblast lineage. Nature Rev. 2011;13:27–38.
15. Maes C. Role and regulation of vascularization processes in endochondral bones. Calcif Tissue Int.2013;92:307–23.
16. Ducy P, Zhang R, Geoffroy V, et al. Osf2/Cbfa1: a tran
scriptional activator of osteoblast differentiation. Cell.
1997;89:747–54.
17. Ducy P, Starbuck M, Priemel M, et al. A Cbfa1‐depend
ent genetic pathway controls bone formation beyond embryonic development. Genes Dev. 1999;13:1025–36.
18. Baek WY, Lee MA, Jung JW, et al. Positive regulation of adult bone formation by osteoblast‐specific transcrip
tion factor osterix. J Bone Miner Res. 2009;24:1055–65.
19. Baek WY, de Crombrugghe B, Kim JE. Postnatally induced inactivation of Osterix in osteoblasts results in the reduction of bone formation and maintenance. Bone.
2010;46:920–8.
20. Zhou X, Zhang Z, Feng JQ, et al. Multiple functions of Osterix are required for bone growth and homeostasis in postnatal mice. Proc Natl Acad Sci U S A. 2010;107:
12919–24.
21. Zhou X, Zhang Z, Feng JQ, et al. Multiple functions of Osterix are required for bone growth and homeostasis in postnatal mice. Proc Natl Acad Sci U S A. 2010;107:
12919–24.
22. Rauch A, Seitz S, Baschant U, et al. Glucocorticoids sup
press bone formation by attenuating osteoblast differen
tiation via the monomeric glucocorticoid receptor. Cell Metab. 2010;11:517–31.
23. Rodda SJ, McMahon AP. Distinct roles for Hedgehog and canonical Wnt signaling in specification, differentiation and maintenance of osteoblast progenitors.
Development. 2006;133:3231–44.
24. Maes C, Kobayashi T, Selig MK, et al. Osteoblast precur
sors, but not mature osteoblasts, move into developing and fractured bones along with invading blood vessels.
Dev Cell. 2010;19:329–44.
25. Baron R, Kneissel M. WNT signaling in bone homeosta
sis and disease: from human mutations to treatments.
Nat Med. 2013;19:179–92.
26. Ono N, Ono W, Nagasawa T, et al. A subset of chondro
genic cells provides early mesenchymal progenitors in growing bones. Nat Cell Biol. 2014;16:1157–67.
27. Yang L, Tsang KY, Tang HC, et al. Hypertrophic chondrocytes can become osteoblasts and osteocytes in endochondral bone formation. Proc Natl Acad Sci U S A.
2014;111:12097–102.
28. Zhou X, von der Mark K, Henry S, et al. Chondrocytes transdifferentiate into osteoblasts in endochondral bone during development, postnatal growth and fracture heal
ing in mice. PLoS Genet. 2014;10:e1004820.
29. Song L, Liu M, Ono N, et al. Loss of wnt/beta‐catenin sign
aling causes cell fate shift of preosteoblasts from osteoblasts to adipocytes. J Bone Miner Res. 2012;27:2344–58.
30. Liu Y, Berendsen AD, Jia S, et al. Intracellular VEGF regulates the balance between osteoblast and adipocyte differentiation. J Clin Invest. 2012;122:3101–13.
31. Sinha P, Aarnisalo P, Chubb R, et al. Loss of Gsalpha early in the osteoblast lineage favors adipogenic differentiation of mesenchymal progenitors and committed osteoblast precursors. J Bone Miner Res. 2014;29:2414–26.
32. Iyer S, Ambrogini E, Bartell SM, et al. FOXOs attenuate bone formation by suppressing Wnt signaling. J Clin Invest. 2013;123:3409–19.
33. Tang Y, Wu X, Lei W, et al. TGF‐beta1‐induced migration of bone mesenchymal stem cells couples bone resorp
tion with formation. Nature Med. 2009;15:757–65.
34. Dobnig H, Turner R. Evidence that intermittent treatment with parathyroid hormone increases bone
References 29 formation in adult rats by activation of bone lining cells.
Endocrinology. 1995;136:3632–38.
35. Kim SW, Pajevic PD, Selig M, et al. Intermittent parathyroid hormone administration converts quiescent lining cells to active osteoblasts. J Bone Miner Res.
2012;27:2075–84.
36. Kim SW, Lu Y, Williams EA, et al. Sclerostin antibody administration converts bone lining cells into active osteoblasts. J Bone Miner Res. 2016;
37. Turner RT, Iwaniec UT, Wong CP, et al. Acute exposure to high dose gamma‐radiation results in transient activation of bone lining cells. Bone. 2013;57:164–73.
38. Matic I, Matthews BG, Wang X, et al. Quiescent bone lining cells are a major source of osteoblasts during adulthood. Stem Cells. 2016;34:2930–42.
39. Sacchetti B, Funari A, Michienzi S, et al. Self‐renewing osteoprogenitors in bone marrow sinusoids can organize a hematopoietic microenvironment. Cell. 2007;131:
324–36.
40. Crisan M, Yap S, Casteilla L, et al. A perivascular origin for mesenchymal stem cells in multiple human organs.
Cell Stem Cell. 2008;3:301–13.
41. Boulais PE, Frenette PS. Making sense of hematopoietic stem cell niches. Blood. 2015;125:2621–9.
42. Morrison SJ, Scadden DT. The bone marrow niche for haematopoietic stem cells. Nature. 2014;505:327–34.
43. Ono N, Kronenberg HM. Bone repair and stem cells.
Curr Opin Genet Dev. 2016;40:103–107.
44. Omatsu Y, Sugiyama T, Kohara H, et al. The essential functions of adipo‐osteogenic progenitors as the hemat
opoietic stem and progenitor cell niche. Immunity.
2010;33:387–99.
45. Greenbaum A, Hsu YM, Day RB, et al. CXCL12 in early mesenchymal progenitors is required for haematopoi
etic stem‐cell maintenance. Nature. 2013;495:227–30.
46. Morikawa S, Mabuchi Y, Kubota Y, et al. Prospective identification, isolation, and systemic transplantation of multipotent mesenchymal stem cells in murine bone marrow. J Exp Med. 2009;206:2483–96.
47. Zhou BO, Yue R, Murphy MM, et al. Leptin‐receptor‐
expressing mesenchymal stromal cells represent the main source of bone formed by adult bone marrow. Cell Stem Cell. 2014;15:154–68.
48. Ding L, Saunders TL, Enikolopov G, et al. Endothelial and perivascular cells maintain haematopoietic stem cells. Nature. 2012;481:457–62.
49. Mizoguchi T, Pinho S, Ahmed J, et al. Osterix marks dis
tinct waves of primitive and definitive stromal progeni
tors during bone marrow development. Developmental Cell. 2014;29:340–9.
50. Mendez‐Ferrer S, Michurina TV, Ferraro F, et al.
Mesenchymal and haematopoietic stem cells form a unique bone marrow niche. Nature. 2010;466:829–34.
51. Ono N, Ono W, Mizoguchi T, et al. Vasculature‐associ
ated cells expressing nestin in developing bones encom
pass early cells in the osteoblast and endothelial lineage.
Developmental Cell. 2014;29:330–9.
52. Park D, Spencer JA, Koh BI, et al. Endogenous bone mar
row MSCs are dynamic, fate‐restricted participants in
bone maintenance and regeneration. Cell Stem Cell.
2012;10:259–72.
53. Kusumbe AP, Ramasamy SK, Adams RH. Coupling of angiogenesis and osteogenesis by a specific vessel subtype in bone. Nature. 2014;507:323–8.
54. Wu JY, Purton LE, Rodda SJ, et al. Osteoblastic regula
tion of B lymphopoiesis is mediated by Gs{alpha}‐
dependent signaling pathways. Proc Natl Acad Sci U S A. 2008;105:16976–81.
55. Colnot C, Zhang X, Knothe Tate ML. Current insights on the regenerative potential of the periosteum: molecu
lar, cellular, and endogenous engineering approaches.
J Orthop Res. 2012;30:1869–78.
56. Kawanami A, Matsushita T, Chan YY, et al. Mice expressing GFP and CreER in osteochondro progenitor cells in the periosteum. Biochem Biophys Res Commun.
2009;386:477–82.
57. Matthews BG, Grcevic D, Wang L, et al. Analysis of alphaSMA‐labeled progenitor cell commitment identi
fies notch signaling as an important pathway in fracture healing. J Bone Miner Res. 2014;29:1283–94.
58. Colnot C. Skeletal cell fate decisions within periosteum and bone marrow during bone regeneration. J Bone Miner Res. 2009;24:274–82.
59. Grayson WL, Bunnell BA, Martin E, et al. Stromal cells and stem cells in clinical bone regeneration. Nat Rev Endocrinol. 2015;11:140–50.
60. Kuznetsov SA, Mankani MH, Gronthos S, et al.
Circulating skeletal stem cells. J Cell Biol. 2001;153:
1133–40.
61. Kuznetsov SA, Mankani MH, Leet AI, et al. Circulating connective tissue precursors: extreme rarity in humans and chondrogenic potential in guinea pigs. Stem Cells.
2007;25:1830–9.
62. Rosada C, Justesen J, Melsvik D, et al. The human umbilical cord blood: a potential source for osteoblast progenitor cells. Calcif Tissue Int. 2003;72:135–42.
63. Otsuru S, Tamai K, Yamazaki T, et al. Bone marrow‐
derived osteoblast progenitor cells in circulating blood contribute to ectopic bone formation in mice. Biochem Biophys Res Commun. 2007;354:453–8.
64. Otsuru S, Tamai K, Yamazaki T, et al. Circulating bone marrow‐derived osteoblast progenitor cells are recruited to the bone‐forming site by the CXCR4/stromal cell‐
derived factor‐1 pathway. Stem Cells. 2008;26:223–34.
65. Pignolo RJ, Kassem M. Circulating osteogenic cells:
implications for injury, repair, and regeneration. J Bone Miner Res. 2011;26:1685–93.
66. Alm JJ, Koivu HM, Heino TJ, et al. Circulating plastic adherent mesenchymal stem cells in aged hip fracture patients. J Orthop Res. 2010;28:1634–42.
67. Eghbali‐Fatourechi GZ, Lamsam J, Fraser D, et al.
Circulating osteoblast‐lineage cells in humans. N Engl J Med. 2005;352:1959–66.
68. Medici D, Shore EM, Lounev VY, et al. Conversion of vascular endothelial cells into multipotent stem‐like cells. Nat Med. 2010;16:1400–6.
69. Kumagai K, Vasanji A, Drazba JA, et al. Circulating cells with osteogenic potential are physiologically mobilized
into the fracture healing site in the parabiotic mice model. J Orthop Res. 2008;26:165–75.
70. Guan M, Yao W, Liu R, et al. Directing mesenchymal stem cells to bone to augment bone formation and increase bone mass. Nat Med. 2012;18:456–62.
71. Kumar S, Ponnazhagan S. Bone homing of mesenchymal stem cells by ectopic alpha 4 integrin expression. Faseb J. 2007;21:3917–27.
72. Kumar S, Ponnazhagan S. Mobilization of bone marrow mesenchymal stem cells in vivo augments bone healing
in a mouse model of segmental bone defect. Bone.
2012;50:1012–18.
73. Toupadakis CA, Granick JL, Sagy M, et al. Mobilization of endogenous stem cell populations enhances fracture healing in a murine femoral fracture model. Cytotherapy.
2013;15:1136–47.
74. Matsumoto T, Kawamoto A, Kuroda R, et al. Therapeutic potential of vasculogenesis and osteogenesis promoted by peripheral blood CD34‐positive cells for functional bone healing. Am J Pathol. 2006;69:1440–57.
31
Primer on the Metabolic Bone Diseases and Disorders of Mineral Metabolism, Ninth Edition. Edited by John P. Bilezikian.
© 2019 American Society for Bone and Mineral Research. Published 2019 by John Wiley & Sons, Inc.
Companion website: www.wiley.com/go/asbmrprimer