Characterization of circulating osteoblast lineage cells in humans

Abstract

We recently identified circulating osteoblastic cells using antibodies to osteocalcin (OCN) or alkaline phosphatase (AP). We now provide a more detailed characterization of these cells. Specifically, we demonstrate that 46% of OCN positive (OCN pos ) cells express AP, and 37% also express the hematopoietic/endothelial marker, CD34. Using two different anti-OCN antibodies and forward/side light scatter characteristics by flow cytometry, we find that OCN pos cells consist of two distinct populations: one population exhibits low forward/side scatter, consistent with a small cell phenotype with low granularity, and a second population has higher forward/side scatter (larger and more granular cell). The smaller, low granularity population also co-expresses CD34, whereas the larger, more granular cells are CD34 negative. Using samples from 26 male subjects aged 28 to 68 years, we demonstrate that the concentration of circulating OCN pos cells increases as a function of age (R = 0.59, P = 0.002). By contrast, CD34 pos cells tend to decrease with age (R = −0.31, P = 0.18); as a consequence, the ratio of OCN pos :CD34 pos cells also increases significantly with age (R = 0.54, P = 0.022). These findings suggest significant overlap between circulating cells expressing OCN and those expressing the hematopoietic/endothelial marker, CD34. Further studies are needed to define the precise role of circulating OCN pos cells not only in bone remodeling but rather also potentially in the response to vascular injury.

Circulating osteogenic cells: Implications for injury, repair, and regeneration

“…(28,29) Eghbali-Fatourechi et al extended these studies to isolate a plastic-nonadherent cell population from peripheral blood. (30,31) Similarly, these investigators employed osteocalcin and ALP antibodies to show that osteocalcinþ cells exhibit mononuclear cell morphology, and up to $50% of these cells express CD34. However, these cells seem to exhibit under standard in vitro culture conditions low growth rates and less robust osteoblast differentiation capacity compared with bone marrow MSCs.…”

“…However, these cells seem to exhibit under standard in vitro culture conditions low growth rates and less robust osteoblast differentiation capacity compared with bone marrow MSCs. (30,31) Unfortunately, plastic adherence is not a standardized procedure and varies with time or the type of plastic surface employed in different laboratories; thus, it may be more appropriate to base ontological or hierarchical relationships on specific cellular phenotypes.…”

Cell Sources for Bone Tissue Engineering: Insights from Basic Science

“…The existence of circulating osteoblast precursors suggested a possible recruitment of these cells in response to bone injury. 20,21 In the study by Kumagai et al, the parabiosis mouse model was used to show that circulating cells were mobilized to the fracture site. 22 Although cells brought by blood vessels expressed the osteoblast marker alkaline phosphate, they were found as bone-lining cells, but did not integrate within new bone as osteocytes.…”

Circulating Osteogenic Cells

“…Eghbali-Fatourechi et al extended these studies to isolate a plastic-nonadherent cell population from peripheral blood [27] , demonstrating that OCN + cells exhibit mononuclear cell morphology and that up to apprxoimately 50% of these cells express CD34 [28] . However, these cells seem to exhibit under standard in vitro culture conditions low growth rates and less robust osteoblast differentiation capacity compared with bone marrow MSCs [27] .…”