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. 2009 Jan;174(1):239–247. doi: 10.2353/ajpath.2009.080627

Figure 5.

Figure 5

Model for coordinating site-specific recruitment of OC and OB progenitors to bone remodeling sites. The model summarizes the main features recognized in our study (see text). A bone remodeling site involves joint OC and OB activities on the bone surface [bone remodeling unit (BRU) or basic multicellular unit (BMU)2]. These activities were proposed to occur in a BRC separated from the bone marrow by an OB lineage cell canopy.16 According to our study, this BRC is part of a more complex entity because the OB lineage cell canopy is connected to capillaries. This structural entity directs OC and OB progenitors to the bone remodeling site, as evidenced, respectively, by direct observations on OC progenitors and by lack of bone formation when the BRC canopy is disrupted in diseased bone. It remains to be elucidated where OB progenitors are positioned and how the capillaries communicate with the BRC.