Figure 7: FGF23 expression is a feature of osteocytes in bone packets of early secondary mineralization.

(A) Schematic depiction of phases of mineral accumulation in newly formed bone packets. The dotted line marks the transition between primary and secondary mineralization. (B) Trabecular bone section demonstrating immunohistochemical staining for FGF23. The red arrow points to area of bone with a cluster of FGF23 expressing osteocytes; the blue arrow points to an area of peripheral bone with active bone formation. (C) Backscattered electron image of bone core adjacent to the FGF23-stained section shown in (B). Heterogeneously mineralized bone matrix is observed with dark gray areas corresponding to low and bright gray to high mineral contents. Bone packets are circled in white; calcium content of each (weight % Ca) is written adjacent to each circumscribed packet. Red font is used to indicate weight % Ca of the FGF23-containing packet. (D) Image (C) with red pixels depicting areas of bone with mineral content below 16 weight % calcium and with bone packets numbered to correspond to mineral accumulation phases depicted in (A). (E) An enlarged image of bone packet #1 (i.e. an area of active bone formation). (F) Confocal laser scanning microscopic fluorescence image from the area of bone packet #1 demonstrating tetracycline incorporation at the mineralization front.