Fig 1: OPN in resorption pits. a A low magnification FE-SEM view of an osteoclast resorbing bone slice. b–e Example images of low- and high-magnification views of a resorption pit in human bone and carbonated hydroxyapatite with a detector capable of visualizing gold particles. Gold particle labeled OPN (arrows) is evident as bright dots. f OPN particle count data. bmbone bone marrow derived osteoclasts on bone, pbbone peripheral blood derived osteoclasts on bone, bmca bone marrow derived osteoclasts on carbonated hydroxyapatite, pbca peripheral blood derived osteoclasts on carbonated hydroxyapatite, pit bottom of resorption pit, surf intact surface; there were approximately two to three times more OPN particles within a resorption pit than outside of it (*p < 0.05) indicating that osteoclasts deposit OPN specifically into the resorption pit. On bone slices, osteoclasts move during resorption and create a trail of resorbance, whereas while resorbing carbonated hydroxyapatite, the cell does not move and only resorbs in a downward direction, creating a round pit. Figure scale bar lengths: a 10 µm, b and c 2 µm, d and e 400 nm