Detection of cathepsin K mRNA and protein in sections of human peri-implantation tissues. The techniques used were in situ hybridization with a 35S-labeled anti-sense RNA probe ((a, c) bright field and (b, d) dark field) and (e-h) immunohistochemistry with a rabbit polyclonal antibody to human cathepsin K. Multinuclear cells on the bone surface and some mononuclear cells in the peri-implant tissues adjacent to the bone express high levels of cathepsin K mRNA (panels a [bright field] and b [dark field]). Low levels of cathepsin K mRNA were detected in multinuclear cells associated with polyethylene (PE) particles (panels c [bright field] and d [dark field]). Multinuclear cells on the bone surface (panels e and f) and mononuclear and multinuclear cells associated with PE particles (panels g and h) stain positively for cathepsin K protein. PE particles are easily identified by their strong bi-refringence with polarizing light microscopy.