Utility of irradiated, bone marrow-transplanted, marker tolerant rats for regenerative studies and cell tracking. When hPLAP-tg BMC were injected into the knee joint of irradiated WT (hPLAP-BMT) rats, there is a 2 – 3 cells wide layer of donor cells (arrowheads) on the cartilage surface, 1 week post-injection (A). Inset in (A) shows lack of hPLAP-staining in the articular cartilage of an irradiated WT (hPLAP-BMT) control rat not injected with transgenic BMC. Four weeks post-injection, transgenic donor cells have adopted a fibroblast-like (arrowheads) or chondrocyte-like (arrows) appearance, and show incorporation into the surrounding chondrocyte matrix of host origin (B). Five-μm-thick undecalcified MMA sections of ethanol-fixed knee joints stained for hPLAP activity after heat pretreatment and counterstained with nuclear fast red. Bar = 50 μm.