To further define the mechanisms involved in the preservation of bone mass in aged Sam68−/− mice, we prepared sections from plastic-embedded femora and tibia to evaluate osteoblast and osteoclast activity (Figure 5). In situ enzyme histochemical staining for alkaline phosphatase (ALP; brown stain) activity was used as a biomarker for osteoblasts and tartrate-resistant ALP (tartrate-resistant acid phosphatase [TRAP]; red stain) activity as a marker for osteoclasts. Little difference was seen between Sam68+/+ (Figure 5A and 5B) and Sam68−/− (Figure 5C and 5D) mice at 4 months of age. ALP- and TRAP-positive cells were reduced in the 12-month-old Sam68+/+ mice (Figure 5E and 5F) and remained unchanged in 12-month-old Sam68−/− mice (Figure 5G and 5H). The reduction in both osteoblast and osteoclast activity in the 12-month-old Sam68+/+ mice argued against bone being lost primarily due to a relative increase in osteoclast over osteoblast activity, as seen in high turnover disease [49].