To examine the functional role of membrane‐bound SLF, we evaluated the growing skeletons of WT and SLF mutant (Sl/Sl^d) mice that do not produce this protein using DXA, bone histomorphometry, cell culture, and flow cytometry. Deletion of membrane‐bound SLF delays bone growth, decreases bone mass and BMD, impairs osteoblast function, and increases osteoclast surface in young mice.

Introduction: Mutations at the murine steel locus lead to a defect in the development of hematopoietic stem cells, mast cells, and germ cells. Two isoforms of steel factor (SLF), soluble and membrane‐associated, have been reported. Soluble SLF increases osteoclast formation and activity in cell culture. The effects of deletion of membrane‐bound SLF on bone metabolism in mice have yet to be determined and are the subject of this study.

Materials and Methods: Five‐, 7‐, and 12‐week‐old male and 5‐week‐old female WCB6F1/J‐Kitl^Sl/Kitl^Sl‐d (Sl/Sl^d) mice and wildtype (WT) littermates were used. BMD and bone mass, growth, architecture, and turnover were evaluated by DXA (males and females) and histomorphometry (males only). Primary osteoblasts isolated from humeri of 5‐week‐old male WT and Sl/Sl^d mice were used to determine osteoblast function, and bone marrow cells from tibias and femurs of these mice were analyzed to determine cell surface expression of osteoclast precursors.

Results and Conclusions: Young Sl/Sl^d mice grew more slowly, had a reduced bone mass, and had shorter bones than WT littermates. Male mutants had significantly decreased whole body BMD in all age groups, largely because of a reduction in BMC. Tibial cross‐sectional, cortical, and marrow area of cortical bone and cancellous bone volume was reduced in the mutants at all ages. The osteopenia in Sl/Sl^d was caused by increased osteoclast surface at all ages and decreased osteoblast surface at 5 weeks of age. [³H]thymidine incorporation studies showed that proliferation of osteoblasts derived from mutant mice was significantly suppressed (56%). Moreover, a decrease in mineralization was observed in Sl/Sl^d osteoblast culture. Fluorescence‐activated cell sorting analysis of bone marrow cells from Sl/Sl^d mice revealed a 65% increase in the percentage of c‐Fms⁺CD11b⁺RANK⁺ cells compared with WT controls. These findings suggest that membrane‐bound SLF/c‐Kit signaling plays a role in the regulation of peak bone mass.