Osteoclasts are nearly non-existent in mutant osteopetrotic (op/op) mice due to defects in the expression of the macrophage colony-stimulating factor (M-CSF). Thus, areal distribution of the biological apatite (BAp) c-axis in a femoral cross-section of the op/op mouse was investigated using a microbeam X-ray diffraction system in order to understand the role of osteoclasts on formation of BAp preferential alignment.
The incident X-ray beam was focused to 20 pm in diameter. The diffraction analysis was performed at 20 pm displacement intervals from the periosteal surface along the radial axis of the anterior portion at a cross-section of the center of the femoral diaphysis. The op/op mouse and its normal littermate used as a control were 12 weeks old.
The lack of osteoclasts induces both abnormality of the skeletal system and calcification of the medullary cavity, which are typical features of osteopetrosis. Preferential alignment of the BAp c-axis in the op/op mouse always shows a lower degree than that in the control mouse, regardless of the distance from the periosteum. Moreover, areal distributions of preferential alignment of the BAp c-axis on the femoral cross-section show quite different tendencies between the op/op and control mice. The preferential alignment of BAp gradually increases towards the periosteal surface in the op/op mouse due to the intramembranous ossification, while that in the control mouse is the lowest near the cortical envelope on the cross-section. This is because the decrease in the number of osteoclasts suppresses the normal modeling, resulting in degradation of preferential alignment of the BAp c-axis as a bone quality parameter in the op/op mouse.