Bone Matrix Maturation in a Rat Model of Intra-Cortical Bone Remodeling

Matrix maturation within cortical bone is an important but oft-neglected component of bone remodeling because of the lack of a suitable small animal model. Intra-cortical remodeling can be induced in rodents by feeding virgin or lactating animals a low-calcium diet. The current study aimed to determine which of these two models is most suitable for studying intra-cortical matrix maturation. We compared intra-cortical remodeling in female rats fed a normal calcium diet (virgin/normal Ca), a low-calcium diet (virgin/low Ca), or a low-calcium diet during lactation (lactation/low Ca). The low-calcium diet was administered for 23 days (induction phase) followed by return to normal calcium for 30 days (recovery phase). At the end of induction, the virgin/normal Ca and virgin/low-Ca animals had no difference in cortical porosity, but the lactation/low-Ca animals had elevated cortical porosity at various diaphyseal sites in the femur and tibia. The distal femoral site had the greatest amount of induced porosity in the size range of rat secondary osteons. Neither global mineralization nor tissue age-specific mineral-to-matrix ratio in the bone formed during recovery were affected in the lactation/low-Ca rats. Serum calcium levels did not differ from controls, but phosphate levels were slightly elevated, consistent with the rapid recovery of lost bone mass. We conclude that the lactation/low-Ca model represents a means to increase intra-cortical remodeling in adult rats with no apparent detrimental effect on matrix maturation. This model will provide researchers with a new tool to study matrix maturation throughout the cortex.