A histomorphometric, structural, and immunocytochemical study of the effects of diet-induced hypocalcemia on bone in growing rats.

Despite several studies on the effect of calcium deficiency on bone status, there is relatively little information on the ensuing histological alterations. To investigate bone changes during chronic hypocalcemia, weanling rats were kept on a calcium-free diet and deionized water for 28 days while control animals were fed normal chow. The epiphyseal-metaphyseal region of the tibiae were processed for histomorphometric, histochemical, and structural analyses. The distribution of bone sialoprotein (BSP), osteocalcin (OC), and osteopontin (OPN), three noncollagenous bone matrix proteins implicated in cell-matrix interactions and regulation of mineral deposition, was examined using postembedding colloidal gold immunocytochemistry. The experimental regimen resulted in serum calcium levels almost half those of control rats. Trabecular bone volume showed no change but osteoid exhibited a significant increase in all its variables. There were a multitude of mineralization foci in the widened osteoid seam, and intact matrix vesicles were observed in the forming bone. Many of the osteoblasts apposed to osteoid were tartrate-resistant acid phosphatase (TRAP)- and alkaline phosphatase-positive, whereas controls showed few such TRAP-reactive cells. Osteoclasts in hypocalcemic rats generally exhibited poorly developed ruffled borders and were inconsistently apposed to bony surfaces showing a lamina limitans. Sometimes osteoclasts were in contact with osteoid, suggesting that they may resorb uncalcified matrix. Cement lines at the bone-calcified cartilage interface in some cases were thickened but generally did not appear affected at bone-bone interfaces. As in controls, electron-dense portions of the mineralized matrix showed labeling for BSP, OC, and OPN but, in contrast, there was an abundance of immunoreactive mineralization foci in osteoid of hypocalcemic rats. These data suggest that chronic hypocalcemia affects both bone formation and resorption.