Matrix vesicles and calcification of rachitic rat osteoid

Tibiae from rachitic weanling rats were studied during healing to examine the mechanism of bone mineralization. Rickets was induced by feeding the animals a low phosphate, low vitamin D diet for five weeks. Calcification was reinstituted in three ways; group I animals received 1.0 ml of NaH2PO4 i.p. and the rachitogenic diet in a darkened room; group II animals were placed in a lighted room and given standard laboratory chow; group III animals were placed in lighted room, given standard laboratory chow and received 1.0 ml of 0.1M NaH2PO4 i.p. Group I healed slowly while II and III were found to heal rapidly and at nearly identical rates. Groups II and III revealed a significantly elevated serum alkaline phosphatase activity and became hyperphosphatemic as the rickets healed suggesting a more vigorous recovery. Ultrastructurally, numerous matrix vesicles were noted in unhealed rachitic bone matrix and these structures acquired mineral upon initiation of healing. Vesicle-associated mineral aggregates increased in size penetrating through and beyond the vesicle membrane and were incorporated into the advancing mineralization front. By 48 hrs. post-healing initiation, the osteoid borders were almost completely mineralized in groups II and III. These observations suggest that in addition to pre-existing mineral, matrix vesicles can also serve as nucleating sites in the osteoid of post-fetal bone.