Bone growth in the neonatal rat. II. Biochemical aspects of bone mineral incorporation.

Long bones and mandibles from neonatal rats were studied to characterize the mechanisms regulating their growth and to describe the patterns of bone mineral acquisition. Mineral uptake was determined by measuring the rates of incorporation of radioactive calcium (45Ca) into three metabolic pools within, or intimately surrounding, the bone. The incorporation into the pool of protein-bound calcium was low throughout the neonatal period, accounting for no more than 1% of the total calcium uptake during this period. The ionized calcium concentration, which was low throughout, increased at the time when formation of the mineral phase was found to be high. The largest fraction of the mineral, the complexed calcium, showed a gradually increasing rate of incorporation, lasting approximately until the age of weaning. The two bony tissues differed somewhat in the slopes at which the incorporation rates changed. From these studies of bone growth, it appears that the process of bone mineralization starts around the time of birth, gradually gains intensity and is maximal between approximately 10 and 18 days. It is speculated that the ionized calcium fraction may be the precursor of mineral formation.