Age and magnetic effects on ectopic bone formation induced by purified bone morphogenetic protein

A mixture of heparin-Sepharose-purified bovine bone morphogenetic protein (BMP) and type I atellocollagen was implanted in the subcutaneous tissues of 4-week, 10-month and 18-month-old rats. The implants were removed at 7, 14 and 21 days after implantation. The effects of rat age on ectopic bone formation were evaluated on the explants using H&E staining, morphometric analysis, alkaline phosphatase (ALP) activity and calcium (Ca) content determination, as well as immunohistochemical staining of type IV collagen present in the basement membrane of blood vessels. On day 14 and 21, bone was observed in 4-week and 10-month-old rats but the amount of bone formed in the later was less than in the former. In 18-month-old rats, bone was first found focally in very limited regions of the explants on day 21 and the amount of bone was much less than in 4-week-old rats. At all periods ALP activity was higher in younger rats. On day 7, there were more blood vessels in the explants of 4-week-old rats than in those of 10- or 18-month-old rats. On day 14 and 21, more blood vessels were found in the central regions of the explants in 4-week-old rats than in the same regions in 10- or 18-month-old rats. The findings in the present study indicated that the rate and quantity of ectopic bone formation were reduced, and that the difference in blood vessel distribution might be related to the reduction in ectopic bone formation in aged rats, and suggest that the difference in blood vessel distribution is related to ectopic bone formation. Magnetism can stimulate ectopic bone formation induced by BMP.