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

A mixture of heparin-Sepharose-purified bovine bone morphogenetic protein a nd type I atellocollagen was implanted in the subcutaneous tissues of 4-wee k, 10-month and 18-month-old rats. The implants were removed at 7, 14 and 2 1 days after implantation. The effects of rat age on ectopic bone formation were evaluated on the explants using haematoxylin-eosin staining, morphome tric analysis, alkaline phosphatase activity and calcium content determinat ion, as well as immunohistochemical staining of type TV collagen present in the basement membrane of blood vessels. On day 14 and 21, bone was observe d in 4-week and 10-month-old rats, bur the amount of bone formed in the lat ter was less than in the 4-week-old rats. In 18-month-old rats, bone was fi rst 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, akal ine phosphatase activity was higher in younger rats. On day 7, there were m ore blood vessels in the explants of 4-week-old rats than in those of 10-mo nth or 18-month-old rats. On day 14 and 21, more blood vessels were found i n the central regions of the explants in 4-week-old rats than in the same r egions in 10-month or 18-month-old rats. The findings in the present study indicate that the rate and quantity of ectopic bone formation are reduced i n aged rats, and suggest that the difference in blood vessel distribution i s related to this reduction in ectopic bone formation.