Morphological and Immunohistochemical Study of Ectopic Bone Formation Induced by Crude BMP in 4-Week-Old and 6-Month-Old Rats

Abstract

The mixture of crude BMP and type I collagen as a carrier was implanted in the subcutaneous tissues of 4-week-old and 6-month-old rats. The effect of aging on ectopic bone formation was evaluated after H＆E and type II collagen immunohistochemical staining. On day 7, type II collagen was detected around chondrocytes in 4-week-old rats while in 6-month-old rats, chondrocytes were absent and type II collagen was negative. On day 14, a large number of chondrocytes were observed with strong staining to type II collagen in 4-week old rats but in 6-month-old rats, chondrocytes were hardly seen. On day 21, a large number of trabecular bones with hematopoietic bone marrows were observed in 4-week-old rats but in 6-month-old rats, hematopoietic bone marrow was absent. The absence of chondrocytes on day 7 and their sparsity on day 14 in 6-month-old rats indicates that crude BMP may induce ectopic bone formation mainly through the intramembranous ossification mode in 6-month-old rats but in 4-week-old rats, mainly through endochondral ossification. The results suggested that the differentiation of mesenchymal cells to hard tissue forming cells was delayed and bone formation was quantitatively less in 6-month-old rats than in 4-week-old rats, and that the difference in the ossification mode could be one of the reasons causing the delay and reduction of hard tissue formation in 6-month-old rats.