The nature and function of mononuclear cells on the resorbed surfaces of bone in the reversal phase during remodeling

In a reversal phase of bone remodeling many mononuclear cells appear on the resorbed surfaces of bone with characteristic reversal lines as revealed b y transmission electron microscopy (TEM). However, these mononuclear cells have been variously hypothesized or reported. The present study examined th e TEM features on the resorbed surfaces of three calcified connective tissu es, and aimed to clarify the nature and function of the mononuclear cells i n a reversal phase. Dentine slices cultured with isolated osteoclasts, huma n deciduous teeth, and rat mandibles were used in this study. Specimens wer e fixed, decalcified, and then embedded in Epon 812, and sectioned into 0.1 -mum-thick ultrathin sections. The ultrathin sections were stained with ura nyl acetate and lead citrate, and then examined by TEM. Many sharply pointe d collagen fibrils with striation were observed exposed on the resorbed sur faces of cultured dentine slices, but there were neither cells nor reversal lines. The same features were observed on the root dentine surfaces of hum an deciduous teeth. Under many mononuclear cells in a reversal phase of rem odeling, reversal lines were seen on the resorbed surfaces of rat mandibles , but there were no striated collagen fibrils exposed on the bone surfaces. The alternation of the TEM features on the resorbed surfaces before and af ter the participation of mononuclear cells in a reversal phase of remodelin g suggests the nature and function of these cells: they participate in both degrading the demineralized and disrupted matrix left on the resorbed surf aces and forming reversal lines there.