Basic fibroblast growth factor and transforming growth factor beta-1 expression in the developing dura mater correlates with calvarial bone formation

Numerous studies have found dura mater-calvarial mesenchyme interactions during calvarial bone induction; however, the exact molecular mechanisms governing these inductive events remain unknown. Recent studies have implicated basic fibroblast growth factor (FGF-2) and transforming growth factor-beta1 (TGF-beta1) in regulating bone formation. The purpose of this study was, therefore, to investigate the expression of FGF-2 and TGF-beta1 during calvarial bone formation in rats. Eight rats were killed on embryonic days 14, 18, and 20 and neonatal day 1 (n = 32). Four animals at each time point were analyzed by in situ hybridization, and the remainder were analyzed by immunohistochemistry. The results indicated that the dura mater underlying the developing calvarial bone strongly expressed FGF-2 and TGF-beta1 mRNA at all time points examined. In contrast, minimal growth factor expression was noted in the overlying calvarial mesenchyme until embryonic day 18, but it increased significantly with increasing age. Importantly, FGF-2 and TGF-beta1 mRNA expression in the dura mater underlying the developing calvarium preceded and was significantly greater than expression in the calvarium mesenchyme (p < 0.05). Interestingly, minimal expression of FGF-2 and TGF-beta1 mRNA was noted for all time points in the dura mater underlying the posterior frontal suture and within the posterior frontal suture connective tissue (p < 0.01 when compared with the dura mater underlying the developing calvarium). Immunohistochemical findings closely paralleled mRNA expression, with intense staining for FGF-2 and TGF-beta1 in the dura mater underlying the developing calvarial mesenchyme. Increasing FGF-2 and TGF-beta1 staining was noted within calvarial osteoblasts with increasing age, particularly in cells located near the endocranial surface (i.e., in contact with the developing dura mater). These findings, together with the known biologic functions of FGF-2 and TGF-beta1, implicate these growth factors in the regulation of calvarial bone growth by the developing dura mater. The possible mechanisms of this interaction are discussed.