Immature versus mature dura mater: II. Differential expression of genes important to calvarial reossification.

The ability of immature animals and newborns to orchestrate successful calvarial reossification is well described. This capacity is markedly attenuated in mature animals and in humans greater than 2 years of age. Previous studies have implicated the dura mater as critical to successful calvarial reossification. The authors have previously reported that immature, but not mature, dural tissues are capable of elaborating a high expression of osteogenic growth factors and extracellular matrix molecules. These findings led to the hypothesis that a differential expression of osteogenic growth factors and extracellular matrix molecules by immature and mature dural tissues may be responsible for the clinically observed phenotypes (i.e., immature animals reossify calvarial defects; mature animals do not). This study continues to explore the hypothesis through an analysis of transforming growth factor (TGF)-beta3, collagen type III, and alkaline phosphatase mRNA expression. Northern blot analysis of total RNA isolated from freshly harvested immature (n = 60) and mature (n = 10) dural tissues demonstrated a greater than three-fold, 18-fold, and nine-fold increase in TGF-beta3, collagen type III, and alkaline phosphatase mRNA expression, respectively, in immature dural tissues as compared with mature dural tissues. Additionally, dural cell cultures derived from immature (n = 60) and mature dura mater (n = 10) were stained for alkaline phosphatase activity to identify the presence of osteoblast-like cells. Alkaline phosphatase staining of immature dural cells revealed a significant increase in the number of alkaline phosphatase-positive cells as compared with mature dural tissues (p < 0.001). In addition to providing osteogenic humoral factors (i.e., growth factors and extracellular matrix molecules), this finding suggests that immature, but not mature, dura mater may provide cellular elements (i.e., osteoblasts) that augment successful calvarial reossification. These studies support the hypothesis that elaboration of osteogenic growth factors (i.e., TGF-beta33) and extracellular matrix molecules (i.e., collagen type III and alkaline phosphatase) by immature, but not mature, dural tissues may be critical for successful calvarial reossification. In addition, these studies suggest for the first time that immature dural tissues may provide cellular elements (i.e., osteoblasts) to augment this process.