The Impact of Age Upon Healing: Absolute Quantification of Osteogenic Genes in Calvarial Critical-Sized Defects

The current study was performed to elucidate changes in growth factor expression over time in critical-sized calvarial defects in rats from infancy to skeletal maturity.

Critical-sized parietal defects of 5, 6, and 8 mm were created in postnatal day 6 (P6), postnatal day (P20), and postnatal day (P84) adult rats, respectively. Dura was harvested at 3, 7, or 14 days after surgery, and serial micro-computed tomography imaging was performed through 12 weeks postoperatively. Absolute quantitative polymerase chain reaction was performed for Bone Morphogenic Protein-2 (BMP-2), Fibroblast Growth Factor-2 (FGF-2), Insulin-like Growth Factor-1 (IGF-1), and Transforming Growth Factor-β1 (TGF-β).

The P6 (6-d-old) rats showed the greatest difference in gene expression between the dura derived from the defect side and the dura derived from the control side, demonstrating significant differences in TGF-β1, BMP-2, IGF-1, and FGF-2 at various time intervals. Absolute gene expression in the defect dura was highest in the P6 rats and declined with age. Significant differences were noted at limited time points in the P20 rats for TGF-β1 and BMP-2 as well as in the P84 rats for TGF-β1. TGF-β1 was the only gene studied that showed significant differences at postoperative days 3, 7, and 14 in varying age groups.

The P6 rats have a higher osteogenic potential accompanied by a more vigorous alteration in growth factor expression compared with the P20 or P84 rats. Decrease in BMP-2 and FGF-2 as well as relative increase in TGFβ-1 messenger RNA were observed in healing defects. These data provide valuable insight into the mechanism of healing of critical-sized defects and may be of use to engineer factor-releasing implants to correct skull defects.