Regenerative versus reparative healing in tendon: a study of biomechanical and histological properties in fetal sheep.

Previous studies have shown fetal tissues heal in a regenerative fashion without scar formation. The objective of this study is to compare the healing properties of adult and fetal tendons. Time-mated pregnant ewes at 80-85 days of gestation were utilized. A partial, midsubstance tenotomy was performed in the lateral extensor fetal tendons, and analogous tenotomies were created in the maternal limbs. One week after injury, the fetal and adult animals were sacrificed, and tendons were histologically and mechanically evaluated. Immunohistochemical staining for transforming growth factor beta isoform 1 (TGF-beta1) was performed. Histologically, a gap with granulation tissue and inflammatory cells was visible in the site of wounding in the adult tendons. In the fetal tendons, no abnormalities were noted in the wound, with reconstitution of collagen architecture. TGF-beta1 expression was low in fetal but upregulated in the adult wounds. No significant differences were found in the biomechanical properties between groups. We identified regenerative healing properties in injured fetal tendon, while adult tendon tissue healed reparatively with scar formation. Fetal tendons demonstrated a limited recovery of mechanical properties after injury that was no better than that of the adult tendons at seven days. A better understanding of the mechanisms of fetal healing may lead to novel therapeutic strategies in the clinical setting.