Repairing of rabbit skull defect by dehydrothermally crosslinked collagen sponges incorporating transforming growth factor β1

Collagen sponges of various biodegradabilities were prepared by dehydrothermal crosslinking at 140°C for different time periods. When the collagen sponges were radioiodinated and implanted subcutaneously into the back of mice, the radioactivity remaining at the implanted site decreased with time; the longer the time of dehydrothermal crosslinking, the slower the radioactivity decrement. The radioactivity following the subcutaneous implantation of collagen sponges incorporating ¹²⁵I-labeled transforming growth factor (TGF)-β1 also decreased with time. The time profile of both the radioactivity remainings was in good accordance to each other, irrespective of the crosslinking time. This indicates that the TGF-β1 incorporated in the sponges was released as a result of sponge biodegradation. Potential of collagen sponges incorporating 0.1 μg of TGF-β1 in repairing the defect of rabbit skulls was evaluated in a stress-unloaded state. Bone repairing was induced by application of the collagen sponges incorporating 0.1 μg of TGF-β1 whereas that of free TGF-β1 at the same dose and TGF-β1-free, empty collagen sponges were ineffective. The bone defect was histologically closed by the bone tissue newly formed 6 weeks after application. Bone mineral density (BMD) analysis revealed that the collagen sponge incorporating TGF-β1 enhanced the BMD value at the bone defect to a significantly great extent compared with other agents. A maximum enhancement of BMD was observed for the collagen sponge incorporating TGF-β1 which was prepared by dehydrothermal crosslinking for 6 h. It was concluded that the TGF-β1 incorporated in the collagen sponge was released in a biologically active form as a result of sponge biodegradation, resulting in enhanced bone repairing at the skull defect. It is possible that for too slowly degraded sponges, the remaining physically impairs the bone repairing at the skull defect. Induction of bone repairing would not be achieved through a rapid release of TGF-β1 from too fast-degraded sponge.