Gene expression at graft-host interfaces of cortical bone allografts and autografts

The healing processes of autogenous and frozen allogeneic cortical bone grafts were compared in rats. Two unicortical defects created in the proximal metaphysis of the tibia were autografted or allografted with blocks of cortical bone. The biomechanical properties of the graft-host junction and the time-related changes of the expression of genes coding for Type I, II, III, and X collagens and osteonectin were determined at one, two, four, and eight weeks. The strength of the allograft-host bone union was lower than that of autografts at the early stage of incorporation but the difference diminished by eight weeks. Northern analysis of graft mRNAs demonstrated a strong expression of Type I collagen and osteonectin genes at the beginning of autograft incorporation. The allografts showed a lower expression of the corresponding genes at one and two weeks, but the difference diminished thereafter. The gene coding for Type III collagen showed a temporary expression at two weeks both in allografts and autografts, probably because of the formation of inflammatory fibrovascular stroma provided by the host bone at the graft-host interface. No expression of cartilage-specific Type II and Type X collagens were observed, demonstrating that the healing in both grafts proceeded through primary bone healing. Compared with cortical-cortical junctions of autografts, the graft-host interfaces of allografts exhibited a reduced osteoinductive process and a slower increase of union strength.