INTRAMEMBRANOUS TRABECULAR GENERATION IN NORMAL BONE

The ability of trabeculae to reform following localized ablation may p rovide further insight into the sequence of events in cancellous regen eration. Histological features of cancellous repair were examined in t he iliac crest of aged female sheep at intervals after removal of a l- cm diameter biopsy. Comparison was made with normal intramembranous tr abecular formation in the foetal lamb. The first immature trabeculae t o form in the defects within 3 weeks were exclusively intramembranous, not endochondral, and the systematic process was indistinguishable fr om that in the intact growing foetal lamb. In both the young and old s keleton, two features were prominent. First, the damaged endosteum of the sheep functioned like the intact periosteum of the lamb to produce orderly migrating arrays of discrete coarse collagenous fibres, 5-25 mu m thick, which penetrated the surrounding soft tissues to form a po larised preliminary framework. Without this structure, primary trabecu lar development did not take place. Throughout subsequent bone apposit ion the preliminary framework, which bonded hard to soft tissues and n ew bone to old, remained largely unmineralised. Second. intratrabecula r resorption channels divided the established, thickened primary bars into networks of mature secondary trabeculae. It is concluded that the two features are central and universal to trabecular proliferation an d may provide a morphological basis for future trabecular restitution of the depleted elderly skeleton.