Intramembranous trabecular generation in normal bone.

The ability of trabeculae to reform following localized ablation may provide further insight into the sequence of events in cancellous regeneration. Histological features of cancellous repair were examined in the iliac crest of aged female sheep at intervals after removal of a 1-cm diameter biopsy. Comparison was made with normal intramembranous trabecular formation in the foetal lamb. The first immature trabeculae to form in the defects within 3 weeks were exclusively intramembranous, not endochondral, and the systematic process was indistinguishable from that in the intact growing foetal lamb. In both the young and old skeleton, 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 microns thick, which penetrated the surrounding soft tissues to form a polarised preliminary framework. Without this structure, primary trabecular development did not take place. Throughout subsequent bone apposition the preliminary framework, which bonded hard to soft tissues and new bone to old, remained largely unmineralised. Second, intratrabecular 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 and may provide a morphological basis for future trabecular restitution of the depleted elderly skeleton.