Periosteal changes in mechanically stressed rat caudal vertebrae.

When a caudal vertebra is stressed by looping the tail, remodelling results with increased formation of bone on the inner (concave) side of the loop and decreased formation on the corresponding outer (convex) side. The initial morphological changes in periosteum under stress are examined by histology, autoradiography and transmission electron microscopy. Vessel damage appears minimal and thus seems unlikely to be a trigger for the remodelling process. On stress application the connective tissue relationships in the fibrous component of the periosteum are altered immediately but changes in the osteogenic layer are delayed. On the inner side, the midzone between the cellular periosteum and the fibrous periosteum becomes drawn out and enlarged, with reorientation of the cells perpendicular to the bone. This reflects the tension exerted on the bone surface through the elastic recoil of the fibrous periosteum. On the outer side, the midzone becomes narrowed as the taut fibrous periosteum exerts a compressive force on it. The midzone, which shows a delayed response and the greatest structural change with altered stress, may buffer the osteogenic layer and so play an important role in bone remodelling. The results have considerable bearing on the establishment of bone form during normal growth and development.