Study of cell kinetics within evolving secondary Haversian systems.

A study of the origin, proliferation rate and migration of cells within the secondary evolving Haversian systems was undertaken in young adult Beagle dogs. Autoradiographs of serial longitudinal sections prepared from rib biopsies taken from one hour to eleven days after the injection of tritiated thymidine were subjected to semiquantitative analysis as to the time of appearance, number, location and transformation of various labelled cells. Numerous labelled osteoblasts appeared early (at 14-24 hours) in the most proximal closing cone. With time, this zone was seen to have been left behind the advancing cutting cone and the successive generations of osteoblasts. The first labelled osteocytes were seen at nine days after injection, in the distal closing cone. Labelled nuclei within the osteoclasts were few and appeared late (none before 24 hours). It is apparent that each self renewing cell population within these systems (i.e. osteoclasts, osteoblasts and endothelial cells) derives from its own immediate precursor and evolves at its own speed. The mononuclear osteoclasts' precursors divide locally and infrequently and the turnover of osteoclastic nuclei appears to be slow; consequently their life span and that of the osteoclasts appears to be longer than the time of the observation, i.e. 11 days. The proliferation of osteoblasts' precursors and osteoblasts recruitment is rapid. The life span of osteoblasts was found to be indeterminate; some osteoblasts may become osteocytes within a few days while others may continue to deposit bone for several weeks. Since the recruitment of osteoclastic nuclei is slow while that of the osteoblasts is fast, it is unlikely that the osteoclasts in the sites of lamellar bone remodelling modulate into osteoblasts.