INTERNAL REMODELING OF COMPACT BONE 375 



and D, fluorescent image with ultraviolet liglit, revealing deposition 

 of tetracvcline administered 4 days before sacrifice. Treatment of 

 sections of undecalcified bone by these different methods adds 

 greatly to the information to l^e obtained from their study. The 

 autoradiographs and the fluorescent image reveal that both radium 

 and tetracycline are preferentialh' deposited in preosseous tissue, as 

 l:one is laid down in concentric layers in the rebuilding of osteons. 



When visualized in three dimensions, the tunnel, or resorption 

 cavity, is seen to contain blood vessels and connective tissue cells. 

 It is excavated in the long axis of the bone, cutting through old 

 csteons and lamellar bone, without relation to the previous structure. 

 While resorption is in progress, the cavity is lined with osteoclasts; 

 when rebuilding begins, these are replaced by osteoblasts. The 

 tunnel is then filled in, from the walls toward the center, by apposi- 

 tion of bone in successive layers or lamellae, incorporating the 

 csteocvtes, their lacunae, and the canalicules. The formation of new 

 lavers continues until the canal reaches its final diameter, usually 

 approximately 20 microns; at this time the osteoblasts assume a 

 resting form, and are no longer recognizable. 



Using lead as a marker, Vincent ( 1957 ) has determined the rate 

 of formation and of maturation of new osteons. An average resorp- 

 tion cavity in a dog takes roughly 3 weeks to form, this being the 

 period required for tunneling or excavation. The building of the 

 new osteon, including partial minerafization of the organic matrix, 

 requires some 6 to 12 weeks. Primary mineralization, to about 70 

 per cent of the final density, occurs rapidly during and immediately 

 after the deposition of new layers of organic matrix; completion of 

 secondary mineralization, to maximum density, takes much longer 

 and has been found to be incomplete for as long as 18 weeks. 



Frost et al. (1960), using tetracycline as a marker, found the 

 mean osteon formation time in a 57-year-old man to be 5 weeks. The 

 biologic half life of the osteons was calculated as 2.7 years for the 

 femur and 8.6 years for the tilMa. In another study, also with tetracy- 

 cline. Frost and Villanueva (1960) report that an average of 0.9 

 micron of new osseous tissue per da\' is formed in actively forming 

 haversian systems in man. 



As to the formation of resorption cavities, little is known of the 



