INTERNAL REMODELING OF COMPACT BONE 377 



wholly dependent upon parathyroid function; a rate-determining 

 relation between the parathyroids and osteoclastic activity seems 

 likely. It is not known whether in the absence of the parathyroids 

 vitamin D assists in promoting the resorption incident to remodeling; 

 such an influence is a possibility. 



Once a tunnel is completed, there is a reversal in activity, and 

 osteoblastic deposition of bone begins; there is established a con- 

 tinuing supply of preosseous tissue, and deposition of bone mineral 

 in the organic matrix is initiated. It is in these locations that the 

 reactivity of the newly formed tissue is demonstrable by observation 

 of the uptake of radioactive calcium or radium. When new tissue is 

 laid down, gradually filling in the cavity until only enough space is 

 left for the haversian canal, there is always a fresh surface lining 

 the forming haversian systems, available for deposition of mineral. 

 It is in such locations that accretion continues throughout life. 



Deposition of new mineral, as seen bv autoradiographs following 

 administration of Ca^"' or Ra^-'', is concentrated in these newly 

 formed layers of preosseous tissue, lining the haversian systems; 

 these areas have acquired the designation hotspots, as illustrated 

 in Fig. 3. The mineral so deposited is often referred to as exchange- 

 able, labile, or reactive; there is as yet no direct evidence that it is 

 from these locations that mineral leaves the bone to replace that 

 lost in the rapid turnover of the blood calcium. In addition to the 

 concentration in hotspots, there is diffuse deposition of the isotopes, 

 in much lower concentration and more or less uniformly, throughout 

 the compact bone; this has been designated as the diffuse component 

 (Marshall et al., 1959). Of the total amount deposited in the skele- 

 ton, following administration of a single dose of Ca**^ or Ra^^'', ap- 

 proximately one-half is distributed in the diffuse component, while 

 the other half is concentrated in the hotspots, occupying only a small 

 proportion of the total space in the bones. 



Bauer et al. ( 1955 ) accounted for the transfer of calcium between 

 blood and bone, in both directions, by a combination of accretion, 

 resorption, and exchange. The mineral concentrated in the newly 

 forming osteons, as demonstrable by the use of tracer Ca^^ or Ra"^*', 

 is attributed to accretion. Resorption, associated with the presence 

 of osteoclasts, is generally believed to result from an osteolytic action 



