MICRORADIOGRAPHY OF BONE RESORPTION 451 



bone is being remo\ed in the process of remodeling in the metaph- 

 vsis, and in large irregular ca\'ities in cortical bone (Jowsev et cil., 

 1956). Further studies (Jowsev et cil, 1958; Neuman et al., 1960) 

 have shown that the deposition of \'ttrium on the surface of hydroxy- 

 apatite is associated with citric acid production or a change in pH 

 which in turn is produced by injections of parathyroid hormone, 

 which causes resorption of bone in conjunction with the production 

 of osteoclasts (Martin et ah, 1958). Y'*^ may well be a better indica- 

 tion of bone resorption than the presence of osteoclasts, since it 

 depends on the mechanism of tissue resorption for its deposition in 

 bone, the isotope being taken up on bone mineral in areas where 

 the yttrium in the bodv fluid is rendered diffusible by a local increase 

 in acidity. Though deposition of yttrium probably always indicates 

 resorption and its absence in high concentrations indicates lack of 

 resorption, the absence of an osteoclast does not always mean that 

 no resorption is occurring in that area ( Hancox, 1956; Fell and Mel- 

 lanby, 1952; Urist and Deutsch, 1960 ) . Therefore the deposition of 

 yttrium in bone can be used to confirm the characteristic micro- 

 radiographic appearance of a resorbing surface which has been 

 established by comparing the microradiograph with the histological 

 preparation. There should be a high correlation between the sites 

 of deposition of yttrium and the microradiographic appearance of 

 resorption; experimental data confirm this. Autoradiographs of bone 

 sections from a young monkey injected with Y'" 24 hours before 

 death were compared with microradiographs of the same sections. 

 The sites of concentrated Y"^ deposition were correlated with the 

 sites in the microradiograph where the highly calcified surface indi- 

 cated active resorption to be occurring. Accurate measurements on 

 enlarged photographs of the lengths of surface involved showed that 

 the yttrium deposition and microradiographic appearance of re- 

 sorption corresponded in 89 per cent of the areas, the remaining 

 11 per cent being sites either of yttrium deposition which were 

 judged as not showing resorption in the microradiograph or of 

 microradiographic indications of resorption with no concentrated 

 uptake of yttrium. The correlation coefficient between the two sets 

 of results was 0.99. If, on the other hand, the results from the auto- 



