45^2 J. .towsp:y 



radiographic measurements of resorption and the microradiographic 

 measurements of resorption are compared directly, paying no atten- 

 tion to whether the measurements correspond, there is a 99.99 per 

 cent similarity between the results. Therefore, assuming the results 

 from the ) ttrium autoradiographs to be correct, the results from the 

 microradiographs will be 99.99 per cent similar numerically though 

 they will include 10 per cent mistakes. This correlation between the 

 deposition of yttrium and the microradiographic appearance of high 

 density confirms the criteria established for bone resorption by com- 

 parison with the presence of osteoclasts in a histological preparation, 

 and also the quantitative data suggest that very nearlv 100 per cent 

 of all resorption surfaces are being measured by this method. 



These correlative data show that a bone surface that is under- 

 going resorption appears in a microradiograph as an irregular, 

 crenated, highlv calcified surface with the lamellae Iving at all 

 angles to the surface rather than around the cavity ( Fig. 1 ) . 



It is most important to distinguish between a bone surface that 

 is actively being resorbed and appears microscopically crenated and 

 irregular and one where resorption has been taking place but has 

 ceased. The latter surfaces are highly calcified and irregular and 

 have the lamellae running at angles to the surface, like areas of 

 active resorption, but are microscopically smooth and have a narrow 

 rim of high mineralization immediately adjacent to the vascular 

 cavity. 



Apart from the areas of active resorption, yttrium is also taken 

 up diffusely throughout the bone tissue, probably in association with 

 low levels of citric acid, and it is also retained to some extent on 

 the highly mineralized surfaces of completed osteons ( Jowsey et ah, 

 1956, 1958). Therefore yttrium and the rare earths probably cannot 

 be used to measure bone resorption with any certainty b\' methods 

 using in vivo external counting or chemical measurements on large 

 pieces of bone, because, as with the alkaline earths, it is not possible 

 to differentiate between the fraction of isotope retained as a result 

 of the localized intense activity of the bone, that is, the resorption 

 of tissue, and the slow metabolic activity that occurs throughout 

 the bone. 



