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J. JOWSEY 



Fig. 1. Microradiograph of a cross section through the mid-shaft of the 

 femur of a 2-year-old male. Bone formation is occurring at a and b while 

 resorption is occurring at c and d. There are a number of areas of interstitial 

 bone containing enlarged lacvniae (e and /) which are characteristic of woven 

 or rapidly growing bone. ( X 20. ) 



surface from which bone is being removed looks like in, for instance, 

 a microradiograph of a bone section. This evidence comes from a 

 number of sources. The periosteal surface of the metaphvsis of a 

 long bone from a growing animal is an area where removal of tissue 

 must be occurring as the diameter of the shaft becomes smaller. 

 In addition, histological preparations show osteoclasts on such 

 surfaces, and osteoclasts have been observed to resorb bone in tissue 

 culture (Goldhaber, 1960). Such a periosteal surface must therefore 

 be undergoing active resorption. This surface appears in a micro- 

 radiograph as an irregular, highlv calcified surface. 



The resorption of bone has also been associated with the deposi- 

 tion of radioactive isotopes of the rare earths and, in particular, of 

 yttrium. Y^^ has been shown to be localized in vivo in areas where 



