944 



ADVENTURES IN RADIOISOTOPE RESEARCH 



ged after this period, while 29 and 7 per cent respectively were renewed 

 not once, but, at least to some extent, repeatedly. 



The restricted extent of renewal of the skeleton is due to the fact 

 that while the P atoms of the uppermost molecular layer of the bone 

 apatite crystals can promptly interchange with the free P atoms of the 

 plasma (actually not the P atoms, but the phosphate ions interchange), 



Tablk 5. — Extent of Renewal of the Mine- 

 ral Constituents of the Bone 

 IN the Coitrse of 50 Days 



a renewal of the main part of the apatite P can take place only when 

 the crystal is dissolved and when crystals are formed from the plasma ; 

 from a labelled plasma, labelled crystals are formed. This "biological" 



10 mm 



Yia. 5. — Distribution of labelled phosphorus in the incisor of a rat 

 killed 3 days after the administration of the phosphorus. The figures 

 below give the relative amounts of labelled phosphorus present in 

 1 mgm of fresh tissue in the section in question. The figures above 

 crive the length of the section in mm 



recrystallization of the skeleton crystaUites is a comparatively slow pro- 

 cess. Moreover, we have to consider that if only the outer part of the 

 crystal is renewed, this process can often be repeated without affecting 

 deeper molecular layers of the crystal. 



No data are available concerning the extent of renewal of the human 

 skeleton ; the relative rates of renewal of different parts of the skeleton 

 were, however, determined by Erf^''"^ 



