236 ADVENTURES IN RADIOISOTOPE RESEARCH 



blood and the phosphate ions lying on the surface of the apatite crystals 

 of which the bone structure is built. Hand in hand with this exchange 

 goes a biological recrystallization of the skeleton. Crystals of the bone 

 structure go into solution, and the new crystals, which have been formed 

 from the labelled fluid blood, have to be radioactive. The biological 

 recrystallization-— that is, the renewal of the bone skeleton— governs 



Fig. 1. Phosphorus-32 absorption by the tibia of an adult rat 5 minutes 



after intravenous injection (left) and 120 minutes after intravenous 



injection (light). ( X 3) [Photos courtesy of C. P. Leblond 



this exchange process nearly completely after a short time. Autoradio- 

 graphs taken bj^ Leblond and co-workers (9) illustrate clearly that 

 radioactive phosphate is absorbed by the epiphysial plate of the adult 

 rat 5 minutes after injection (compare Fig. 1). After 2 hours, the absorp- 

 tion is very distinct. 



It follows from the classical investigations of Paneth (10) that the 

 ions in the topmost molecular layer fraction of a crystal powder enter 

 into an exchange equilibrium with the ions of a surrounding solution. 

 This statement holds only for a part of the ions which are located in the 

 topmost molecular la^^er of a well-developed crystal surface — of a mine- 

 ral, for example. Even if only a small of the topmost molecular layer 

 of the bone apatite took part in the exchange proceedings, it would 

 be sufficient for the removal of an important part of the phosphorus-32 

 from the plasma which was added to the blood fluid. Three percent of the 

 bone phosphate, or maybe even more, settles in the topmost molecular 

 layer of the bone apatite, and the phosphorus content of the latter is 

 about 700 times greater than that of the blood plasma. If after a while 



