BONE RESORPTION IN TISSUE CULTURE 633 



resorbing factors — including parathyroid extract. Perhaps the en- 

 hancing effect of oxygen is directly related to increased formation 

 of ATP, since the cells involved in bone resorption are oliviously 

 most active and must require large amounts of energ)' to carry out 

 the complicated task of bone resorption. The inhibition of bone 

 resorption in our system by the addition to the medium of 2,4-dini- 

 trophenol, an uncoupler of oxidative phosphorylation, provides ex- 

 perimental evidence favoring this hvpothesis. The influence of 

 oxygen tension on vitamin A-induced resorption and probablv on 

 vitamin D-induced resorption may be explained on a similar basis. 

 Since the oxygen tensions delivered to the bone-resorbing cells 

 in the calvaria were probably within physiological limits, it is 

 possible that regulation of bone resorption in vivo might be accom- 

 plished by variation of local oxygen tension through alterations in 

 the local vasculature. Such a scheme would allow local control or 

 modification ( enhancement or inhibition ) of the effects of a variety 

 of systemically circulating bone resorption stimulators. 



The finding that several D vitamins ( dihydrotachysterol, crystal- 

 line vitamin D2 or Da ) exhibit marked enhancement of bone resorp- 

 tion in our tissue culture system provides additional evidence 

 favoring the concept that vitamin D may have a similar direct 

 effect on bone in vivo. 



Although enhanced bone resorption was demonstrated in our sys- 

 tem by the addition of either parathyroid extract, crystalline vitamin 

 A, dihydrotachysterol, or crystalline vitamin D2 or D3, the difference 

 in results obtained with such closely related compounds as /3-caro- 

 tene and vitamin A emphasizes the specificity of the stimulation. 

 How these compounds alter the metabolism of bone cells must be 

 the subject of continued experimentation. 



Summary 



1. The extent of bone resorption can be regulated in roller-tube 

 cultures containing chick embryo extract by variation of the oxygen 

 tension in the gas phase. 



2. Parathyroid extract, when introduced into the supernatant me- 

 dium from which embryo extract had been eliminated, induced 

 marked and progressive bone resorption which could be detected 



