632 



p. GOLDHABER 



Figs. 32 to 35. Microscopic appearance of 7-day roller-tube cultures gassed 

 with 50 per cent Oo and treated with various concentrations of dihydrotachy- 

 sterol. Note the increased frontal bone resorption (R) as compared with the 

 control (Fig. 32) as the concentration of dihydrotachvsterol is raised from 

 0.001 mg/ml (Fig. 33), to 0.01 mg/ml (Fig. 34), to 0.1 mg/ml (Fig. 35). 

 (All figures X 14.) 



test substance. The fact that elimination of embryo extract from our 

 supernatant fluid results in decreased bone resorption in response to 

 oxygen indicates that the "oxygen effect" with our embryo extract- 

 containing medium depends on the simultaneous presence of some 

 unknown bone-resorbing cofactor in the extract. Paradoxically, the 

 elimination of embryo extract permits parathyroid extract, in the 

 presence of oxygen, to stimulate bone resorption, thereby suggesting 

 the presence of a parathyroid inhibitor in embryo extract. This latter 

 possibility is analogous to the findings of Chen ( 1954 ) concerning 

 the inhibitory effect of embryo extract on insulin in tissue culture. 



Of interest is the finding that the effect of parathyroid extract is 

 directly dependent on the amount of oxygen in the system. This 

 effect of oxygen may be due to a generalized metabolic priming of 

 the target cells, making them more susceptible to the action of bone- 



