38 



M. 0\(EN 



Results 



There was radioactive labelling generally throughout all osteoblasts and pre- 

 osteoblasts. Labelling of fibroblasts was not above background in the autoradiographs 

 studied. The results for grains per nucleus and grains per cytoplasm are shown for 

 osteoblasts in Fig. 2 and for preosteoblasts in Fig. 3. The results for the osteoblasts, 

 Fig. 2, show that there is an initial rapid appearance of labelled RNA in the nucleus 

 which reaches a maximum between one and two hours after injection and then 

 remains approximately constant at this level up to 4 days. The appearance of labelled 

 RNA in the cytoplasm with time occurs more slowly and reaches a maximum at 

 about 24 hours and thereafter there appears to be a slight fall off by 3 and 4 days 

 after injection. Over the time period studied there was no detectable difference in the 

 results for osteoblasts on the surface or in haversian canals. 



Grams/nucleus 



Grams/cytoplasm 



Grains/cell 



Fig. 2. Uptake of ^H-Uridine with time after injection as measured on autoradiographs, in osteoblasts on the 

 periosteal surface of the shaft of a two-week old rabbit femur 



Fig. 3. Uptake of 'H-Uridine with time after injection as measured on autoradiographs, in preosteoblasts on the 



periosteal surface of the shaft of a two-week old rabbit femur. The dashed curves are for the preosteoblasts. 



The curves for the osteoblasts from Fig. 2 are also shown for comparison 



The results for the preosteoblasts, Fig. 3, show a very similar pattern. For com- 

 parison the curves for the osteoblasts from Fig. 2 have been included in Fig. 3. The 

 curves for the grains per nucleus for both the osteoblasts and the preosteoblasts are 



