RNA Synthesis in Growing Bone 39 



not significantly different. The curves for the grains per cytoplasm have a similar 

 shape but the results for the preosteoblasts are considerably below those of the osteo- 

 blasts. In the case of the preosteoblasts the results for the cells on the surface were 

 lower than those in the haversian canals at the two longest time intervals, 3 and 4 

 days. In Fig. 3 the results for the cells on the surface are designated by S, the other 

 points are for the cells in haversian canals. This probably reflects some cell division 

 which takes place mainly in the cells on the periosteal surface. 



An injection of non-radioactive uridine one hour after radioactive uridine had 

 no detectable effect on the curves for RNA in the nucleus or cytoplasm of either cell 

 types. 



Discussion 



A detailed discussion of these results is not possible in the space allotted and a 

 few points only will be made. The present results are in agreement with previous 

 reports concerning the study of RNA synthesis in the cells of bone In so far as a 

 comparison can be made (Burchard et al., 1959; Young, 1963). Previous autoradio- 

 graphic studies similar to the present one, however, have been made on other cell 

 types mostly in vitro. The type of experiment performed has usually involved 

 incubation of the cells for a short period in medium containing the radioactive pre- 

 cursor followed by a period in non-radloactlve medium. Experiments have been 

 carried out mostly on rapidly dividing populations (Hela cells and fibroblasts) 

 though some experiments on a non-multlplying cell (macrophages) have also been 

 made (Watts and Harris, 1959; Harris, 1959; Feinendegen et al., 1960; Perry, 

 1960). In spite of the dift'erence In experimental design It is significant that the 'In 

 vitro' results for different systems and our 'in vivo' results for osteoblasts and pre- 

 osteoblasts show the same common features. The results from both types of experi- 

 ments show that there is an initial rapid labelling of RNA in the nucleus and a time 

 lag before radioactivity Is detected in the cytoplasm. It Is generally accepted that 

 synthesis of most RNA takes place in the nucleus and that cytoplasmic RNA Is 

 derived from this, although there Is still some controversy on this Issue (Prescott, 

 1964). 



It is of interest to determine the amount of RNA turnover which occurs in cells 

 In conjunction with protein synthesis, where turnover of RNA is defined as a 

 balanced process of synthesis and degradation of RNA. It ought to be possible to 

 make a direct measurement of turnover In RNA using a radioactlvely labelled pre- 

 cursor as in the present experiment, by determining the extent to which the radio- 

 active label Is retained In the RNA. Some of the conditions In the present experiment 

 are favourable for such a measurement, for example we can be fairly certain In the 

 case of the osteoblasts that we are looking at the same population of cells over the 

 period of 4 days and that there has been a negligible amount of cell division In these 

 cells. From the upper curve in Fig. 2 it can be seen that about 25'Vn of the labelled 

 RNA turns over In 3 days. However this figure is likely to be a lower limit since 

 reutlllzatlon of degradation products, which is known to occur, will mask the effect 

 of turnover. 



In fact interpretation of experiments of the present kind are very difficult due 

 mainly to 'pool effects' and reutilization phenomena, (Watts and Harris, 1959; 

 Watts, 1964 a; Watts, 1964 b). There Is good evidence, mainly from tissue culture 



