40 J. S. Greenspan, H. J. J. Blackwood 



studies, to show that RNA precursors, such as labelled nucleosides, are first rapidly 

 incorporated into intracellular pools of intermediate-sized molecules. RNA synthesis 

 then proceeds using these labelled intermediates and, depending upon the size of a 

 particular intracellular pool, the labelling of RNA may continue for some time after 

 a pulse of labelled nucleoside has been given. This is referred to as the 'pool effect'. 

 Dilution of these pools by non-radioactive precursors is not always easily obtained 

 and as shown in the present experiment a second injection one hour later of non- 

 radioactive Uridine one hundred fold, failed to produce any detectable effect in the 

 pattern of RNA synthesis and presumably in the dilution of the pool of labelled 

 intermediates. It is clear that the cells in the system being studied show a large pool 

 effect since labelling of RNA does not reach a maximum until 24 hours after injec- 

 tion. 



References 



BuRCHARD, J., R. Fontaine et P. Mandel: Metabolisme des acides ribonuclciques de I'os 



de lapln et de rat in vivo. C. R. Soc. Biol. 153, 334 (1959). 

 Feinendegen, L. E., V. P. Bond, W. W. Shreeve, and R. B. Painter: RNA and DNA 



metabolism in human tissue culture cells studied with tritlated cytldlne. Exp. Cell Res. 



19, 443 (1960). 

 FiARRis, H.: Turnover of nuclear and cytoplasmic ribonucleic acid in two types of animal 



cell, with some further observations on the nucleolus. Biochem. J. 73, 362 (1959). 

 Owen, M.: Cell population kinetics of an osteogenic tissue I. J. Cell Biol. 19, 19 (1963). 

 — , and S. MacPherson: Cell population kinetics of an osteogenic tissue II. J. Cell Biol. 19, 



33 (1963). 

 Perry, R. P.: On the nucleolar and nuclear dependence of cytoplasmic RNA synthesis in 



Hela cells. Exp. Cell Res. 20, 216 (1960). 

 Prescott, D. M.: Cellular sites of RNA synthesis. In Progress in Nucleic Acid Research and 



Molecular Biology. Davidson and Cohn (eds.). New York: Academic Press 1964, p. 33. 

 Watts, J. W.: Turnover of nucleic acids in a multiplying animal cell. Biochem. J. 93, 297 



(1964 a). 

 — Turnover of nucleic acids in a multiplying animal cell. Biochem. J. 93, 306 (1964 b). 

 — , and Fi. Harris: Turnover of nucleic acids in a non-multiplying animal cell. Biochem. J. 



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Histochemical Studies of Chondrocyte Function in the Cartilage 

 of the Mandibular Condyle of the Rat 



J. S. Greenspan, H. J. J. Blackwood 

 Royal Dental Fiospital of London School of Dental Surgery, London, England 



Introduction 



The cartilage of the mandibular condyle has not been used extensively as a site 

 at which to investigate the processes of cell differentiation, maturation and endo- 

 chondral ossification. The investigation reported here is part of a more comprehensive 

 study of the cellular dynamics, cytochemistry and ultrastructure of this tissue. The 

 aim of this study is to extend our knowledge of the normal and pathological behav- 



