RNA Synthesis in Growing Bone 



37 



cells, the osteoblasts, engaged in active protein synthesis, predominantly collagen, 

 (each cell producing on average just less than its own volume of matrix per day) and 

 their precursors, the preosteoblasts, which are presumably a mixed population of cells 

 in different stages of differentiation and cell division. 



(a) 



(b) 



Fig. 1. Diagrammatic representation of the periosteal surface of the shaft of the femur of a two-week old rabbit 

 illustrating (a) the various layers of cells on the hone surf.ice and (b) bone growth over a period of 4 days 



Materials and methods 



Suckling rabbits between one and two weeks of age were chosen according to 

 weight, 110 to 130 grams. Tritiated uridine, (Uridine-5-T, specific activity 24.4 c/mM) 

 obtained from The Radiochemical Centre, Amersham was used as an RNA precursor. 

 An intraperitoneal injection of uridine, 5 //c/gram, was given and the rabbits killed 

 at intervals varying from 15 mins. to 4 days after injection. Autoradiographs were 

 prepared as previously described (Owen, 1963). Exposure times were 10 days. In 

 some cases a second Injection of about one hundred fold non-radioactive uridine was 

 given 1 hour after the initial Injection of the radioactive material. In order to deter- 

 mine the effect of this on the pattern of uptake with time. 



Counts were made of the number of grains per nucleus and per cytoplasm for 

 preosteoblasts and osteoblasts on the bone surface and In haversian canals within a 

 band of bone of width B on the periosteal surface. Fig. 1 a. The width of B varied 

 according to the time interval at which the animal was killed. Up to 8 hours after 

 Injection B was 200 // wide. At the later time Intervals the width of B was increased 

 to take account of bone growth, see Fig. 1 b. Cells on the bone surface, I. e. within 

 the loops out to the first layer of fibroblasts, were counted separately to those within 

 the haversian canals. Counts were corrected for background which was less than 1.0 

 grain per cell. 



