. Calcified tissues; proceedings. Bone; Collagen; Calcification. 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 (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
. Calcified tissues; proceedings. Bone; Collagen; Calcification. 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 (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 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 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. Co
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