36 M. Owen 



Two weeks after transplantation of such cartilaginous fragments, the grafts were 

 taken out of the host and autoradiographs were prepared. Examination of the ossi- 

 fied part of the grafts revealed heavily labelled osteoblasts and osteocytes (Fig. 4). 

 This obviously indicates that these cells originated from cartilage cells. Moreover, 

 heavily labelled periosteal cells were found surrounding the bony cap. This suggests 

 that cartilage cells probably first differentiate into connective tissue cells (peri- 

 chondrium or periosteum) and then form bone afterwards. 



It can, therefore, be concluded that to the list of possible precursors of bone cells, 

 the cartilage cell should be added. 



RNA Synthesis in Growing Bone 



M. Owen 



Medical Research Council, Bone-Seeking Isotopes Research Unit, The Churchill Hospital, 

 Oxford, England 



Introduction 



This paper describes the preliminary results of an autoradiographic study of the 

 pattern of RNA synthesis in cells which are associated with growing bone surfaces. 

 The system studied is the periosteal surface of the shaft of the femur of young 

 actively growing rabbits aged about 2 weeks. A diagrammatic representation of this 

 surface is shown in Fig. 1 a. The bone surface has characteristic loops typical of form- 

 ing haversian systems. Osteoblasts line the surface of the bone and the haversian 

 systems. The layer of cells behind the osteoblasts on the bone surface have been called 

 preosteoblasts, a term also given to cells which are within the haversian canals but 

 not on their surfaces. A layer of fibroblasts with typical elongated nuclei separates 

 the osteogenic cellular layer from nearby muscle. 



In previous work, (Owen, 1963; Owen and Macpherson, 1963) the rate of bone 

 growth was measured using tritiated glycine and it was found that the front of the 

 bone surface advanced at the rate of about 70 // per day. The situation after 4 days 

 is illustrated in Fig. 1 b. The rate of increase of the cell population during bone 

 growth and the rate of movement of cells from one stage to another was also 

 measured. The fibroblasts were shown to have a low rate of cell division and con- 

 sequently contributed a negligible amount to the increase in cell population. The main 

 region of cell proliferation was the pre-osteoblasts on the bone surface. These cells 

 increased at the rate of about 33% per day, and spent on average 3 days on the 

 periosteal surface before going on to become preosteoblasts within haversian canals or 

 osteoblasts on the bone surface. The average time spent by osteoblasts on the bone 

 surface was also three days before they became incorporated within the bone matrix 

 as osteocytes or as osteoblasts in haversian systems. During this period of three days 

 it was also shown that the osteoblasts produced on average between two and three 

 times their own volume of bone matrix. 



We were interested to learn something about the pattern of RNA synthesis in this 

 system consisting as it does of a fairly uniform population of highly differentiated 



