STRUCTURE AND MULTIPLICATION OF BONE CELLS 143 



By slowly changing the focus the relation to other cells at other 

 levels can be established. Since the osseous matrix is unstained 

 or only slightly tinted, the deeply stained bone cells with their 

 anastomosing protoplasmic processes produce the picture of a 

 syncytium. 



a. Shape and size of bone cells 



The shape of the bone cell is usually stellate with a more 

 or less elongated body, but difference in shape depend on the age 

 of the cells. Cross sections of the parietal bone show that the 

 bodies of bone cells are not flat but round or oval. Very young 

 cells are more rounded much like the osteoblast and the processes 

 are short or sometimes even absent. In older cells the stellate 

 shape is very marked The cell body varies from 10 to 18 micra 

 in diameter although some polynucleated cells measure more. 

 In young cells the average diameter is greater than in older 

 cells. The young cells are usually preparing to divide, and this 

 may be the reason for the difference. This seems to be sub- 

 stantiated by the fact that in very young parietal bone of the 

 rat some groups of cells are very large while other groups are 

 rather small. The nuclei of the large cells usually have divided 

 or are about to divide (fig. 6). The groups of smaller cells 

 show signs of having passed through a stage of rapid division 

 (fig. 2). Both of these groups of young cells can be found near 

 the periphery of the growing parietal bone. The difference in 

 size, however, is not. entirely explained by the stage of pro- 

 liferation because a difference in size also exists in older cells 

 of different ages. In the one day old rat the cells at the center of 

 the parietal bone^ — these are the oldest cells — are large, measuring 

 from 15 to 16 micra in diameter (fig. 4). On the other hand the 

 cells near the center of a 15 day old rat measure about 12 to 

 13 micra (fig. 3). Figures 3 and 4 also show the interesting fact 

 that the oldest cells of the 15 day old rat are about 20 micra 

 apart while those of the one day old are separated by only 16 

 micra. The difference is usually greater than is shown by these 

 figures. This difference can also be seen between the peripheral 

 and central cells of a given young bone. Thus the central 



