34 



Marijke E. Holtrop 



Donor mice were injected with ^H-thymidine and fragments of ribcartilage 

 including the zone of cell columns were excised and transplanted according to the 

 usual technique. Autoradiographs of the donor tissue before transplantation showed 

 that tritiated thymidine was readily taken up, predominantly by cells in the zone of 

 cell columns. Twelve days after transplantation the grafts showed endochondral 



. Ribcartiiage 

 17 days of cu 



ng the zone of cell columns derived from a 18-day-old mouse embryo as observed 

 ition on a coaguluni. Hypertrophic cartilage is formed and bone is deposited alongside 



ossification in the manner already described. Autoradiographs showed that hardly 

 any label was left in the zone of the cell columns. But some heavily labelled osteo- 

 blasts, and also some heavily labelled osteocytes were found. 



This experiment shows, therefore, that not only in tissue culture, but also in 

 grafts, the donor tissue is capable of forming bone. 



The question now arises as to whether the bone cells originate from the peri- 

 chondrium or from the cartilage, or from both. 



It would seem reasonable that they had originated from the perichondrium as in 

 fact some perichondrium cells were labelled before transplantation. However, some 

 further observations led us to assume that cartilage cells might also be precursors of 

 bone cells. 



Information concerning this supposition was obtained by repeating the trans- 

 plantation experiments, but this time using grafts of ribfragments of cartilage without 

 any perichondrium at all. Cutting off the perichondrium of mouse ribs proved diffi- 

 cult as they were too small to handle easily. It was found that the ribs of rats could 

 be completely parted from the perichondrium while enough cartilage remained for 

 transplantation. It was still doubtful how these 'maltreated' fragments would behave 

 after transplantation but surprisingly enough, two weeks after transplantation of 

 such fragments endochondral ossification developed In exactly the same way as the 

 grafts with adhering perichondrium had done. 



In order to determine what happened to the cartilage cells during their presence 

 in the host. In the next experiment, the donor tissue was labelled with ^H-thymldlne. 

 Ribfragments including the zone of cell columns but deprived of perichondrium, were 



