The Origin of Bone Cells in Endochondral Ossification 33 



So with this knowledge the question can be asked again — what is the origin of 

 the bone cells? 



For the special circumstances outlined above we can put the question somewhat 

 differently. Did the newly formed bone cells originate from the host (connective 

 tissue and/or muscle), or could they be derived from the donor tissue (cartilage and/or 

 perichondrium)? 



Fig. 



Endochondral ossihc 



1 with, 

 ibcarti 



two weeks after intramus 

 ;e includinsJ the zone of 



ransplantation of a frag- 



To solve this problem it is necessary to distinguish donor cells and host cells. This 

 can be achieved most completely by applying the in vitro cultivation technique. In 

 this way the host is eliminated totally and every development occurring in the culti- 

 vated donor tissue must certainly be brought about by the donor cells. 



Thus, fragments of rlbcartUage similar to those used in the grafts, derived from 

 18-day-old mouse embryos, were cultivated on a coagulum according to a modified 

 watchglass technique. After 17 days of cultivation hypertrophic cartilage cells were 

 formed and alongside this zone a border of bone was deposited (Fig. 2). From these 

 experiments it is obvious that the donor tissue has Indeed the potency to form bone. 

 However, this result does not prove that in grafts the donor tissue also produces 

 bone. In order to get some information on this point donor cells were made dis- 

 tinguishable from host cells by labelling the donor tissue with ^H-thymldlne prior to 

 transplantation; In this way the fate of the donor cells during their presence in the 

 host could be determined. 



Under these circumstances, we have to take Into account the possibility that a 

 donor cell could release its label when dying, and a host cell could then pick up the 

 label. In such event a considerable dilution of the label would, however, take place. 

 For this reason only heavily labelled cells were considered to be derived from the 

 graft. 



3''^' Europ. Symp. on Cal. Tissues 



