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The Origin of Bone Cells in Endochondral Ossification 



Marijke E. Holtrop 

 Laboratorium voor Celbiologie en Histologic, Rijksuniversiteit, Leiden, Nederland 



In developing long bone the area of endochondral ossification consists of a variety 

 of tissues in which various developmental processes take place: cartilage cells arrange 

 themselves into cell columns; hypertrophic cartilage cells are formed; all kinds of 

 bone cells appear, osteoblasts, osteocytes and osteoclasts; the adjacent connective 

 tissue and bloodvessels also take an active part In the process by penetrating and 

 Invading the developing cartilage tissue. 



In this rather complex system It Is very difficult to determine which cells actually 

 differentiate Into bone cells. The possibility to make an experimental approach to this 

 problem came within reach following some transplantation experiments with frag- 

 ments of ribs from young mice. It was found that an Intramuscular transplant of a 

 well defined part of the developing area of a rib (viz. a piece of cartilage including 

 the zone of the cell columns), progressed to complete endochondral ossification within 

 two weeks of transplantation (Fig. 1). First a zone of hypertrophic cartilage cells 

 was formed and a layer of bone was deposited around this zone. Connective tissue 

 and bloodvessels then penetrated the bony border from the outside. Invaded the 

 hypertrophic cartilage cells and finally bone was deposited alongside the remnants of 

 the cartilage matrix. So, following the Intramuscular grafting of pure cartilage 

 together with Its adhering perichondrium, osteoblasts, osteocytes and osteoclasts 

 appeared after some time. 



