Interaction of growth factors regulating chondrocyte differentiation in the developing embryo.

Endochondral ossification is multistep process that is regulated by a complex network of signalling systems. Endochondral ossification is initiated with the condensation of chondrocytes into cartilage elements in which the chondrocytes subsequently progress through stages of proliferation and hypertrophic differentiation. Finally, terminally differentiated chondrocytes undergo apoptosis and are replaced by bone. As hypertrophic differentiation links chondrocyte proliferation with the ossification of the skeletal elements it seems to be one of the critical steps in this process (Fig. 5). Ihh and PTHrP are two signalling molecules that interact in a negative feedback loop regulating the pace of hypertrophic differentiation. In addition Ihh has recently been shown to independently regulate chondrocyte proliferation and the ossification process, thus coordinating three different steps of endochondral bone formation. Two other groups of signalling molecules have been found to interact with Ihh during endochondral ossification. BMP signalling seems to act downstream of Ihh. BMPs might serve as secondary signals downstream of Ihh mediating the Ihh signals to the periarticular perichondrium to induce PTHrP. Alternatively BMP signalling, induced by Ihh, might reciprocally act back on the prehypertrophic chondrocytes, thereby coordinating hypertrophic differentiation with the differentiation of the periosteum. The idea of an interaction of the two signalling systems is supported by the fact that not only BMPs but also their receptors and at least two of the BMP antagonist are expressed in regions that are thought to be targets of Ihh signalling. A third signalling pathway critical for proper bone development is signalling through the FGFR3, which seem to act upstream of both Ihh and BMP signalling. In summary, it becomes more and more obvious that the single steps of endochondral ossification are tightly coordinated. For example signals from the joint region of the cartilage elements play an important role in regulating both chondrocyte proliferation and differentiation and at least some of these signals seem to interact with signals from the hypertrophic region, linking hypertrophic differentiation and proliferation. In addition, signals from the perichondrium/periosteum are thought to interact with signals from the differentiating chondrocytes to coordinate the differentiation of the periosteum with hypertrophic differentiation. Although significant progress has been made during the last years in analysing the signals regulating endochondral ossification in the developing embryo, complete understanding of the control system will require further extensive studies.