I have identified another topic which I feel will turn out to be important in this meeting—the notion of coupling. In all of the stages and processes there is coupling between a variety of events. There is coupling between different tissues (usually epithelia and mesenchyme) in terms of the initial induction of the skeleton and the embryo. There is cell—cell coupling in all sorts of situations — in condensation, differentiation and morphogenesis. There is the cellular coupling that occurs in replacement of cartilage by bone, which involves hypertrophic chondrocytes coupling to the invading vascular system. There is the hormonal coupling that occurs between bone formation and bone resorption: the paradoxical fact that the best way to stimulate bone resorption is to stimulate the cells that lay down bone and have them interact with osteoclasts. Then of course there is extensive coupling in growth, including coupling with other organ systems (musculoskeletal systems), with hormones and with molecules, including a whole variety of growth factors and signalling molecules that are involved in controlling these processes.

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