Forces and pattern in limb morphogenesis

Prior to cartilage and bone formation in the limb bud chondroblasts condense into foci which provide the pattern for subsequent bone development. Formation of these condensations is, finally, a mechanical event, and so it is natural to ask what are the forces responsible for creating them. We have constructed a model for the process of cell aggregation during chondrogenesis which involves the following forces: (1) the passive elasticity of the extracellular matrix (ECM), (2) the osmotic swelling pressure of the ECM, which is generated principally by the hyaluronate (HA) component, (3) the active cell tractions developed by the chondroblasts. By examining the balance of forces between the cells and matrix we find that patterns of cell aggregation can spontaneously arise by an instability mechanism analogous to that which occurs in chemical pattern formation models