They discovered, to their surprise, that rather than being shaped by processes within the skeleton, bone-ridge formation was directly regulated by tendons and muscles in a two-phase procedure. First, the embryonic tendons initiated bone-ridge formation by attaching to the skeleton. This interaction induced the tendon cells to express a specific protein called scleraxis, which in turn, led to the production of another protein, BMP4 - a molecule involved in the onset of bone formation. Blocking BMP4 production in tendon cells prevented deltoid tuberosity bone ridge formation. In the second phase, the subsequent growth and ultimate size of the deltoid tuberosity was directly regulated by muscle activity.

The results demonstrate that tendons play an active role in initiating bone ridge patterning. Zelzer: 'These findings provide a new perspective on the regulation of skeletogenesis in the context of the musculoskeletal system, and they shed light on an important mechanism that underlies the assembly of this system.'

Source: Weizmann Institute of Science