Sesamoid bones are small, flat auxiliary bones generally found embedded within tendons close to joints. Classically, they are thought to arise from within the tendon through a programme dependent on mechanical forces. This model was challenged by Elazar Zelzer and colleagues, who showed that the mouse patella – the largest sesamoid bone – develops from a group of chondroprogenitors adjacent to the femur. The developing patellar structure then detaches from the femur in a mechanical load-dependent fashion. It was, however, unclear whether this mechanism was specific to the patella or reflected a more general developmental programme. Now, Zelzer and co-workers analyse two other sesamoid bones – the lateral fabella of the back of the knee, and the metacarpophalangeal sesamoid in the digit – to gain further insight into how sesamoid bones form. They find that, like the patella, these two sesamoids originate from Sox9/Scx-positive chondroprogenitors, but that, whereas the metacarpophalangeal sesamoid forms in juxtaposition to the bone, the lateral fabella does not. Further, the metacarpophalangeal sesamoid detaches from its parent bone by a different (mechanical load-independent) mechanism from the patella – perhaps reflecting the different kind of joint (cartilaginous versus synovial) at which these bones are found. These data demonstrate a common progenitor type but significant diversity in the mechanisms underlying sesamoid bone development.
