3D representation of the developing chick knee joint: a novel approach integrating multiple components.

The knee joint has a highly complex 3-dimensional (3D) morphology that is sculpted at the interface of the forming long bones as they are generated in the embryo. Although it is clear that regulatory genes guide joint formation, the mechanisms that are responsible for morphogenesis of the knee are poorly understood. Certainly the process involves integration across several tissues and physical/mechanical influences from neighbouring tissues are important. We describe the acquisition of shape in the chick knee joint in detail and show that by HH34 the joint already displays shape characteristics of the adult structure. Through imaging developing cartilage, tendons, ligaments and muscle across developmental stages from HH28-34 we have built 3D representations of the forming structure including the various components important in knee formation. We describe the timing of muscle and tendon development in parallel with the refinement of cartilage shape, showing when and where (tendon attachment points) muscle forces are applied to the cartilage elements. Shape begins to emerge as the tendons are forming (HH30-32) but is fully refined (HH34) in the presence of tendons. The resulting integrated 3D representations of the developing knee across time will serve as the foundation for computational analysis of the mechanical environment, and experimental approaches to investigating morphogenetic mechanisms.