Skeleton

Connective tissue and the skeleton differentiate from the parietal mesoderm of the lateral plate. At S16-17, precartilaginous blastemas (mesenchyme condensations) form along a proximodistal gradient of maturation. From S17-18 onward, these become cartilaginous (cartilaginous models of the future bones). The cartilaginous models are marked off by homogeneous mesenchymal interzones. The interzones develop further to form a three-layered structure. A loose intermediate layer forms between two cartilage-producing layers. The mesenchyme condenses outwith the future joint to form a fibrous capsule. Liquid-filled cavities form at the margin at the border and in the central part of the three-layered interzone. These cavities coalesce to form a homogeneous joint cavity. The cells lining the joint surfaces and the capsule form a flat layer, called the synovial membrane. The synovial membrane disappears from the joint surfaces later on in development. The capsule and the ligaments arise from the mesenchyme surrounding the joint.