Joints form during embryonic development in conjunction with the formation and growth of the associated bones. The embryonic tissue that gives rise to all bones, cartilage, and connective tissues of the body is called mesenchyme.
The mesenchymal stem cells differentiate into chondrocytes that form the hyaline cartilage, and later the cartilaginous model of the bone. This model further transforms into a bone. This process is known as endochondral ossification.
During development, the limbs initially develop as small limb buds that appear on the sides of the embryo around the end of the fourth week. Starting during the sixth week, as each limb bud continues to grow and elongate, areas of mesenchyme within the bud begin to differentiate into the hyaline cartilage that will form models for each of the future bones. The synovial joints will form between the adjacent cartilage models, in an area called the joint interzone. Cells at the center of this interzone region undergo cell death to form the joint cavity, while surrounding mesenchymal cells will form the articular capsule and supporting ligaments. The process of endochondral ossification, which converts the cartilage models into bone, begins by the twelfth week of embryonic development. At birth, ossification of much of the bone has occurred, but the hyaline cartilage of the epiphyseal plate will remain throughout childhood and adolescence to allow for bone lengthening. Hyaline cartilage is also retained as the articular cartilage that covers the surfaces of the bones at synovial joints.
This text is partially adapted from Openstax, Anatomy and Physiology 2e, Section 9.7: Development of Joints
