Figure 2.

A and B, Embryonic development of the human intervertebral disc and articular joint. A, Starting in week 5, the intervertebral discs are formed by the notochord and sclerotome. The sclerotome (on the outside) forms the outer annulus and surrounds the notochord (inside), which will eventually partition into the nuclei pulposi. A transition zone between notochord and sclerotome characterizes the inner annulus fibrosus. B, In week 6, the limb bud is formed by mesoderm covered by ectoderm and starts to grow outwards in order to form arms and legs. The formation of articular joints occurs later in embryonic development (ie, in week 8), and is characterized by the interzone (light blue), which is a group of mesenchymal cells that form an interspace