Stages in embryonic IVD formation.
A. E8.5-E10.5: The node (N) is formed under the paraxial mesoderm (PM) and enlongates to form the notochord (NC).
B. E10.5-E11.5: The mesenchyme around the notochord forms a continuous perichordial tube (PT).
C. E12.5-E15.5: Somitic mesenchymal condensates differentiate to form the vertebrae, leading to segmentation of the notochord and formation of the perichordal disc. The vertebral body (VB) and annulus fibrosus (AF) originate from the somites, while the nucleus pulposus (NP) originates from the notochord.
D. E16.5-E17.5: Segments of the differentiated notochord are incorporated in the intervertebral disc (IVD).