The notochord represents the earliest fetal axial skeleton, extending from the Rathke pouch to the coccyx. It is a primitive cell line from which the skull base and vertebral column develop. The notochord is cylindrical and is replaced by sclerotomes that produce cartilage, and subsequently bone.
The nucleus pulposus of the intervertebral discs represents residual notochord. In non-human species, cells of the notochord alongside the mucoid nucleus pulposus continue to exist after adulthood. By contrast, human notochord cells are replaced by the age of four by chondrocyte-like ones, whose origin is unspecified. Because these cells synthesise proteoglycans and produce large quantities of collagen, the nucleus pulposus is dehydrated, opacified and rendered firmer 3.
Remnants of the notochord usually remain near the midline and are entrapped in bone. Chordomas are a malignant tumour that can develop from the notochord remnants.
Vestigial non-neoplastic notochordal remnants are seen in up to 2% of cadavers, usually in the midline in the following locations:
sacrococcygeal regions
smaller scattered foci in the spine (especially C2 and lumbar spine)
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