"Thornwaldt cysts"
"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 years 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."