Your spinal discs hide the remains of a rod every chordate shares
Long before you had a backbone, you had a notochord: a springy rod running head to tail through the embryo. It is the defining feature of the whole chordate group. In humans it mostly vanishes as vertebrae form, but its descendants live on as the jelly-like core of every spinal disc.
The notochord forms from embryonic mesoderm during gastrulation. It is built like a pressurised hose: a core of swollen, vacuole-filled cells packed with glycoproteins, wrapped in two collagen and elastin sheaths wound in opposite helices. The angle of those fibres decides whether extra pressure makes the rod shorter and fatter or longer and thinner. It sits along the body's midline, beneath the nerve cord and above the gut.
Its biggest job is as a signalling centre. The notochord releases a protein called sonic hedgehog, which marks the belly side of the embryo and tells nearby cells to become motor neurons. Experiments proved the point dramatically: transplanting a second notochord beside the upper side of the neural tube, the opposite of its usual position, makes motor neurons appear where sensory cells would normally form. In fish and amphibians it also triggers a temporary structure needed to build the dorsal aorta.
Chordates treat the rod very differently. Lancelets keep it for life as their main skeleton, and in amphioxus it pokes out beyond the front of the nerve tube, helping the animal burrow into the sediment where it filter-feeds. Tunicates have one only as larvae. Sturgeon and paddlefish hold on to it after the embryo stage, and in every vertebrate except the hagfish it gives way to bone or cartilage. In humans, remnants are replaced by cartilage-like cells by age 4, though leftover notochord cells are the probable source of a rare cancer, chordoma.
The rod is ancient. Pikaia seems to have had an early version, and Cambrian animals such as Haikouichthys carried true notochords. How it first evolved is unsettled: a 2015 review found two ideas revived by molecular evidence, one that chordates invented it from scratch by rewiring developmental genes, the other that it descends from a muscle strip, the axochord, in worm-like ancestors.
Source: Notochord
