1) The notochord is the defining structure of the phylum Chordata, which includes all vertebrates. In its "mature" state, the notochord is a rod of large cells constrained by a thick extracellular sheath and positioned between the developing spinal cord and gut. Turgor pressure generated by notochord cells exerted against the sheath gives the notochord stiffness, imparting the mechanical properties required for one of its main functions as the central axial skeletal element of the developing embryo.
2) The notochord exists transiently during the life of most vertebrates. During its short life, however, the notochord plays another critical role for developing embryos,.producing a variety of secreted growth factors, such as Sonic hedgehog, which instruct surrounding tissues to acquire particular differentiated fates. A number of embryological experiments, for example, tell us that, without notochord-derived signals, motorneurons in the spinal cord would not form, the pancreas insulin-producing cell would not differentiate, and the body muscles would not be produced properly. Without the axial skeletal support of the notochord, many lower vertebrate larvae, such as fish fry and tadpoles, become severely shortened and cannot swim or feed properly. Thus many of the mutations affecting zebrafish notochord development are named after the seven dwarves.
