Notochord

Lamprey notochord and vacuolated core cells

The notochord, a slender elastic-like rod, is one of four biological features that draw together a wide range of animals into a single grouping named Chordata.  Along with the notochord, the dorsal nerve cord, pharyngeal slits and postanal tail are characteristics shared by an assemblage that includes both humans and the sac-like sea squirt.  Vertebrates make up the vast majority of the animals represented but the larval form of the marine sea squirt gives them admission to this distinguished club, as well. 

45 hour old chick embryo with notochord

The notochord is a hydrostatic organ with a tough outer wall enclosing a fluid core.  This gives it lateral elasticity while enabling it to resist any axial compression.  Anchored to this rod, that extends nearly the length of the organism, is a series of segmented muscles used, in most instances, to give the animal the means of propulsion through the water.  The contraction of muscles on one side and then to the next provides alternating lateral pressure against the surrounding substrate.  The resulting undulating motion propels the animal forward.  Once the muscles relax after contracting on one side of the body the springy notochord acts to straighten the body out.  The notochord acts as the antagonist against the muscles’ action, enhancing the sweeping of the tail from side to side. 

Lamprey notochord extending beneath brain

The hydrostatic nature of the notochord prevents the compression of the animal’s axis which would severely hinder its ability to swim.  This pressure is provided by fluid residing between the notochord’s core cells or by core cells swollen with vacuoles containing fluid.  These vacuolated cells are wrapped tight within a sheath of tough, fibrous tissue.  Under these conditions the inner fluid is held fixed, unable to flow. 

Zebrafish embryo with notochord, segmented muscles

The notochord may persist in more primitive chordates but in the case of bony fish and terrestrial vertebrates this rod is replaced by the vertebral column.  In these instances the notochord appears as a structure used as a scaffold around which the embryonic body can grow.  It makes its appearance early when the mesodermal layers at the dorsal midline differentiates into the chordamesoderm tissue.  This gives rise to the notochord as well as further stimulating the differentiation of the overlying ectoderm into producing the central nervous system.   It is consequently above the body’s main central cavity, or coelom, and beneath the dorsal nerve cord.

Human vertebrae with notochord derived discs

The notochord does not necessarily disappear.  In adult mammals it has transformed into a series of intervertebral disks.  These form circular pads that lie between the successive vertebrae.  Each pad is a fibrocartilage tissue that encloses a gel-like core, called the nucleus pulposus, providing a cushion between the connected bony vertebrae.  If you’ve ever suffered a slipped, ruptured or crushed disc you know how important these structures can be to your general well-being and a healthy frame of mind.