Definitions

notochord

notochord

[noh-tuh-kawrd]
notochord, in biology, supporting rod running most of the length of animals of the phylum Chordata and present at varying times in the life cycle. Composed of large cells packed within a firm connective tissue sheath, the notochord lies between the neural tube (spinal cord) and the gut. The division of the phylum Chordata into subphyla is based on the structure of the notochord and the time of life in which it is present: In the subphylum Urochordata (tunicates) the notochord characterizes the larval, swimming, stage of the animals and does not extend into the head; in the subphylum Cephalochordata (lancelets) the notochord extends to the extreme tip of the head in both young and adults; and in the subphylum Vertebrata the notochord becomes surrounded by skeletal vertebrae during embryonic development—in higher vertebrates it is present in the early embryo only and is later completely replaced by the vertebrae.
The notochord is a flexible, rod-shaped body found in embryos of all chordates. It is composed of cells derived from the mesoderm and defines the primitive axis of the embryo. In lower vertebrates, it persists throughout life as the main axial support of the body, while in higher vertebrates it is replaced by the vertebral column. The notochord is found on the ventral surface of the neural tube.

Notochords were the first "backbones", as well, serving as support structures in chordates that lacked a bony skeleton. The very first vertebrates, such as Haikouicthys, had only a notochord. Embryos of vertebrates have notochords today, as embryonic development often happens to follow a pattern similar to the ancestral evolution of the modern animal's traits. Notochords were advantageous to primitive fish-ancestors because they were a rigid structure for muscle attachment, yet flexible enough to allow more movement than, for example, the exoskeleton of the dominant animals of that time. In humans, they eventually develop into the nucleus pulposus of the intervertebral discs.

In the adult human, a notochord remnant is the apical ligament of the atlanto-axial joint.

Development of the notochord

Notogenesis is the development of the notochord by the epiblasts that make up the floor of the amnion cavity (Human Embryology). The notochord arises as a pouch from the mesoderm.

The notochord in neural development

Research into the notochord has played a key role in understanding the development of the central nervous system. By transplanting and expressing a second notochord near the dorsal neural tube, 180 degrees opposite of the normal notochord location, one can induce the formation of motoneurons in the dorsal tube. Motoneuron formation generally occurs in the ventral neural tube, while the dorsal tube generally forms sensory cells.

The notochord secretes a protein called sonic hedgehog homolog (SHH), a key morphogen regulating organogenesis and having a critical role in signaling the development of motoneurons. The secretion of SHH by the notochord establishes the ventral pole of the dorsal-ventral axis in the developing embryo.

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