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Notochord

 

 
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Notochord
 
 
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The notochord is a flexible, rod-shaped body found in embryo
Embryo

An embryo is a multicellular organism ploidy eukaryote in its earliest stage of development, from the time of first cell division until birth, Egg , or germination....
s of all chordate
Chordate

Chordates are a group of animals that includes the vertebrates, together with several closely related invertebrates. They are united by having, at some time in their life cycle, a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail....
s. It is composed of cell
Cell (biology)

The cell is the structural and functional unit of all known Life organisms. It is the smallest unit of an organism that is classified as living, and is often called the building bricks of life....
s derived from the mesoderm
Mesoderm

One of the three germ layers found in the embryos of animals more complex than cnidarians, making them triploblastic. Mesoderm forms in the embryo during gastrulation when some of the cells migrating inward to form the endoderm, produce an additional layer that lies between the endoderm and the ectoderm....
 and defines the primitive axis of the embryo
Embryo

An embryo is a multicellular organism ploidy eukaryote in its earliest stage of development, from the time of first cell division until birth, Egg , or germination....
. In lower vertebrate
Vertebrate

Vertebrates are members of the subphylum Vertebrata, chordates with Vertebras or Vertebral columns. The grouping sometimes includes the hagfish, which have no vertebrae, but are genetically quite closely related to lampreys, which do have vertebrae....
s, it persists throughout life as the main axial support
Axial skeleton

The axial skeleton consists of the 80 bones in the head and trunk of the human body. It is composed of five parts; the human skull, the ossicles of the inner ear, the hyoid bone of the throat, the chest, and the vertebral column....
 of the body, while in higher vertebrates it is replaced by the vertebral column
Vertebral column

In human anatomy, the vertebral column is a column of 24 vertebrae, the sacrum, intervertebral discs, and the coccyx situated in the dorsum aspect of the torso, separated by spinal discs....
. The notochord is found on the ventral
Anatomical terms of location

Standard anatomical terms of location are employed in sciences dealing with the anatomy of animals to avoid ambiguities which might otherwise arise....
 surface of the neural tube
Neural tube

In the developing vertebrate, the neural tube is the embryo's precursor to the central nervous system, which comprises the brain and spinal cord....
.

Notochords were the first "backbones", as well, serving as support structures in chordates that lacked a bony skeleton.






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The notochord is a flexible, rod-shaped body found in embryo
Embryo

An embryo is a multicellular organism ploidy eukaryote in its earliest stage of development, from the time of first cell division until birth, Egg , or germination....
s of all chordate
Chordate

Chordates are a group of animals that includes the vertebrates, together with several closely related invertebrates. They are united by having, at some time in their life cycle, a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail....
s. It is composed of cell
Cell (biology)

The cell is the structural and functional unit of all known Life organisms. It is the smallest unit of an organism that is classified as living, and is often called the building bricks of life....
s derived from the mesoderm
Mesoderm

One of the three germ layers found in the embryos of animals more complex than cnidarians, making them triploblastic. Mesoderm forms in the embryo during gastrulation when some of the cells migrating inward to form the endoderm, produce an additional layer that lies between the endoderm and the ectoderm....
 and defines the primitive axis of the embryo
Embryo

An embryo is a multicellular organism ploidy eukaryote in its earliest stage of development, from the time of first cell division until birth, Egg , or germination....
. In lower vertebrate
Vertebrate

Vertebrates are members of the subphylum Vertebrata, chordates with Vertebras or Vertebral columns. The grouping sometimes includes the hagfish, which have no vertebrae, but are genetically quite closely related to lampreys, which do have vertebrae....
s, it persists throughout life as the main axial support
Axial skeleton

The axial skeleton consists of the 80 bones in the head and trunk of the human body. It is composed of five parts; the human skull, the ossicles of the inner ear, the hyoid bone of the throat, the chest, and the vertebral column....
 of the body, while in higher vertebrates it is replaced by the vertebral column
Vertebral column

In human anatomy, the vertebral column is a column of 24 vertebrae, the sacrum, intervertebral discs, and the coccyx situated in the dorsum aspect of the torso, separated by spinal discs....
. The notochord is found on the ventral
Anatomical terms of location

Standard anatomical terms of location are employed in sciences dealing with the anatomy of animals to avoid ambiguities which might otherwise arise....
 surface of the neural tube
Neural tube

In the developing vertebrate, the neural tube is the embryo's precursor to the central nervous system, which comprises the brain and spinal cord....
.

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
Nucleus pulposus

Nucleus pulposus is the jelly-like substance in the middle of the spinal disc. It is the remnant of the notochord. It functions to distribute hydraulic pressure in all directions within each Annulus fibrosus disci intervertebralis under compressive loads....
 of the intervertebral discs.

Development of the notochord

Notogenesis is the development of the notochord by the epiblast
Epiblast

In embryology, the epiblast is a tissue type derived from the inner cell mass. It lies above the hypoblast.In mammalian embryogenesis, the columnar cells of the epiblast are adjacent to the trophoblast, while the cuboidal cells of the hypoblast are closer to the Blastocoele....
s that make up the floor of the amnion
Amnion

The amnion is a membranous sac that surrounds and protects an embryo. It is developed in reptiles, birds, and mammals, which are hence called ?Amniote?; but not in amphibians and fish, which are consequently termed ?Anamniota?....
 cavity (). The notochord arises as a pouch from the mesoderm.

The notochord forms during gastrulation
Gastrulation

Gastrulation is a phase early in the development of animal embryos, during which the morphology of the embryo is dramatically restructured by cell migration....
 and soon after induces the formation of the neural plate
Neural plate

In human embryology, formation of neural plate is the first step of neurulation. It is created by a flat thickening opposite to the primitive streak of the ectoderm....
 (neurulation
Neurulation

Neurulation is a part of organogenesis in vertebrate embryos. Steps of neurulation include the formation of the dorsal nerve cord, and the eventual formation of the central nervous system....
), synchronizing the development of the neural tube
Neural tube

In the developing vertebrate, the neural tube is the embryo's precursor to the central nervous system, which comprises the brain and spinal cord....
. On the ventral
Anatomical terms of location

Standard anatomical terms of location are employed in sciences dealing with the anatomy of animals to avoid ambiguities which might otherwise arise....
 aspect of the neural groove an axial thickening of the endoderm
Germ layer

A germ layer is a group of cell s, formed during animal embryogenesis. Germ layers are particularly pronounced in the vertebrates; however, all animals more complex than sea sponge produce two or three primary tissue layers ....
 takes place. (In bi-pedal chordates, e.g. humans, this surface is properly referred to as the anterior
Anatomical terms of location

Standard anatomical terms of location are employed in sciences dealing with the anatomy of animals to avoid ambiguities which might otherwise arise....
 surface). This thickening appears as a furrow (the chordal furrow) the margins of which anastimose (come into contact), and so convert it into a solid rod of polygonal-shaped cells (the notochord) which is then separated from the endoderm.

In higher vertebrates, it extends throughout the entire length of the future vertebral column, and reaches as far as the anterior end of the midbrain, where it ends in a hook-like extremity in the region of the future dorsum sellę of the sphenoid bone
Sphenoid bone

The sphenoid bone is a bone situated at the base of the skull in front of the temporal bone and basilar part of the occipital bone.The sphenoid bone is one of the seven bones that articulate to form the orbit ....
. Initially it exists between the neural tube and the endoderm of the yolk-sac, but soon becomes separated from them by the mesoderm
Germ layer

A germ layer is a group of cell s, formed during animal embryogenesis. Germ layers are particularly pronounced in the vertebrates; however, all animals more complex than sea sponge produce two or three primary tissue layers ....
, which grows medially and surrounds it. From the mesoderm surrounding the neural tube and notochord, the skull
Skull

The skull is a bone structure found in the head of many animals. The skull supports the structures of the face and protects the head against injury....
, vertebral column, and the membrane
Meninges

The meninges is the system of Mesotheliums which envelops the central nervous system. The meninges consist of three layers: the dura mater, the arachnoid mater, and the pia mater....
s of the brain
Brain

The brain is the center of the nervous system in all vertebrate, and most invertebrate, animals. Some primitive animals such as cnidarian and echinoderm have a decentralized nervous system without a brain, while sponges lack any nervous system at all....
 and medulla spinalis are developed.

Postembryonic vestige of the notochord is found in the nucleus pulposus
Nucleus pulposus

Nucleus pulposus is the jelly-like substance in the middle of the spinal disc. It is the remnant of the notochord. It functions to distribute hydraulic pressure in all directions within each Annulus fibrosus disci intervertebralis under compressive loads....
 of the intervertebral disks, but not in the vertebral bodies
Vertebra

A vertebra is an individual bone in the flexible column that defines vertebrate animals. The vertebral column encases and protects the spinal cord, which runs from the base of the cranium down the dorsal side of the animal until reaching the pelvis....
, from which notochordal cells usually regress entirely. In humans, by the age of 4, all notochord residue is replaced by a population of chondrocyte
Chondrocyte

Chondrocytes are the only cells found in cartilage. They produce and maintain the cartilaginous matrix, which consists mainly of collagen and proteoglycans....
-like cells of unclear origin. Persistence of notochordal cells within the vertebra may cause a pathologic condition- persistent notochordal canal
Congenital vertebral anomaly

Congenital vertebral anomalies are a collection of malformations of the vertebral column in animals. Most are not clinically significant, but they can cause compression of the spinal cord by deforming the vertebral canal or causing instability....
. They are also found to persist in the nasopharyngeal space and, in such an unusual instance, may give rise to Tornwaldt's cyst.

The notochord in neural development

Research into the notochord has played a key role in understanding the development of the central nervous system
Central nervous system

The central nervous system is the part of the nervous system that functions to coordinate the activity of all parts of the bodies of multicellular organisms....
. By transplanting and expressing a second notochord near the dorsal
Anatomical terms of location

Standard anatomical terms of location are employed in sciences dealing with the anatomy of animals to avoid ambiguities which might otherwise arise....
 neural tube, 180 degree
Degree (angle)

A degree , usually denoted by ? , is a measurement of plane angle, representing 1/360 of a Turn ; one degree is equivalent to p/180 radians....
s 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
Cell (biology)

The cell is the structural and functional unit of all known Life organisms. It is the smallest unit of an organism that is classified as living, and is often called the building bricks of life....
.

The notochord secretes a protein called sonic hedgehog homolog
Sonic hedgehog

Sonic hedgehog homolog is one of three proteins in the mammalian Hedgehog family, the others being DHH and Indian Hedgehog . SHH is the best studied ligand of the hedgehog ....
 (SHH), a key morphogen
Morphogen

A morphogen is a substance governing the pattern of tissue development and, in particular, the positions of the various specialized cell types within a tissue....
 regulating organogenesis
Organogenesis

In embryogenesis, organogenesis is the process by which the ectoderm, endoderm, and mesoderm develop into the internal organs of the organism. Internal organs initiate development in humans within the 3rd to 8th weeks in utero.The germ layers in organogenesis differ by three processes: folds, splits, and condensation....
 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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