The
notochord is a flexible, rod-shaped body found in
embryoAn embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
s of all
chordateChordates are animals which are either vertebrates or one of several closely related invertebrates. They are united by having, for at least some period of their life cycle, a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail...
s. It is composed of
cellThe cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of an organism that is classified as living, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos...
s derived from the
mesodermIn humans, the mesoderm is one of the three primary germ cell layers - the other two are the ectoderm and endoderm - in the very early embryo. The mesoderm is the middle layer. It differentiates to gives rise to a number of tissues and structures including bone, muscle, connective tissue, and the...
and defines the primitive axis of the
embryoAn embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
. In some chordates, it persists throughout life as the main
axial supportThe 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 rib cage, and the vertebral column...
of the body, while in most vertebrates it is replaced by the
vertebral columnIn human anatomy, the vertebral column is a column usually consisting of 33 vertebrae, the sacrum, intervertebral discs, and the coccyx situated in the dorsal aspect of the torso, separated by spinal discs...
. The notochord is found
ventralStandard anatomical terms of location are employed in science which deal with the anatomy of animals to avoid ambiguities which might otherwise arise. They are not language-specific, and thus require no translation...
to the
neural tubeIn 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" serving as support structures in chordates such as
Haikouicthys. 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. 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. In
tetrapodTetrapods are vertebrate animals having four feet, legs or leglike appendages. Amphibians, reptiles, dinosaurs/birds, and mammals are all tetrapods, and even the limbless snakes are tetrapods by descent...
s, they eventually develop into the
nucleus pulposusNucleus 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 disc under compressive loads. The nucleus pulposus consists of chondrocytes, collagen fibrils, and...
of the intervertebral discs.
Development of the notochord
Notogenesis is the development of the notochord by the
epiblastIn 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
amnionThe amnion is a membrane building the amniotic sac that surrounds and protects an embryo. It is developed in reptiles, birds, and mammals, which are hence called “Amniota”; but not in amphibians and fish , which are consequently termed “Anamniota”. The primary function of this is the protection of...
cavity (
Human Embryology). The notochord arises as a pouch from the mesoderm.
The notochord forms during
gastrulationGastrulation is a phase early in the development of most animal embryos, during which the morphology of the embryo is dramatically restructured by cell migration. Gastrulation varies in different phyla...
and soon after induces the formation of the
neural plateIn 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....
(
neurulationNeurulation 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. The process begins when the notochord induces the formation of the central nervous system by signaling the...
), synchronizing the development of the
neural tubeIn 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
ventralStandard anatomical terms of location are employed in science which deal with the anatomy of animals to avoid ambiguities which might otherwise arise. They are not language-specific, and thus require no translation...
aspect of the neural groove an axial thickening of the endoderm takes place. (In bi-pedal chordates, e.g. humans, this surface is properly referred to as the
anteriorStandard anatomical terms of location are employed in science which deal with the anatomy of animals to avoid ambiguities which might otherwise arise. They are not language-specific, and thus require no translation...
surface). This thickening appears as a furrow (the chordal furrow) the margins of which anastomose (come into contact), and so convert it into a solid rod of polygonal-shaped cells (the notochord) which is then separated from the endoderm.
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 sellæ of the
sphenoid boneThe sphenoid bone is an unpaired 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, which grows medially and surrounds it. From the mesoderm surrounding the neural tube and notochord, the
skullThe skull is a bony 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
membraneThe meninges is the system of membranes 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
brainThe brain is the center of the nervous system in all vertebrate, and most invertebrate, animals. Some primitive animals such as jellyfish and starfish 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 pulposusNucleus 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 disc under compressive loads. The nucleus pulposus consists of chondrocytes, collagen fibrils, and...
of the intervertebral disks, but not in the
vertebral bodiesA vertebra is an individual bone in the flexible column that defines vertebrate animals, e.g. humans. 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 there, vertebra continue into...
, from which notochordal cells usually regress entirely. In humans, by the age of 4, all notochord residue is replaced by a population of
chondrocyteChondrocytes 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. 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 systemThe central nervous system is the part of the nervous system that functions to coordinate the activity of all parts of the bodies of bilaterian animals—that is, all animals more advanced than sponges or jellyfish. In vertebrates, the central nervous system is enclosed in the meninges. It contains...
. By transplanting and expressing a second notochord near the
dorsalStandard anatomical terms of location are employed in science which deal with the anatomy of animals to avoid ambiguities which might otherwise arise. They are not language-specific, and thus require no translation...
neural tube, 180
degreeA degree , usually denoted by ° , is a measurement of plane angle, representing 1⁄360 of a full rotation; one degree is equivalent to π/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
cellsThe cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of an organism that is classified as living, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos...
.
The notochord secretes a protein called
sonic hedgehog homologSonic hedgehog homolog is one of three proteins in the mammalian signaling pathway family called hedgehog, the others being desert hedgehog and Indian hedgehog . SHH is the best studied ligand of the hedgehog signaling pathway. It plays a key role in regulating vertebrate organogenesis, such as...
(SHH), a key
morphogenA morphogen is a hypothetical substance governing the pattern of tissue development and, in particular, the positions of the various specialized cell types within a tissue...
regulating
organogenesisIn animal development, 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...
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.
The structure of the notochord
The notochord is composed primarily of a core of glycoproteins, encased in a sheath of collagen fibers wound into two opposing
helicesA helix is a special kind of space curve, i.e. a smooth curve in three-space. As a mental image of a helix one may take the spring...
. The angle between these fibers determines whether increases pressure in the core will result in shortening and thickening versus lengthening and thinning.
Organisms with a notochord
- Amphioxus
- Hagfish
Hagfish are marine craniates of the class Myxini, also known as Hyperotreti. Myxini is the only class in the clade Craniata that does not also belong to the subphylum Vertebrata...
- Lamprey
A lamprey is a parasitic marine animal with a toothed, funnel-like sucking mouth. Translated directly, their name means stone lickers . While lampreys are well known for those species which bore into the flesh of other fish to suck their blood, these species make up the minority...
- Sturgeon
Sturgeon is the common name used for some 26 species of fish in the family Acipenseridae, including the genera Acipenser, Huso, Scaphirhynchus and Pseudoscaphirhynchus. The term includes over 20 species commonly referred to as sturgeon and several closely related species that have distinct common...
- African lungfish
The African lungfishes are the genus Protopterus and constitute the four species of lungfish found in Africa. Protopterus is the sole genus in the family Protopteridae.-Description:...
- Tadpole
A tadpole, polliwog , or pollywiggle is the wholly aquatic larval stage in the life cycle of an amphibian, particularly of a frog or toad.- Appellation :The name "tadpole" is from Middle English taddepol, made up of the elements tadde, "toad", and...
s