What is the difference between a vertebrate and a notochord

Unlike vertebrates, urochordates and cephalochordates never develop a bony backbone. Members of Urochordata are also known as tunicates. The name tunicate derives from the cellulose-like carbohydrate material, called the tunic, which covers the outer body of tunicates. Although tunicates are classified as chordates, only the larval form possesses all four common structures. Adults only maintain pharyngeal slits and lack a notochord, a dorsal hollow nerve cord, and a post-anal tail.

Urochordates : a This photograph shows a colony of the tunicate Botrylloides violaceus. Most tunicates are hermaphrodites. After hatching, a tunicate larva swims for a few days until it finds a suitable surface on which it can attach, usually in a dark or shaded location. It then attaches via the head to the surface and undergoes metamorphosis into the adult form, at which point the notochord, nerve cord, and tail disappear.

Most tunicates live a sessile existence on the ocean floor and are suspension feeders. The primary foods of tunicates are plankton and detritus. Suspended material is filtered out of this water by a mucous net pharyngeal slits and is passed into the intestine via the action of cilia. The anus empties into the excurrent siphon, which expels wastes and water.

Tunicates are found in shallow ocean waters around the world. Members of Cephalochordata possess a notochord, dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail in the adult stage. Extinct members of this subphylum include Pikaia , which is the oldest known cephalochordate.

Pikaia fossils were recovered from the Burgess shales of Canada and dated to the middle of the Cambrian age, making them more than million years old. Extant members of Cephalochordata are the lancelets, named for their blade-like shape. Lancelets are only a few centimeters long and are usually found buried in sand at the bottom of warm temperate and tropical seas. Like tunicates, they are suspension feeders. With notochord and paired muscle blocks, the lancelet and Pikaia may belong to the chordate group of animals from which the vertebrates have descended.

Cephalochrodates : The lancelet, like all cephalochordates, has a head. Adult lancelets retain the four key features of chordates: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. Water from the mouth enters the pharyngeal slits, which filter out food particles. The filtered water then collects in the atrium and exits through the atriopore. Both genomic and fossil evidence suggests that vertebrates evolved from craniates, which evolved from invertebrate chordates.

The clade Craniata is a subdivision of Chordata. Members of Craniata posses a cranium, which is a bony, cartilaginous, or fibrous structure surrounding the brain, jaw, and facial bones. The clade Craniata includes all vertebrates and the hagfishes Myxini , which have a cranium but lack a backbone. Hagfish are the only known living animals that have a skull, but not a vertebral column. Hagfish : Although it lacks a backbone, the hagfish is a member of the Craniata clade because it possesses a bony skull.

Clade Craniata : Craniata, including this fish Dunkleosteus , are characterized by the presence of a cranium, mandible, and other facial bones. Vertebrates are members of the subphylum Vertebrata, the clade Craniata, and the phylum Chordata. Vertebrates display the four characteristic features of chordates, but they are named for the vertebral column composed of a series of bony vertebrae joined together as a backbone.

In adult vertebrates, the vertebral column replaces the embryonic notochord. Vertebrates : Vertebrata are characterized by the presence of a backbone, such as the one that runs through the middle of this fish. All vertebrates are in the Craniata clade and have a cranium.

In the phylum Chordata, the closest relatives of the vertebrates are the invertebrate chordates. Based on the molecular analysis of vertebrate and invertebrate genomes genomics , scientists can determine the evolutionary history of different phylogenetic groups.

According to these genomic analyses, vertebrates appear to be more closely related to the lancelets cephalochordates than to the tunicates urochordates. This suggests that the cephalochordates first diverged from urochordates, and that vertebrates subsequently diverged from the cephalochordates.

This hypothesis is further supported by the fossil of a million-year-old organism with a brain and eyes like a vertebrate, but without the skull found in a craniate. A comparison of the genomes of a lancelet, tunicate, lamprey, fish, chicken, and human confirmed that two whole-genome duplications occurred in the early history of the Vertebrata subphylum.

Both fossil and genomic evidence suggests that vertebrates arose during the Cambrian explosion. The upper 24 vertebrae are separated by intervertebral disks while the lower 9 vertebrae are fused with each other in adults, forming the sacrum and coccyx.

The first seven articulating vertebrae are cervical vertebrae while the next 12 vertebrae are thoracic vertebrae. Moreover, we call the final 5 articulating vertebrae lumbar vertebrae.

In humans, the vertebral column is responsible for transmitting body weight while walking and standing. Notochord refers to a cartilaginous, skeletal rod, supporting the body in all embryonic and some adult chordate animals while vertebral column refers to the flexible column, extending from the neck to tail, made of a series of bones. Thus, this is the main difference between notochord and vertebral column. Furthermore, notochord occurs in all chordates during the embryonic stage and in adult lower chordates while vertebral column only occurs in the adult higher chordates.

Hence, this is an important difference between notochord and vertebral column. Also, another difference between notochord and vertebral column is that the notochord is made up of cartilages while the vertebral column is made up of bones.

Moreover, notochord occurs between the dorsal nerve cord and the gut while vertebral column occurs surrounding the nerve cord. The notochord is a rod-like supportive structure present in the dorsal part of the body. In fact, it is one of the main distinguishing features of chordates.

However, lower chordates have it throughout their lifetime while it develops into the vertebral column in higher chordates known as vertebrates after the embryonic stage. In other species, homologous structures have been found: the embryonic shield of teleost fish, Hensen's node in the chick and the node of mouse embryos all possess essentially the same activities as Spemann and Mangold's dorsal organiser Beddington, ; Oppenheimer, ; Waddington, The functions and activities of the dorsal organiser are complex and have been discussed in detail elsewhere Harland and Gerhart, For this discussion, it is useful to consider the relationship between specification of notochord fate and dorsal organiser activities.

First,however, what are the distinct morphological stages that the dorsal mesendoderm progresses through on its way to becoming a mature notochord? The first major transition is from dorsal organiser to chordamesoderm. During early gastrula stages, the chordamesoderm, which is the direct antecedent of the notochord, becomes morphologically and molecularly distinct from other mesoderm.

Cellular rearrangements involving the mediolateral intercalation and convergence of cells towards the dorsal midline, force the chordamesoderm into an elongated stack of cells. Genetic screens in zebrafish have identified two loci, floating head flh and bozozok dharma — Zebrafish Information Network , as being essential for this transition to occur Amacher and Kimmel, ; Fekany et al.

As development proceeds, chordamesoderm cells acquire a thick extracellular sheath and a vacuole. Osmotic pressure within the vacuole acts against the sheath, gives the notochord its characteristic rod-like appearance, and provides mechanical properties that are essential for the proper elongation of embryos and for the locomotion of invertebrate chordates and many vertebrate species Adams et al.

This transition, from chordamesoderm to mature notochord, requires a host of loci that have been identified in zebrafish genetic screens Odenthal et al. A comparison of notochords. A At 24 hours post fertilisation hpf , the zebrafish notochord red occupies a considerable volume of the embryo.

B This is especially apparent in cross-section, where the area of the notochord is nearly equal to that of the neural tube. C By contrast, in an embryonic day E 9 mouse embryo, the notochord is proportionally smaller, D taking up a cross-sectional area of only a fraction of the neural tube.

A Lateral view of live 24 hpf zebrafish embryo. B Trunk-level cross section of a 24 hpf zebrafish embryo. C Three dimensional reconstruction of an E9 mouse. White line indicates level of section shown in D, with notochord highlighted in red. Studies of several early-acting zebrafish genes have been helpful in investigating the relationship between dorsal organiser activity and specification of notochord fate. It is clear that organiser activity, per se,is separable from the specification of notochord fate.

For example, bozozok mutant embryos lack a morphologically distinct shield, and both bozozok and floating head mutant embryos fail to form a notochord Fekany et al. Yet in both types of mutant embryo, the embryonic axes form and embryos possess clear DV and anterior-posterior AP identities Fekany et al.

Similarly, when organiser tissue is surgically ablated, the resulting embryos develop with clearly defined axes, but lack notochord and prechordal mesoderm,despite the fact that the ablated tissue can induce the formation of a complete second axis in another host Saude et al. Expression of flh mRNA is a good prospective marker of notochord fate Gritsman et al.

In early gastrula zebrafish embryos, flh is expressed superficially within the organiser region. Simultaneously, another homeodomain-encoding gene, goosecoid gsc is expressed in deep organiser tissues. The distinct activities of deep versus superficial organiser have been measured in transplantation assays.

In these experiments, deep tissue,which expresses gsc, was found to induce secondary axes that possess only head, whereas superficial flh -expressing tissue was found predominantly to produce secondary axes that possess only trunk and tail Fig.

The specification of notochord and prechordal fates are, however, controlled by the earlier acting processes — dorsal specification and mesoderm induction — that are also responsible for specifying dorsal organiser activities De Robertis and Kuroda, ; Kimelman and Griffin, ; Sokol, One key event during chordate early development is the induction of mesoderm.

Defined today in terms of characteristically expressed genes that are known to correlate with future mesodermal fate, we now know many of the molecules involved in mesoderm induction, largely owing to classical embryological experiments carried out by Pieter Nieuwkoop Boterenbrood and Nieuwkoop, ; Gerhart, ; Kimelman and Griffin, Using frog embryos, Nieuwkoop found that, when left on its own, the animal pole region, the animal cap, of a frog embryo would form only a ciliated epidermis; the yolky vegetal cells would form only endoderm.

In combination, however, Nieuwkoop found that the vegetal-derived factors could convert animal cap cells to a mesoderm fate. In the past 15 years, this assay has led to the discovery of the signalling pathway that underlies mesoderm induction Green and Smith, ; Schier and Shen, ; Smith et al.

We now know that Nodal and Nodal-related proteins are secreted and serve to induce mesoderm formation. Importantly, the response of animal cap cells to Nodal is graded so that different levels of Nodal signalling lead to different mesodermal and axial mesendodermal fates.

High levels of Nodal signalling specify the deep gsc -expressing cell fates, while lower levels specify flh -expressing prospective chordamesoderm Gritsman et al. Initially characterised by zygotic loss-of-function, oep mutant embryos were found to be cyclopean but to possess a notochord Schier et al. The oep gene product, however, is also supplied maternally, and mutants lacking both zygotic and maternal Oep lack all endoderm and mesoderm, apart from a small amount of muscle that forms in the tail Gritsman et al.

The type of mesoderm formed in response to inductive signals is dose dependent. This was first appreciated in Xenopus animal cap experiments, where different concentrations of activin were found to elicit different mesodermal fates Green et al. More recently this dose dependence with respect to Nodal signalling was revealed when maternal-zygotic oep MZ oep mutant zebrafish embryos were compared with zygotic oep mutants Gritsman et al.

The major defect in zygotic oep mutants, in which low-level Nodal signalling still occurs, is the lack of gsc -expressing prospective prechordal mesendoderm, which is the tissue that requires the highest level of Nodal signalling for proper specification. Indeed, this tissue, which only transiently expresses gsc in oep mutants, instead turns on flh expression and becomes chordamesoderm. The maternal supply of Oep protein is likely to be depleted with time in these embryos, suggesting that one difference between prechordal and notochord specification is that the prechordal tissue requires persistent Nodal signalling, while transient Nodal signalling is sufficient for chordamesoderm specification.

Notochord specification is a stable property of the superficial dorsal organiser. This is most clearly indicated by the observation that when prospective chordamesoderm is transplanted anywhere into an equivalently staged host, the transplanted tissue will form notochord Saude et al. Separable organiser activities. A,B The zebrafish dorsal organiser is the embryonic shield between the arrowheads , and is shown in A lateral and B animal-pole views.

C Deep organiser tissue expresses gooscoid gsc , while D superficial tissue expresses floating head flh mRNA. E By separating deep grey and superficial red organiser tissues and grafting the individual pieces, superficial tissue was found to induce a secondary axis that possessed only trunk and tail F , while deep tissue induced a secondary axis that possessed only a head G.

F,G Embryos are stained for sonic hedgehog expression by in situ hybridisation. Arrows indicate F anterior limit of secondary axis and G posterior limit of secondary axis. The Nodal signalling pathway is required for specification of dorsal mesendodermal fates and for early mesoderm induction. It is not, however,required for dorsal specification or neural induction.

Although endoderm and virtually all mesoderm fails to form in zebrafish MZ oep mutants and in mutants simultaneously lacking the Nodal ligands Squint and Cyclops, these embryos nevertheless posses DV and AP axes Feldman et al. Consistent with its structural role in vertebrate development, the notochord shares many features with cartilage. It expresses many genes that are characteristic of cartilage, such as those that encode type II and type IX collagen, aggrecan, Sox9 and chondromodulin Dietz et al.

There is,however, one clear difference between chondrogenesis and notochord formation. Chondrocytes normally secrete a highly hyrdrated extracellular matrix, which gives cartilage its main structural properties Knudson and Knudson, By contrast, while notochord cells produce a thick basement membrane sheath,they retain hydrated materials in large vacuoles Adams et al.

These vacuoles allow notochord cells to exert pressure against the sheath walls,which gives the notochord its structural properties Adams et al. There may be a direct relationship between notochord and cartilage in which cartilage has evolved to secrete certain materials; by rerouting those materials to an internal vacuole, notochord cells have co-opted some of the essential structural properties of cartilage to their needs.

The ultimate fate of the notochord also emphasises the relatedness of notochord and cartilage. During endochondral bone formation, the type II collagen-rich extracellular matrix of cartilage is deposited with type X collagen, which signals the eventual replacement of cartilage by bone Linsenmayer et al.

Similarly, during the development of vertebrae, notochord that runs through the middle of each vertebra first expresses type X collagen and is then replaced by bone Linsenmayer et al. Between the vertebrae, the notochord does not express type X collagen and is not replaced by bone, but becomes the centre of the intervertebral disc — the nucleus pulposis Aszodi et al.

Thus, notochord can become ossified in a fashion similar to cartilage. Consistent with this view, in mutant mice that lack type II collagen, the notochord is not replaced by bone, presumably because the type II collagen network is required for proper deposition of type X collagen. By definition, the notochord arose with the chordates; however, there are some possible hints to its origin in hemichordates.

Hemichordates, along with echinoderms and chordates, constitute the three monophyletic phyla of the Deuterostomia Jefferies, ; Peterson et al. Morphologically, several features of the hemichordates suggest a close relationship with the chordates. For example, in the head of hemichordates, there is a structure called the stomochord, which bears some structural resemblance to the notochord. There is, however, little direct evidence of a notochord in hemichordates.

A more definitive answer is likely to come from an analysis of the expression of canonical notochord genes in hemichordates Lowe et al. An analysis of the expression of brachyury , a gene normally associated with notochord development, in the hemichordate Ptychodera flava revealed, however, that brachyury is never expressed in the stomochord Peterson et al. As such, an analysis of cartilage-specific genes could also be helpful in elucidating the relationship between these structures.

In contrast to hemichordates, the ascidians have definitive notochords. A wide variety of ascidian species have been studied by developmental biologists, and much attention has been given to the notochord of ascidian larvae Satoh, In one study, a comparison was drawn between two closely related ascidian species, Molgula oculata and Molgula occulta ; the first sports a conventional ascidian tail, the other is tailless Swalla and Jeffery, In hybrid crosses, the tail of the normally tailless species is restored and a gene called Manx , which encodes a zinc-finger protein, was identified by differential gene expression analysis between the two species.

The disruption of Manx expression by antisense oligonucleotides in the tail-bearing species leads to tail loss Swalla and Jeffery, ; Swalla et al. More recent screens have identified a host of notochord-specific genes and mutations that affect notochord development in ascidians Di Gregorio and Levine, ; Jeffery, ; Moody et al.

One process that is important to notochord development is convergent extension. A variety of studies in vertebrates have implicated the planar cell polarity PCP pathway in the control of convergence and extension during gastrulation. In zebrafish, for example, embryos in which PCP has been disrupted exhibit a widened chordamesoderm and a subsequent defect in tail notochord development Hammerschmidt et al.

A mutation that affects ascidian notochord development has been characterised by positional cloning and was found to encode a homologue of Prickle, which is a PCP component known to be involved in vertebrate convergent extension Carreira-Barbosa et al.

The notochord has several well-established roles in patterning surrounding tissues. As these issues have been comprehensively reviewed elsewhere Cleaver and Krieg, ; Dodd et al. Perhaps the best characterised is the role of the notochord in patterning the neural tube. A series of experiments involving both the transplantation and the removal of the notochord during development showed that the notochord can signal the formation of the floor plate, which is the ventral-most fate of the spinal cord Placzek et al.

Among the signals secreted by the notochord are the Hedgehog Hh proteins. Sonic Hedgehog, in particular, induces a range of ventral spinal cord fates in a graded fashion while simultaneously suppressing the expression of characteristically dorsal genes Placzek et al.

Reinforcing and maintaining earlier developmental events,notochord signals are also involved in establishing LR asymmetry; the ablation of the notochord in Xenopus gastrulae results in randomisation of asymmetry Danos and Yost, ; Lohr et al. In teleosts, notochord-derived Hh signals control the formation of the horizontal myoseptum, as well as specifying slow-twitch muscle fates Barresi et al.

Notochord-derived signals are important for specifying the formation of the dorsal aorta Cleaver et al. Finally, the notochord is important for the normal development of early endoderm and the pancreas Cleaver and Krieg, ; Kim et al. Although the patterning roles of the notochord are essential for normal vertebrate development, the notochord also has an essential structural role.

The notochord is the main axial skeletal element of the chordate early embryo;without a fully differentiated notochord embryos fail to elongate Odenthal et al. For many species, this results in the inability to swim properly, to escape predation and to feed.

Some understanding of this function of the notochord has been derived from studies of zebrafish mutations Driever et al. A number of loci involved in notochord formation were identified in several large-scale systematic screens for mutations affecting zebrafish embryogenesis Fig. As I have discussed, a few of the loci identified in these screens have been found to be important for the formation of the chordamesoderm, highlighting important signalling roles of the notochord.

Most loci, however, have been found to be profoundly important for structural aspects of notochord function. To understand notochord structure, it is helpful to consider the notochord as part of a mechanical system required for locomotion Fig.

Zebrafish embryos,for example, are able to flip their tails within 1 day of fertilisation, and hatch and swim within 3 days. By analogy, the notochord is like a fire hose,possessing a strong but flexible sheath that can resist high hydrostatic pressures. Consider then a situation in which the fire hose is filled with water balloons, each pushing against the other and against the sheath.

In such an arrangement, the fire hose would be elongated and stiff, but able to bend in any direction. Finally, with cables running along the top and bottom of the inflated fire hose, the cables would resist any upward or downward bending of the hose, and any force acting on the fire hose would deflect it laterally. For the zebrafish embryo, the equivalent of the fire hose is the peri-notochordal basement membrane and the water balloons are the vacuolated notochord cells. Running along the top of the notochord is the floor plate and along the bottom is the hypochord.

Consistent with this structural role, both the floor plate and the hypochord express a variety of cartilage proteins,such as type II collagen Yan et al. In addition, mutations in genes such as oep and cyclops ndr2 — Zebrafish Information Network , which lead to substantial loss of floor plate, also produce embryos with a profound downward curvature Hatta, ; Hatta et al.

Thus, although not yet directly established, it is possible that the floor plate and hypochord act as cables on the respective dorsal and ventral sides of the notochord. In zebrafish, the formation of the peri-notochordal basement membrane and vacuole are each affected by two phenotypic classes of mutations.

One class,originally found to affect both notochord and brain development, comprises the bashful, grumpy and sleepy loci. The second class of mutations was originally found to affect notochord and melanophore development early in embryogenesis, leading eventually to catastrophic cell death throughout mutant embryos. The coatomer complex would normally be considered to perform a eukaryotic housekeeping function.

It is surprising that loss of coatomer activity in zebrafish embryos would lead to a notochord phenotype.