What does segmentation mean in biology

Annelids The first segmented animals to evolve were the annelid worms, phylum Annelida. These advanced coelomates are assembled as a chain of nearly identical segments, like the boxcars of a train. The great advantage of such segmentation is the evolutionary flexibility it offers—a small change in an existing segment can produce a new kind of segment with a different function.

Thus, some segments are modified for reproduction, some for feeding, and others for eliminating wastes. Two-thirds of all annelids live in the sea about 8, species, including the bristle; most of the rest—some 3, species—are earthworms. The basic body plan of an annelid is a tube within a tube: The digestive tract, is suspended within the coelom, which is itself a tube running from mouth to anus. There are three characteristics of annelid body organization: Repeated segments.

The body segments of an annelid are visible as a series of ringlike structures running the length of the body, looking like a stack of doughnuts. In each of the cylindrical segments, the excretory and locomotor organs are repeated. The body fluid within the coelom of each segment creates a hydrostatic liquid-supported skeleton that gives the segment rigidity, like an inflated balloon. Muscles within each segment pull against the fluid in the coelom.

Because each segment is separate, each can expand or contract independently. When an earthworm crawls on a flat surface, for example, it lengthens some parts of its body while shortening others. Specialized segments. The anterior front segments of annelids contain the sensory organs of the worm.

Elaborate eyes with lenses and retinas have evolved in some annelids. One anterior segment contains a well-developed cerebral ganglion, or brain. Because partitions separate the segments, it is necessary to provide ways for materials and information to pass between segments. A circulatory system carries blood from one segment to another, while nerve cords connect the nerve centers located in each segment with each other and the brain.

Arthropods A profound innovation marks the origin of the body plan characteristic of the most successful of all animal groups, the arthropods, phylum Arthropoda. This innovation was the development of jointed appendages. The name arthropod comes from two Greek words, arthros, jointed, and podes, feet. All arthropods have jointed appendages. Thus while lampreys form eight pouches and nine arches, many chondrichthyans and all actinopterygians form six pouches and seven arches, while amphibia form five pouches and six arches and amniotes four pouches and five arches.

Although segmentation of the endoderm provides the framework for pharyngeal development, interactions between the populations that contribute to the arches is also important for their full realisation.

In particular, neural crest cells play a key role as they are the source of the skeletal elements that will underpin the later identity of the arches. In gnathostomes, the most anterior arch will form the jaws, the second the hyoid and the more posterior arches are either gill bearing in fish or subsumed into the larynx in amniotes.

Again, the identity of the skeletal elements is dependent upon Hox genes. With the formation of the somites, the subdivision of the hindbrain and the generation of the pharyngeal arches one can observe the iteration of parts along the long axis of the body. However, as is apparent from our consideration of how these processes are directed, they differ fundamentally from each other.

There are few, if any, common mechanisms. Somites are generated sequentially over a long period using a clock and wave front mechanism acting within the presomitic mesoderm, which is to some extent replenished during axis extension.

The rhombomeres form via internal subdivision of a specified region and over a very short period. The rhombomeres also do not emerge in an anteroposterior sequence with the first segments that are formed being r3, r4 and r5 [ 34 ]. Finally, the pharyngeal segments form through the outpocketing of the pharyngeal endoderm to form the pharyngeal pouches which contact the overlying ectoderm and thus delineate the anterior and posterior limits of the arches.

There are, however, some superficial similarities such as the utilisation of some of same molecules, but these are often not employed in the same way. Thus while a posterior to anterior gradient of FGF is important for somitogenesis [ 5 ], this is not true of pharyngeal or hindbrain segmentation.

Another similarity lies in the identity of the segments being dependent on Hox genes, but this reflects the more general role of these genes in anteroposterior patterning of the body. Hox genes assign identity to segmented and unsegmented regions, such as the lateral plate mesoderm, alike. The segmentation of the paraxial mesoderm to generate individually packaged somites underpins the locomotory strategies of the vertebrates, resulting in the formation of separate bilateral bocks of muscle lying either side of an articulated backbone.

This arrangement is essential for lateral undulatory locomotion of fish and many tetrapods. In contrast, the segmentation of the hindbrain generates subdivisions within a contiguous region which allows for seamless connections between the different hindbrain nuclei and ongoing connections, and for through traffic that connects higher brain centres with the spinal cord.

Lastly, the segmentation of the pharynx relates to its activities in feeding and respiration. The two most anterior pharyngeal segments of the gnathostomes will contribute to the jaw apparatus and the more posterior segments will form gills with an abundant vasculature and thus perform respiratory functions.

To further clarify the relationships between the vertebrate segmentation processes; somitogenesis, rhombomeric subdivisions of the hindbrain and pharyngeal arches, it is important to ask about their evolutionary origins. Vertebrates are chordates and one defining feature of this phylum is the presence of segmented muscle blocks.

We might therefore expect somitogenesis to be a shared characteristic of the chordates. Yet somites are lacking in urochordates and while they do form in cephalochordates this process seems to be somewhat distinct from that described in vertebrates [ 51 ].

The more anterior somites in amphioxus form as bilateral pairs by enterocoelus evagination of the wall of the archenteron while the posterior somites form by schizocoely, alternating between the left and right sides.

It has also been shown that, while the very anterior somites are dependent upon FGF signalling, most of the other somites forming by enterocoely and those formed by schizocoely are FGF insensitive. However, the lineages leading to the extant representatives of the chordate subphyla diverged a very long time ago and thus ancestral characteristics may have been lost or obscured.

Moving outside the chordates, and considering other deuterostomes one cannot find evidence for somites. Thus, it is reasonable to assume that somitogenesis evolved with the chordates but has undergone major modification in the different chordate lineages.

Gene expression studies in other chordates and in hemichordates have established that a region of the nervous system expressing anterior Hox genes, and thus homologous to the vertebrate hindbrain domain, exists in these groups [ 52 ]. There are, however, no indications that rhombomeres exist outside vertebrates.

There is neither morphological nor molecular evidence to support segmentation of the nervous system in an analogous region. For example, amphioxus has a single Krox20 gene but it is not expressed segmentally in the developing nervous system [ 53 ]. Pharyngeal segmentation is, however, relatively ancient. As with somites, pharyngeal gill slits are characteristic of the chordates and it is clear that the simple perforations of the pharynx seen in other chordates such as amphioxus are homologous to the endodermal segmentation of the vertebrate pharynx.

The pharyngeal pouches of vertebrates express a Pax-Six-Eya regulatory network as does the pharyngeal endoderm in amphioxus [ 54 , 55 ]. Recent results from the hemichordate Saccoglossus kowalevskii have shown that pharyngeal segmentation is likely to be a general feature of deuterostomes [ 57 ].

Furthermore, although echinoderms lack gill slits, this is likely to result from secondary loss as fossil evidence has shown that the earliest echinoderms were bilateral and did possess gill slits [ 58 ], which further indicates that pharyngeal segmentation is a characteristic of the deuterostomes. An important point that emerges from this is that there was no ancestral process of segmentation that was co-opted by each of these processes.

The three segmental systems of the vertebrates each arose de novo at different points during evolution Figure 2. The most ancient is pharyngeal segmentation, and that is a feature of the deuterostomes, with somitogenesis following with the emergence of the chordates and finally rhombomere formation and the evolution of the vertebrates.

The evolutionary history of segmentation in the vertebrate lineage. Three instances of segmentation are found in extant vertebrates that are conserved with different invertebrate groups. Pharyngeal segmentation can be dated to the deuterostome ancestor, while somitogenesis dates to the chordate ancestor and rhombomeric organisation of the hindbrain to the vertebrate stem. There have been many discussions as to the evolutionary origin s of segmentation and there are two key issues here that must be confronted.

The first is whether or not there is any evidence to support homology between the manifestations of segmentation seen in vertebrates with those displayed by other bilaterian clades. Attempts to homologize between segmentation in vertebrates and that seen in arthropods and annelids, have been strongly affected by their time. In the late s and early s comparisons were invariably drawn between the mechanisms underpinning the segmentation of the hindbrain and those directing segmentation in Drosophila.

Both involved specification via transcription factor hierarchies and both resulted in the formation of lineage restricted compartments. However, as our molecular understanding of somitogenesis advanced it became more common to draw comparisons between that process and other modes of arthropod segmentation. For example, it was noted that segmentation in spiders involves Notch and Delta signalling [ 59 ]. Yet, for both of these comparisons, our extensive knowledge of the developmental processes underpinning rhombomere formation and somitogenesis would indicate that the highlighted commonalities are but simply superficial similarities.

The mechanisms underpinning the segmentation of the Drosophila embryo are quite different from those in vertebrates. Drosophila segments are formed within a syncytium by a transcription factor cascade. While rhombomeres are formed via cell sorting, using Eph-ephrin signalling, lying downstream of a very different system of signalling molecules and transcriptional effectors.

Furthermore, rhombomeres evolved with the vertebrates Figure 2 ; they are clearly lacking in other deuterostomes, and thus it is unlikely that they could have a common origin with the segmental patterning systems of arthropods.

The formation of somites and the segments of many arthropods do share the characteristic that they are generated sequentially and that this is tied to axis elongation. But, as such, some of the shared features associated with somite formation and arthropod segmentation may indicate more general conserved bilaterian features, such as the involvement of a posterior wnt-secreting growth zone [ 60 , 61 ].

It is also very possible that the association between notch signalling and segments is the result of this very ancient signalling pathway becoming subservient to the segmentation processes in animals that organise their body plan in such a manner, and indeed as we have pointed out Notch signalling is not required for somite formation in vertebrates.

Finally, it should be stressed again that somites are a chordate feature Figure 2 and are not found in other deuterostomes and thus it seems again unlikely that somitogenesis could have a common origin with any mode of arthropod segmentation.

The second issue is the relative paucity of segmentation within the bilateria and its implications. As Hannibal and Patel point out, if segmentation is difficult to evolve, this would suggest that it had a single origin but that it was subsequently lost by the great majority of animal phyla and only retained in a few [ 2 ].

One conclusion following from this hypothesis would be that segmentation is readily dispensable in the generation of a functional body plan. By contrast, if segmentation is relatively easy to evolve then one would expect to observe unrelated, non-homologous, instances of segmentation in different phyla.

Our discussion of the segmented systems of vertebrates would point us towards the latter option. We find that there is no single process of segmentation and, that in the lineage leading to the vertebrates, segmented structures evolved at least three times independently, in different germ layers and using different mechanics, at least three times.

Thus it would seem that it is relatively easy to evolve segmentation. Hannibal and Patel make the excellent point that there is no merit in talking about segmentation without being explicit about what is being discussed.

Thus with regards to segmentation in vertebrates, it is unhelpful to talk generally of segmentation and to lump together the processes of somitogenesis, rhombomere formation and pharyngeal arch development; these are chalk and cheese comparisons. It is more correct and useful to discuss how somites form, how rhombomeres emerge and how pharyngeal arches are generated. Furthermore, as Hannibal and Patel note, it is incredibly difficult to arrive at a precise definition of segmentation and we would argue that this is because there is no single process of segmentation.

Consequently, all definitions of segmentation are superficial; that is, repetition of structures along the main body axis - there is nothing deeper to be indicated. An analogous situation is that of wings - what is a wing? It is a structure that allows an animal to fly. Wings are a feature of flies, birds and bats but the definition of a wing has to be superficial because it describes non-homologous structures.

Thus, many of the problems that arise with the concept of segmentation, and that we have discussed here, ultimately reflect a problem of terminology. The names that we apply to biological processes do not necessarily indicate anything beyond being useful appellations.

Of course this is the problem of homoplasy and the only route to resolving this is to map any given biological process to the phylogeny. Tautz D: Segmentation. Dev Cell. Pourquie O: Vertebrate segmentation: from cyclic gene networks to scoliosis. Dubrulle J, Pourquie O: fgf8 mRNA decay establishes a gradient that couples axial elongation to patterning in the vertebrate embryo. Nat Cell Biol. Article Google Scholar. Dev Dynam. Nat Genet. Ozbudak EM, Lewis J: Notch signalling synchronizes the zebrafish segmentation clock but is not needed to create somite boundaries.

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