Segmentation in biology is the division of some

animal Animals are multicellular, eukaryotic organisms in the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motilit ...

and

plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclud ...

body plan A body plan, ( ), or ground plan is a set of morphological features common to many members of a phylum of animals. The vertebrates share one body plan, while invertebrates have many. This term, usually applied to animals, envisages a "blueprin ...

s into a series of repetitive segments. This article focuses on the segmentation of

body plans, specifically using the examples of the taxa

Arthropoda Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...

Chordata A chordate () is an animal of the phylum Chordata (). All chordates possess, at some point during their larval or adult stages, five synapomorphies, or primary physical characteristics, that distinguish them from all the other taxa. These fiv ...

, and Annelida. These three groups form segments by using a "growth zone" to direct and define the segments. While all three have a generally segmented body plan and use a growth zone, they use different mechanisms for generating this patterning. Even within these groups, different organisms have different mechanisms for segmenting the body. Segmentation of the body plan is important for allowing free movement and development of certain body parts. It also allows for regeneration in specific individuals.

Definition

Segmentation is a difficult process to satisfactorily define. Many taxa (for example the molluscs) have some form of serial repetition in their units but are not conventionally thought of as segmented. Segmented animals are those considered to have organs that were repeated, or to have a body composed of self-similar units, but usually it is the parts of an organism that are referred to as being segmented.

Embryology

Segmentation in animals typically falls into three types, characteristic of different

arthropods Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...

vertebrates Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...

, and

annelids The annelids (Annelida , from Latin ', "little ring"), also known as the segmented worms, are a large phylum, with over 22,000 extant species including ragworms, earthworms, and leeches. The species exist in and have adapted to various ecolo ...

. Arthropods such as the fruit fly form segments from a field of equivalent cells based on

transcription factor In molecular biology, a transcription factor (TF) (or sequence-specific DNA-binding factor) is a protein that controls the rate of transcription of genetic information from DNA to messenger RNA, by binding to a specific DNA sequence. The fu ...

gradients. Vertebrates like the

zebrafish The zebrafish (''Danio rerio'') is a freshwater fish belonging to the minnow family ( Cyprinidae) of the order Cypriniformes. Native to South Asia, it is a popular aquarium fish, frequently sold under the trade name zebra danio (and thus often ...

use oscillating

gene expression Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product that enables it to produce end products, protein or non-coding RNA, and ultimately affect a phenotype, as the final effect. The ...

to define segments known as

somites The somites (outdated term: primitive segments) are a set of bilaterally paired blocks of paraxial mesoderm that form in the embryonic stage of somitogenesis, along the head-to-tail axis in segmented animals. In vertebrates, somites subdivide i ...

. Annelids such as the

leech Leeches are segmented parasitic or predatory worms that comprise the subclass Hirudinea within the phylum Annelida. They are closely related to the oligochaetes, which include the earthworm, and like them have soft, muscular segmented bodie ...

use smaller

blast cell In cell biology, a precursor cell, also called a blast cell or simply blast, is a partially differentiated cell, usually referred to as a unipotent cell that has lost most of its stem cell properties. A precursor cell is also known as a proge ...

s budded off from large teloblast cells to define segments.

Arthropods

Although ''

Drosophila ''Drosophila'' () is a genus of flies, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species ...

'' segmentation is not representative of the

arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...

phylum in general, it is the most highly studied. Early screens to identify genes involved in cuticle development led to the discovery of a class of genes that was necessary for proper segmentation of the ''Drosophila'' embryo. To properly segment the ''Drosophila'' embryo, the

anterior Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

- posterior axis is defined by maternally supplied transcripts giving rise to gradients of these proteins. This gradient then defines the expression pattern for

gap gene A gap gene is a type of gene involved in the development of the segmented embryos of some arthropods. Gap genes are defined by the effect of a mutation in that gene, which causes the loss of contiguous body segments, resembling a gap in the norm ...

s, which set up the boundaries between the different segments. The gradients produced from gap gene expression then define the expression pattern for the

pair-rule gene A pair-rule gene is a type of gene involved in the development of the segmented embryos of insects. Pair-rule genes are expressed as a result of differing concentrations of gap gene proteins, which encode transcription factors controlling pair- ...

s. The pair-rule genes are mostly

s, expressed in regular stripes down the length of the embryo. These transcription factors then regulate the expression of

segment polarity gene A segmentation gene is a generic term for a gene whose function is to specify tissue pattern in each repeated unit of a segmented organism. Animals are constructed of segments; however, Drosophila segments also contain subdivided compartments. The ...

s, which define the polarity of each segment. Boundaries and identities of each segment are later defined. Within the arthropods, the body wall, nervous system, kidneys, muscles and body cavity are segmented, as are the appendages (when they are present). Some of these elements (e.g. musculature) are not segmented in their sister taxon, the

onychophora Onychophora (from grc, ονυχής, , "claws"; and , , "to carry"), commonly known as velvet worms (due to their velvety texture and somewhat wormlike appearance) or more ambiguously as peripatus (after the first described genus, '' Peripatus ...

Annelids: Leech

While not as well studied as in ''

'' and

, segmentation in the

has been described as “budding” segmentation. Early divisions within the leech embryo result in teloblast cells, which are stem cells that divide asymmetrically to create bandlets of blast cells. Furthermore, there are five different teloblast lineages (N, M, O, P, and Q), with one set for each side of the midline. The N and Q lineages contribute two blast cells for each segment, while the M, O, and P lineages only contribute one cell per segment. Finally, the number of segments within the embryo is defined by the number of divisions and blast cells. Segmentation appears to be regulated by the gene

Hedgehog A hedgehog is a spiny mammal of the subfamily Erinaceinae, in the eulipotyphlan family Erinaceidae. There are seventeen species of hedgehog in five genera found throughout parts of Europe, Asia, and Africa, and in New Zealand by introducti ...

, suggesting its common evolutionary origin in the

ancestor An ancestor, also known as a forefather, fore-elder or a forebear, is a parent or (recursively) the parent of an antecedent (i.e., a grandparent, great-grandparent, great-great-grandparent and so forth). ''Ancestor'' is "any person from whom ...

of arthropods and annelids. Within the annelids, as with the arthropods, the body wall, nervous system, kidneys, muscles and body cavity are generally segmented. However, this is not true for all of the traits all of the time: many lack segmentation in the body wall, coelom and musculature.

Chordates: zebrafish and mouse

Although perhaps not as well studied as ''

'', segmentation in

, chicks, and mice is actively studied. Segmentation in chordates is characterized as the formation of a pair of

on either side of the midline. This is often referred to as

somitogenesis Somitogenesis is the process by which somites form. Somites are bilaterally paired blocks of paraxial mesoderm that form along the anterior-posterior axis of the developing embryo in segmented animals. In vertebrates, somites give rise to skeleta ...

. In chordates, segmentation is coordinated by the

clock and wavefront model The clock and wavefront model is a model used to describe the process of somitogenesis in vertebrates. Somitogenesis is the process by which somites, blocks of mesoderm that give rise to a variety of connective tissues, are formed. The model descri ...

. The "clock" refers to the periodic oscillation of specific genes, such as Her1, a hairy/Enhancer of split- gene. Expression starts at the posterior end of the embryo and moves towards the

. The wavefront is where the somites mature, defined by a gradient of FGF with somites forming at the low end of this gradient. In higher vertebrates including mouse and chick, but not zebrafish, the wavefront also depends upon retinoic acid generated just anterior to the caudal FGF8 domain which limits the anterior spreading of FGF8; retinoic acid repression of Fgf8 gene expression defines the wavefront as the point at which the concentrations of both retinoic acid and diffusible FGF8 protein are at their lowest. Cells at this point will mature and form a pair of somites. The interaction of other signaling molecules, such as

myogenic regulatory factors Myogenic regulatory factors (MRF) are basic helix-loop-helix (bHLH) transcription factors that regulate myogenesis: MyoD, Myf5, myogenin, and MRF4. These proteins contain a conserved basic DNA binding domain that binds the E box DNA motif. They ...

, with this gradient promotes the development of other structures, such as muscles, across the basic segments. Lower vertebrates such as zebrafish do not require retinoic acid repression of caudal Fgf8 for somitogenesis due to differences in gastrulation and neuromesodermal progenitor function compared to higher vertebrates.

Other taxa

In other taxa, there is some evidence of segmentation in some organs, but this segmentation is not pervasive to the full list of organs mentioned above for arthropods and annelids. One might think of the serially repeated units in many

Cycloneuralia Cycloneuralia is a clade of ecdysozoan animals including the Scalidophora (Kinorhynchans, Loriciferans, Priapulids) and the Nematoida ( nematodes, Nematomorphs). It may be paraphyletic, or may be a sister group to Panarthropoda. Or perhaps P ...

, or the segmented body armature of the chitons (which is not accompanied by a segmented coelom).

Origin

Segmentation can be seen as originating in two ways. To caricature, the 'amplification' pathway would involve a single-segment ancestral organism becoming segmented by repeating itself. This seems implausible, and the 'parcellization' framework is generally preferred – where existing organization of organ systems is 'formalized' from loosely defined packets into more rigid segments. As such, organisms with a loosely defined metamerism, whether internal (as some molluscs) or external (as onychophora), can be seen as 'precursors' to eusegmented organisms such as annelids or arthropods.

References

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