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Segmentation (biology)
Encyclopedia
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
and plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
body plans into a series of repetitive segments. This article will focus on the segmentation of animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
body plans, specifically using the examples of the phyla Arthropoda, Vertebrata, and Annelida. These three phyla 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 phyla, different organisms have different mechanisms for segmenting the body. Segmentation of the body plan is important for allowing different regions of the body to develop differentially for different uses.
While many details will be discussed here, there are other details about the process of establishing such a segmented body pattern in morphogenesis
Morphogenesis
Morphogenesis , is the biological process that causes an organism to develop its shape...
.
Definition
Segmentation is a difficult term to satisfactorily define. Many taxa (for instance the molluscs) have some form of serial repetition in their units, but are not conventionally thought of as segmented. Truly segmented animals would be considered to have organs that were repeated, or having a body composed of self-similar units, but talking of a "segmented organism" is vague: rather, one should refer to which parts of the organism are segmented.Animals
Segmentation in animals typically falls into three types characteristic of the different phylaPhylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
: Arthropoda, Vertebrata, and Annelida. The three will be discussed here in using an example from each phyla, drosophila
Drosophila
Drosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...
, zebrafish, and leech
Leech
Leeches are segmented worms that belong to the phylum Annelida and comprise the subclass Hirudinea. Like other oligochaetes such as earthworms, leeches share a clitellum and are hermaphrodites. Nevertheless, they differ from other oligochaetes in significant ways...
, respectively. Drosophila form segments from a field of equivalent cells based on transcription factor
Transcription factor
In molecular biology and genetics, a transcription factor is a protein that binds to specific DNA sequences, thereby controlling the flow of genetic information from DNA to mRNA...
gradients. Zebrafish, and other vertebrates, use oscillating gene expression to define segments known as somites. Leech
Leech
Leeches are segmented worms that belong to the phylum Annelida and comprise the subclass Hirudinea. Like other oligochaetes such as earthworms, leeches share a clitellum and are hermaphrodites. Nevertheless, they differ from other oligochaetes in significant ways...
embryos, and other annelids, use smaller cells budded off from teloblast cells to define segments.
Arthropods: Drosophila
Although DrosophilaDrosophila
Drosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...
segmentation is not representative of the arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
phylum in general, it is the most highly studied. The drosophila as a model organism is ideal for genetic screens. 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-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
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 normal body plan...
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 genes.The pair-rule genes are mostly transcription factor
Transcription factor
In molecular biology and genetics, a transcription factor is a protein that binds to specific DNA sequences, thereby controlling the flow of genetic information from DNA to mRNA...
s, expressed in regular stripes down the length of the embryo. These transcription factors then regulate the expression of segment polarity gene
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. In the fruit fly Drosophila melanogaster, segment polarity genes help to define the anterior and posterior polarities within each embryonic parasegment by...
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.
Annelids: Leech
While not as well studied as in drosophilaDrosophila
Drosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...
and zebrafish, segmentation in leech
Leech
Leeches are segmented worms that belong to the phylum Annelida and comprise the subclass Hirudinea. Like other oligochaetes such as earthworms, leeches share a clitellum and are hermaphrodites. Nevertheless, they differ from other oligochaetes in significant ways...
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, suggesting its common evolutionary origin in the ancestor 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
Although perhaps not as well studied as DrosophilaDrosophila
Drosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...
, segmentation in zebrafish is actively studied. The process is similar in zebrafish and other chordates, such as the chicken
Chicken
The chicken is a domesticated fowl, a subspecies of the Red Junglefowl. As one of the most common and widespread domestic animals, and with a population of more than 24 billion in 2003, there are more chickens in the world than any other species of bird...
and the mouse
Mouse
A mouse is a small mammal belonging to the order of rodents. The best known mouse species is the common house mouse . It is also a popular pet. In some places, certain kinds of field mice are also common. This rodent is eaten by large birds such as hawks and eagles...
. Segmentation in chordates is characterized as the formation of a pair of somites on either side of the midline. This is often referred to as somitogenesis
Somitogenesis
Somitogenesis is the process by which somites are produced. Somites are bilaterally paired blocks of mesoderm that form along the anterior-posterior axis of the developing embryos of segmented animals, often originating in an anterior to posterior direction. In vertebrates, somites give rise to...
.
In the zebrafish, segmentation is coordinated by the “clock and wavefront.” The “clock” refers to the periodic oscillation of specific genes, such as Her1, a hairy/Enhancer of split- gene. Expression starts that the posterior end of the embryo and moves towards the anterior. The “wavefront” refers to the location where the somites mature, which is defined by gradients of retinoic acid
Retinoic acid
Retinoic acid is a metabolite of vitamin A that mediates the functions of vitamin A required for growth and development. Retinoic acid is required in chordate animals which includes all higher animals from fishes to humans...
and FGF
Fibroblast growth factor
Fibroblast growth factors, or FGFs, are a family of growth factors involved in angiogenesis, wound healing, and embryonic development. The FGFs are heparin-binding proteins and interactions with cell-surface associated heparan sulfate proteoglycans have been shown to be essential for FGF signal...
. Retinoic acid is high in the anterior and FGF is high in the posterior; the “wavefront” is the point at which both concentrations are at their lowest. Cells at this point will mature and form a pair of somites.