page_title: Segmentation (biology)
section_title: Chordates: zebrafish and mouse
hierarchical_section_title: Segmentation (biology) / Animals / Chordates: zebrafish and mouse
caption_attribution_description: English: Diagram of zebrafish somitogenesis, created by Emily Ho, adapted from Dequéant and Pourquié, 2008.
context_page_description: Segmentation in biology is the division of some animal and plant body plans into a series of repetitive segments. 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.
context_section_description: Although perhaps not as well studied as Drosophila, segmentation in zebrafish, chick, and mouse is actively studied. 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.
  In chordates, segmentation is coordinated by the clock and wavefront model. 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 anterior. 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. Elaboration of this process with other signalling chemicals allows for structures such as muscles to span 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.
caption: Zebrafish form segments known as somites through a process that is reliant upon gradients of retinoic acid and FGF, as well as periodic oscillation of gene expression.
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