Segment (biology)

Segmentation in biology refers to the division of some animal and plant body plans into a series of repetitive segments. This article focuses on the segmentation of animal body plans, specifically using the examples of the taxa Arthropoda, Chordata, 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.

Segmentation is a difficult process 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. 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.

Segmentation in animals typically falls into three types characteristic of different arthropods, vertebrates, and annelids. The arthropod fruit fly forms segments from a field of equivalent cells based on transcription factor gradients. The vertebrate zebrafish uses oscillating gene expression to define segments known as somites. The annelid leech uses smaller blast cells budded off from large teloblast cells to define segments.

Although Drosophila segmentation is not representative of the arthropod 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.

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