To define Segmentation in simple terms would be division of animal bodies into repetitive segments. Segments are comprised of a number of discrete structures developmentally derived from both ectoderm and mesoderm. Segmented animals are found in Annelida, Arthropoda and chordata phylas which are not closely related according to recent phylogenic data. Segmentation allows for a high degree of specialization of bodily regions.
Among Chordata, segmentation is well characterized in vertebrates. Segments or Somites in vertebrates are transient mesodermal structures that form sequentially along the anterior- posterior axis with a regular spatial and temporal periodicity. Molecular data available so far indicate that vertebrate somitogenesis is regulated by a segmentation clock. Notch signaling pathway is a central component of the vertebrate segmentation clock. Notch pathway genes like Notch,Delta and hairy are shown to play a vital role in somite formation in various vertebrates.Genes that are regulated by the segmentation clock exhibit oscillating patterns that cycle with a period equivalent to the time it takes to form one somite.
Segmentation in Arthropoda:
Drosophila melonogaster helped a great deal in understanding segmentation in Arthropoda.Thanks to some landmark work by Nusslein-Volhard and Wieschaus in early 80’s which led to discovery of Segmentation genes (gap, pair-rule and segment polarity genes) which divide the Drosophila embryo into small repetitive segments. Most arthropods generate segments progressively from a posterior growth zone (Short germ band development) in a cellular environment a deviation from Drosophila where all segments are formed simultaneously (long germ band development). Unlike vertebrates Drosophila doesn’t make use of Notch signalling pathway during segmentation process. Even though expression pattern of segmentation genes differ among different arthropods subphylas like myriapods ,chelicerates ,crustaceans and other insects but same genes are used for segment formation throughout the phylum of arthropods suggesting that ancestors of arthropods were segmented.
Drosophila has evolved a different pathway for segmentation by avoiding Notch pathway’s involvement but its not that all arthropods use same strategy. Work from Wim Damen’s lab in early 2000 has shown that the spider Cupiennius salei uses Notch and two Delta orthologues, display segmental expression in the posterior growth zone prior to segment formation. In addition, Delta and hairy exhibit dynamic striped expression in the ectodermal growth zone before and during segmentation.This proves that there are some commonalities between arthropods and vertebrates in using Notch pathway genes during segmentation. Drosophila of course might have lost Notch’s segmentation function and evolved a different strategy for making segments.
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