Left-right asymmetry

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In developmental biology, left-right asymmetry (LR asymmetry) is the process in early embryonic development that breaks the normal symmetry in the bilateral embryo. In vertebrates, left-right asymmetry is established early in development at a structure called the left-right organizer (the name of which varies between species) and leads to activation of different signalling pathways on the left and right of the embryo.^[1] This in turn causes several organs in adults to develop LR asymmetry, such as the tilt of the heart, the different number of lung lobes on each side of the body, and the position of the stomach and spleen on the right side of the body.^[2] If this process does not occur correctly in humans it can result in heterotaxy or situs inversus.

LR asymmetry is pervasive throughout all animals, including invertebrates. Examples of invertebrate LR asymmetry include the large and small claws of the fiddler crab, asymmetrical gut coiling in Drosophila melanogaster, and dextral (clockwise) and sinistral (counterclockwise) coiling of gastropods. This asymmetry can be restricted to a specific organ or feature, as in the crab claws, or be expressed throughout the entire body as in snails.