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This article is about the concept in biology. For the concept in physics, see Symmetry breaking. Symmetry breaking is the process by which uniformity is broken, or the number of points to view invariance are reduced, to generate a more structured and improbable state. That is to say, symmetry breaking is the event where symmetry along a particular axis is lost to establish a polarity. Polarity is a measure for a biological system to distinguish poles along an axis. This measure is important because it is the first step to building complexity. For example, during organismal development, one of the first steps for the embryo is to distinguish its dorsal-ventral axis. The symmetry-breaking event that occurs here will determine which end of this axis will be the ventral side, and which end will be the dorsal side. Once this distinction is made, then all the structures that are located along this axis can develop at the proper location. As an example, during human development, the embryo needs to establish where is ‘back’ and where is ‘front’ before complex structures, such as the spine and lungs, can develop in the right location (where the lungs is placed ‘in front’ of the spine). This relationship between symmetry breaking and complexity was articulated by P.W. Anderson. He speculated that increasing levels of broken symmetry in many-body systems correlates with increasing complexity and functional specialization. In a biological perspective, the more complex an organism is, the higher number of symmetry-breaking events can be found. Without symmetry breaking, building complexity in organisms would be very difficult. The importance of symmetry breaking in biology is also reflected in the fact that it’s found at all scales. Symmetry breaking can be found at the macromolecular level, at the subcellular level and even at the tissues and organ level. It’s also interesting to note that most asymmetry on a higher scale is a reflection of symmetry breaking on a lower scale. Cells first need to establish a polarity through a symmetry-breaking event before tissues and organs themselves can be polar. For example, left-right body axis asymmetry in vertebrates is thought to be determined by asymmetry of cilia rotation during early development, which will produces a constant, unidirectional flow. This flow at the cellular level is the symmetry-breaking event that determines the asymmetry at the body-axis level. There are several examples of symmetry breaking that are currently being studied. One of the most studied examples is the cortical rotation during ''Xenopus'' development, where this rotation acts as the symmetry-breaking event that determines the dorsal-ventral axis of the developing embryo. This example is discussed in more detail below. Another example that involves symmetry breaking is the establishment of dendrites and axon during neuron development, and the PAR protein network in ''C. elegans''. It is thought that a protein called shootin-1 determines which outgrowth in neurons eventually becomes the axon, at it does this by breaking symmetry and accumulating in only one outgrowth. The PAR protein network works under similar mechanisms, where the certain PAR proteins, which are initially homogenous throughout the cell, break their symmetry and are segregated to different ends of the zygote to establish a polarity during development. ==Cortical Rotation== Cortical rotation is a phenomenon that seems to be limited to ''Xenopus'' and few ancient teleosts, however the underlying mechanisms of cortical rotation have conserved elements that are found in other chordates. Research in this area is on-going and changes to the model described below are to be expected. In fact, the origin of asymmetry in cell division, cell polarity and the mechanism that breaks the symmetry continue to be topics of intense research. Since the early 1990s, many discoveries have been made leading to a sound model of the mechanism for symmetry breaking. This article will focus solely on symmetry breaking in the ''Xenopus'' embryo, an animal model that has wide application. A sperm can bind a ''Xenopus'' egg at any position of the pigmented animal hemisphere; however once bound this position then determines the dorsal side of the animal. The dorsal side of the egg is always directly opposite the sperm entry point. The reason being the sperm's centriole acts as an organizing center for the egg’s microtubules. While this observation has been known for quite some time, the question of how all of this works is more complicated. The molecular mechanisms driving dorsal-ventral asymmetry are a fine example of simplicity and complexity inherent in biology. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Symmetry breaking and cortical rotation」の詳細全文を読む スポンサード リンク
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