Self-reinforcing loop in which the mathematical sign of the net gain around the feedback loop is positive: input ‘A’ produces more of ‘A’. An intracellular or transmembrane molecule that is localised in a polarized manner and is essential for polarity. An extracellular signal that orients the direction of polarity but is not necessarily essential for polarization. For example, an equation that depends on the concentration of a factor raised to the power of 2 or 3.

A process whose output is not directly proportional to its input. The ability of two sets of molecules to inhibit the activity or localisation of one another.

The tendency of a molecule to increase its activity non-linearly according to its concentration. Yet, how polarity determinants manage to organise their own polarised locations within cells remains a major unsolved problem.Ĭooperativity. These polarity determinants can orient a whole host of cellular functions, such as cell shape, cell adhesion, cell migration, cell division, cell fate determination, and the uptake and release of molecules. However polarity is manifested, cells rely on molecular polarity determinants (see Glossary, Box 1) that localise to specific domains of the plasma membrane and then act to polarise the action of other cellular systems ( Etienne-Manneville and Hall, 2002 Mellman and Nelson, 2008 Knoblich, 2010 St Johnston and Ahringer, 2010 Goodrich and Strutt, 2011 McCaffrey and Macara, 2011 Vichas and Zallen, 2011). At one extreme are highly polarised neurons with clearly segregated dendritic and axonal domains at the other are round cells, such as those in budding yeast, that display obvious polarity only during certain phases of their life cycle. For example, cells of an epithelial sheet display both apico-basal and planar polarity, while migrating mesenchymal cells have a clear front-to-back organisation. Different cell types employ polarity to orient their behaviour in a variety of different ways. Cell polarity is a fundamental feature of almost all cells.

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