Development and alteration of polarity.

Overall polarization of the plasma membrane of epithelial cells is the sum of the individual polarizations of its components. These individual polarizations in turn, may vary independently in degree (apical/basolateral ratio) and may be expressed at different stages of the cell cycle. They occur in response to cell contacts, nature of the support, and presence of triggering hormones; once established, polarizations may be subject to disruption and resorting. Epithelial cells transcytose receptors, insert membrane mechanisms during a particular period of the cell cycle, remove and relocate misplaced membrane components, and even completely reverse their polarity in the presence of well established TJs. TJs are not responsible for polarization but, ironically, they should be regarded as a result of the polarization process itself (31). The polarization of single cells, such as neurons and muscle cells mentioned at the beginning of this article, may represent extreme cases of cells that polarize but do not produce TJs. However, if an asymmetrically inserted protein is subsequently released from underlying anchoring structures (e.g. the cytoskeleton) to become free (e.g. 50% of Na,K-ATPase in MDCK cells), then the TJ may play a role in confining the free fraction to the apical or to the basolateral region. But even if TJs fail to completely segregate membrane components, mechanisms can restore polarization as in the case of the Na,K-ATPase trapped on the apical side. Lipid polarization seems to depend on the existence of the fence like character of the TJs and to the best of our knowledge lipid polarization is only found in epithelial cells with well established TJs.