Tight junctions and apical/basolateral polarity

Summary Physiological studies led to devise models of epithelial cells in which the membrane does not have its molecules distributed homogeneously, but polarized towards the apical or towards the basolateral regions. For a while, it was assumed that the TJ, acting as a fence between the two regions, would be responsible for this asymmetry. However, today the information available indicates not only that polarization may proceed independently of the TJ, but that this structure itself may attain its precise location due to a polarization process. Nevertheless, TJs may play a role in restricting to the apical or to the basolateral region those molecules that are free to diffuse in the plane of the cell membrane (e.g., lipids and protein that are not attached to cytoplasmic or extracellular structures).