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  • Epithelial cells have two kind of membranes;
    apical (luminal) membrane which fronts lumen and
    basolateral membrane which sits on the basement membrane.

    Both membrane have different carrier proteins, protein channels, and pumps. Na-K-ATPase enzyme (Na-K pump), for instance, is located on the basolateral membrane. This assymmetry is called as polarity.

  • It means that the nucleus in each of the epithelial cells is on the same side (usually on the side of the cell that contacts the basement membrane).

    For example, if you look at this:

    http://missinglink.ucsf.edu/lm/IDS_101_histo_resource/images/246×10-cell_structure_labelled.jpg

    In the image, there are two (and you can see a bit of a third) finger-like extensions called villi. Epithelial cells line their border (the box that’s drawn zooms in on them) and you can see that in each cell, the nucleus (the dark pink/purple circles) is always on the same side.

    Usually this is an indication that the tissue is healthy and normal. Loss of epithelial cell polarity is one of the very early signs that cancer may eventually develop.

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