This way up
Direction is important in cells’ development and function
A cell’s development – including its direction – is constantly influenced by the cells surrounding it. An epithelial sheet, for example, is asymmetric. One face (the apical surface) is exposed to the watery contents of the gut or to the air in the lungs. The opposite face (the basal surface) sits on supporting layers of collagen and connective tissue. Cells that secrete molecules into the gut need different membrane proteins at the top and bottom, and so do those specialised for absorption. The cell keeps track of which end is which, so molecules go the right way.
A more complex example is found in the ear, where a type of epithelial cell in the inner ear turns vibrational signals into electrical impulses so we can hear. These hair cells, which have a bundle of fine cilia, have a top and bottom but have to be arranged in the right direction along another axis as well. If they lose this orientation, or planar polarity, the sense of hearing may be impaired.Lead image:
University of Edinburgh/Wellcome Images CC BY NC