Cell biology of lens epithelial cells

The epithelium of the vertebrate lens plays a critical role in tissue homeostasis and maintenance of lens clarity. The epithelium is the most metabolically active region of the lens and contains all the mitotically active cells. Cell division in the epithelium occurs exclusively in the germinative zone, a swathe of cells encircling the lens just above the equator. Fibroblast growth factor, a molecule with a demonstrated role in lens fiber cell differentiation, may promote epithelial cell division although other growth factors likely contribute. The organization of cells within the lens epithelium has often been likened to a cobblestone pattern. However, recent three-dimensional imaging studies have revealed that individual epithelial cells have a complex, polarized anatomy, with morphologically distinct apical and basolateral domains. The apical membrane is delineated by a hybrid junctional complex consisting of adherens junctions and tight junctions. Adherens junctions play a critical role in epithelial organization and loss of nectins or cadherins, two core components of adherens junctions, has catastrophic consequences for lens organization and transparency. Tight junctions, the apical-most junctional element, restrict the paracellular flow of ions into the lens but also serve as scaffolds for an assemblage of important polarity proteins. Targeted disruption of the partitioning defective (Par) family of polarity proteins results in loss of apical cell junctions and promotes epithelial-to-mesenchymal transition.