Limbal basal epithelial cells are not homogeneous, but consist of limbal stem cells and transit amplifying cells (TACs). Although a number of markers are specifically enriched in limbal basal epithelial cells, no definitive molecular markers distinguishing the limbal stem cells from the TAC cells have been identified. As a self-renewing tissue, the limbal stem cells maintain the corneal epithelium throughout life and are thus essential for maintaining corneal transparency and vision. These undifferentiated stem cells have a high proliferative potential, and give rise to TACs, which have limited proliferative capacity. Some of the TACs give rise to post-mitotic cells, which, in turn, terminally differentiate into limbal superbasal cells, which are then shed from the surface. TACs also migrate from the limbus towards the cuboidal basal layer of the central cornea and become corneal basal epithelial cells. From this layer, cells committed to terminal differentiation migrate outwards into the wing-shaped superbasal cell layer, which gives rise to several layers of flattened, squamous superficial cells that are eventually sloughed off.
Many markers that identify limbal stem cells are also selective for TACs located in the basal limbus, making isolation of a pure stem cells population near impossible.
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