Strategies of epithelial repair: modulation of stem cell and transit amplifying cell proliferation.

Using double labeling techniques, we studied the replication of corneal epithelial stem cells that reside exclusively in the limbal zone, and their progeny transit amplifying cells. We show that corneal epithelial stem cells can be induced to enter DNA synthesis by wounding and by TPA. We demonstrate the existence of a hierarchy of TA cells; those of peripheral cornea undergo at least two rounds of DNA synthesis before they become post-mitotic, whereas those of central cornea are capable of only one round of division. However, the cell cycle time of these TA cells can be shortened and the number of times these TA cells can replicate is increased in response to wounding. These results thus demonstrate three strategies of epithelial repair: (i) stem cell replication, (ii) the unleashing of additional rounds of cell proliferation that remain as an untapped reserve under normal circumstances, and (iii) enhancement of TA cell proliferation via a shortening of the cycling time.