A hierarchy of proliferative cells exists in mouse lens epithelium: implications for lens maintenance.

PURPOSE To determine the distribution of slow-cycling cells, which are detected as label-retaining cells (LRCs), in mouse lens epithelium during postnatal development. METHODS Pregnant BALB/c mice were injected intraperitoneally (twice daily) with tritiated thymidine (3H-TdR), beginning at 17 days of gestation until birth. At birth, the in utero-labeled neonatal mice were injected subcutaneously with 3H-TdR (twice daily) for 3 days. Mice were killed weekly for the first month and then at 3-week intervals up to 18.5 weeks (chase periods). Eyes were removed and processed for autoradiography. In living mice, small scrape wounds were made on the anterior surface of the lens of mice that had been "chased" for 18.5 weeks. Twenty-four hours later, wounded mice received a single injection of BrdU. RESULTS Immediately after the in utero/postnatal labeling period, 100% of the lens epithelial cells incorporated 3H-TdR, and all were heavily labeled. With time, the number of LRCs declined so that only 13% of the lens epithelial cells were labeled at 18.5 weeks. At this time the heaviest labeled cells were exclusively found in the central zone and represented 2% to 3% of the total LRCs. In contrast, lightly labeled cells were found in both the central and germinative zones. After wounding, the heavily labeled LRCs incorporated BrdU, indicating that these cells were healthy and could be recruited to proliferate. CONCLUSIONS The heavily labeled LRCs, located exclusively in the central region, represent cells that divide very infrequently during homeostasis (putative stem cells); on perturbation, these cells can proliferate. The lightly labeled LRCs, located in the central and germinative zones, cycle more frequently than the heavily labeled ones. These LRCs may be phenotypically indistinguishable from stem cells and maintain the normal proliferative needs of the lens. A third population of actively cycling cells exists primarily in the germinative zone and represents the transit amplifying cells, which have a limited proliferative potential.