A Hierarchy of Proliferative Cells Exists in Mouse Lens Epithelium: Implications on Lens Maintenance

Abstract

Abstract: : Purpose: The lens is unique in that cells are continuously produced but never lost; therefore this tissue expands throughout the lifetime of the organism. The single layered lens epithelium is responsible for supplying the lens with all of its cells and thus could be considered as a self– or continuously–renewing system. Since such systems are governed by stem cells, one would expect that stem cells might reside in the lens epithelium. Epithelial stem cells are normally slow–cycling in vivo, and can be identified experimentally as "label–retaining cells (LRCs)". In this study we determine the distribution of LRCs in mouse lens epithelium (LE) during post–natal development. Methods: To identify slow–cycling cells (LRCs), we injected pregnant Balb/c mice intraperitoneally (2x daily), with tritiated thymidine (³H–TdR; 5µCi/gm) beginning at 17 days gestation until birth. At birth, we injected the in utero–labeled neonatal mice, subcutaneously with ³H–TdR (5µCi/gm, 2x daily) for 3 days. Mice (n=4) were sacrificed weekly for the first month and then at three week intervals up to 4.5 months. LE cells that retained ³H–TdR were detected by autoradiography. Cells containing 3–5 grains per nuclei were scored as lightly labeled, nuclei with 6–9 grains were scored as moderately labeled and nuclei with > than 10 grains were heavily labeled. We identified rapidly cycling cells by a single subcutaneous injection of BrdU. Results: We observed that 100% of the LE cells incorporated ³H–TdR immediately after the in–utero/post–natal labeling period, and all were heavily labeled. After a 6 week chase, 50% of the LE cells still retained ³H–TdR; however, only 20% of these LRCs were heavily labeled, and most were restricted to the central zone. The number of ³H–TdR–labeled LE cells continued to decline over time so that only 30% of the LE cells were labeled at 18 weeks. At this time the heaviest labeled cells were exclusively found in the central zone and represented 13% of the total LRCs. In contrast, the moderately and lightly labeled cells were found in both the central and germinative zones. Rapidly cycling (BrdU–labeled) cells were primarily restricted to the germinative zone. Conclusions: All cells were equally labeled following exposure to ³H–TdR, thus the most heavily labeled LRCs, observed after the 18 week chase represent the slowest cycling (putative stem cells) cells, which were confined to the central zone. The more lightly labeled LRCs, distributed throughout the epithelial layer, represent stem cell progeny. This distribution pattern indicates that the lens is maintained by a slow, continuous flow of cells from the central to the germinative zone.

Keywords: proliferation • anatomy