Purpose:
The lens of the eye grows throughout life. To better understand the processes that drive lens growth we are developing a stochastic model of epithelial cell proliferation in the mouse lens.
Methods:
Lens epithelial cell number was determined from orthographic projections of three dimensional confocal data sets. Region-specific mitotic indices were calculated from EdU- and Draq5-stained samples. Cell lineages were visualized in intact lenses using an inducible GFP expression system. Epithelial cell proliferation was modelled mathematically as a Markov branching process.
Results:
The number of cells in the lens epithelium decreased from approximately 50,000 at two weeks of age to less than 40,000 by eight weeks, after which the population stabilized. At eight weeks, the zone of highest mitotic activity was located anterior to the lens equator. This germinative zone (GZ) was 300 microns wide and consisted of two sub-peaks in which mitotic index was elevated. A pre-germinative zone (PGZ), in which the mitotic index was 10-fold lower than in the GZ, was also evident, extending a further 500 microns towards the anterior pole. Inducible expression of GFP was used to track cell lineages. Months after expression was induced, clusters of GFP-labeled cells were observed near the epithelial margin and broad bands of GFP-expressing fiber cells were present in the body of the lens. Labeling index data and cell lineage results are currently being integrated into a stochastic model of epithelial cell proliferation.
Conclusions:
The experiments support a model in which the fate of an epithelial cell (to divide, to perdure, or to die) is a function solely of its latitudinal position in the epithelium. In the adult lens, where the epithelial cell population is constant, cell proliferation in the PGZ and GZ is predicted to result in mitotic pressure pushing cells towards the equator at ever-increasing velocities. We have termed this the "penny pusher" phenomenon, after the popular arcade game. This mode of growth appears to be distinct from stem cell driven proliferation, as seen for example in the corneal epithelium. Finally, clonal expansion in the proliferative regions suggest that large groups of fiber cells are derived from individual epithelial cell progenitors. This may have implications for cortical cataract formation in humans.
Keywords: proliferation • computational modeling • microscopy: confocal/tunneling