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Steven Bassnett, Yanrong Shi; Substructure of the germinative zone of the mouse lens. Invest. Ophthalmol. Vis. Sci. 2014;55(13):744.
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© ARVO (1962-2015); The Authors (2016-present)
The germinative zone (GZ) of the lens epithelium contains the mitotically-active cell population and serves as the growth engine of the lens. Despite its key role, the GZ is geographically ill-defined. As part of ongoing efforts to model lens growth, we analyzed cell proliferation patterns within the GZ across the mouse life span.
Mice (2 weeks to 46 months of age) were given an IP injection of EdU to label S-phase cells. 1 hr after EdU administration mice were euthanized by CO2 inhalation and whole lenses were processed for confocal microscopy. The polar and equatorial regions of the lens were imaged in three dimensions. Maximum intensity projections were generated from stacks of optical sections and analyzed by computer-assisted morphometry. Age-dependent changes in epithelial cell population and age/latitude-dependent variations in cell size and mitotic index were computed.
Over the age range studied here, equatorial and polar diameters of the mouse lens increased by >40%. Despite a concomitant increase in lens surface area, the number of epithelial cells decreased from approximately 50,000 at 2 weeks to 40,000 at six months. This decline was paralleled by an increase in the surface area of individual cells that was first evident in the central epithelium and subsequently spread towards the equator. Analysis of mitotic index within the GZ revealed a decline in peak index from 7% at 2 weeks to <2% at 46 months. At all ages studied, the variation in mitotic index with distance from the equator had a bimodal profile, with the first peak located 150 micrometers from the epithelial margin and the second peak located approximately 280 micrometers from the margin. Long-term labeling protocols indicated that while most mitoses produced daughter cells that became lens fibers, some (notably those located in the distal, pre-germinative zone (pre-GZ)) where incorporated into the epithelial population.
Our data suggest that the GZ can be divided into two sub-regions. The bimodal distribution of S-phase cells within the GZ is not consistent with a model in which a posterior/anterior concentration gradient of a single diffusible mitogen regulates epithelial cell proliferation. We also identify a pre-GZ (in which the mitotic index is significantly lower than in the GZ) that extends a further 300-400 micrometers anterior to the GZ. The majority of lens epithelial cells are located within either the GZ or pre-GZ.
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