Reorganization of centrosomal marker proteins coincides with epithelial cell differentiation in the vertebrate lens

The differentiation of epithelial cells in the vertebrate lens involves a series of changes that includes the degradation of all intracellular organelles and a dramatic elongation of the cells. The latter is accompanied by a substantial remodelling of the cytoskeleton and changes in the distribution of the actin, microtubule and intermediate filament cytoskeletons during lens cell differentiation have been well documented. There have, however, been no studies of microtubule organizing centres (MTOCs) and specifically centrosomes during lens cell differentiation. We have investigated the fate of the centrosomal MTOCs during cellular differentiation in the bovine lens using γ-tubulin, ninein, centrin 2 and centrin 3 as markers. Our studies show that these markers oscillate between a clear centrosome-based association in epithelial cells and a defocused cluster in lens fibre cells. Our data further reveal a transient loss of signal for the typical centrosomal marker γ-tubulin as the lens epithelial cells begin to differentiate into lens fibre cells. This marker apparently disappears in the most distal epithelial cells at the lens equator, only to reappear in early lens fibre cells. The changes in γ-tubulin distribution are mirrored by the other centrosomal markers, centrins 2 and 3 and ninein that also show a similar transient loss of their signals and subsequent clustering at the apical ends of differentiating fibre cells. The transient loss of staining for these centrosomal markers in the most posterior epithelial cells is a distinctive feature that precedes lens cell elongation. The dramatic reorganization of MTOC markers coincides with gap junction reorganization as seen by the loss of connexin 43 (α1-connexin) in these lens epithelial cells suggesting that these events mark a significant change preceding subsequent cell elongation and differentiation into fibre cells.