CHANGES IN THE NUCLEOLAR AND COILED BODY COMPARTMENTS PRECEDE LAMINA AND CHROMATIN REORGANIZATION DURING FIBER CELL DENUCLEATION IN THE BOVINE LENS

Nuclear elimination accompanies differentiation in such specialized ce ll types such as erthyrocytes and lens fibre cells. It also accompanie s apoptosis which has suggested that similar processes could operate i n both. Denucleation occurs in the lens in order to reduce light scatt er and this process is often disrupted in cataract. Using the adult bo vine lens as a model system, nuclear changes accompanying denucleation are described with particular emphasis on the lamina, nucleolar and c oiled body compartments in lens nuclei. Nuclear shape, chromatin reorg anization and chromatin breakdown were also monitored to correlate the timing of events. Rearrangement of both A-and B-type nuclear lamins o ccured in parallel with chromatin condensation and preceded changes in nuclear shape. The earliest changes detected in this study occured in the coiled body and nucleolar compartments using coilin and fibrillar in antibodies respectively, suggesting that a shutdown in transcriptio n is an early event in denucleation. Fibrillarin redistributed from an open floret pattern to several condensed spots which gradually decrea sed in intensity and eventually disappeared. Coilin, however, was loca lized in several microfoci prior to being reorganized into fewer large r foci. Prior to chromatin condensation, coilin redistributed to the n ucleolar compartment and was absent from nuclei where chromatin had be gun to condense. Such nuclei were positive by TUNEL staining. In contr ast to the nucleus, mitochondrial degradation in lens fibre cells was a rapid process and involved a relatively sharp transition between pos itive and negative fibre cells for two mitochondrial specific markers, BAP 37 and prohibitin. A link between the changes in the nuclear lami na and chromatin with the initiation of mitochondrial fragmentation wa s also observed. Therefore, it is possible that the signal for the ini tiation of denucleation could originate from the mitochondria as propo sed for apoptosis. Differences between apoptosis and lens fibre cell d enucleation were noted and included the timescale of nuclear changes a s well as the persistence of a nuclear remnant. These studies suggest that transcriptional shutdown precedes lamina reorganization and chrom atin breakdown during lens fibre cell denucleation.