NUCLEAR RIBONUCLEOPROTEIN-CONTAINING STRUCTURES UNDERGO SEVERE REARRANGEMENT DURING SPONTANEOUS THYMOCYTE APOPTOSIS - A MORPHOLOGICAL-STUDYBY ELECTRON-MICROSCOPY

The rearrangement of nuclear ribonucleoprotein (RNP)-containing struct ures during spontaneous thymocyte apoptosis has been investigated by e lectron microscopy. Along with chromatin margination and condensation, RNPs segregate into the central portion of the nucleus, in the form o f heterogeneous clusters of granules which contain interchromatin and perichromatin granules, perichromatin fibrils, nucleolar components, a nd probably coiled bodies. In parallel with progressive chromatin cond ensation and karyorrhexis, granule clusters are then extruded into the cytoplasm and are finally released at the cell surface as membrane-bo und cytoplasmic debris, sometimes in association with apparently undam aged organelles such as centrioles. It is Likely that this RNP segrega tion may correlate with a severe impairment of protein synthesis. A si milar phenomenon was observed in elongating spermatids, when transcrip tional arrest is induced during the process of reversible silencing of the male genome. It may be hypothesized that segregation into heterog eneous granule clusters could be a common mechanism to remove redundan t RNP-containing structures from the cell.