Nuclear ribonucleoprotein-containing structures undergo severe rearrangement during spontaneous thymocyte apoptosis....

The rearrangement of nuclear ribonucleoprotein (RNP)-containing structures during spontaneous thymocyte apoptosis has been investigated by electron microscopy. Along with chromatin margination and condensation. RNPs segregate into the central portion of the nucleus, in the form of heterogeneous clusters of granules which contain interchromatin and perichromatin granules, perichromatin fibrils, nucleolar components, and probably coiled bodies. In parallel with progressive chromatin condensation and karyorrhexis, granule clusters are then extruded into the cytoplasm and are finally released at the cell surface as membrane-bound cytoplasmic debris, sometimes in association with apparently undamaged organelles such as centrioles. It is likely that this RNP segregation may correlate with a severe impairment of protein synthesis. A similar phenomenon was observed in elongating spermatids, when transcriptional arrest is induced during the process of reversible silencing of the male genome. It may be hypothesized that segregation into heterogeneous granule clusters could be a common mechanism to remove redundant RNP-containing structures from the cell.