APOPTOSIS INDUCED BY METHYLAZOXYMETHANOL IN DEVELOPING RAT CEREBELLUM- ORGANIZATION OF THE CELL-NUCLEUS AND ITS RELATIONSHIP TO DNA AND RIBOSOMAL-RNA DEGRADATION

We present a cytological and biochemical study of the cell death of gr anule cell precursors in developing rat cerebellum following treatment with the cytotoxic agent methylazoxymethanol (MAM) during the first p ostnatal week. The density of apoptotic figures per square millimeter progressively increases after 6, 12, 24 and 44 h of treatment, whereas cells immunoreactive for proliferating cell nuclear antigen tend to d isappear in the external granular layer (EGL). DNA migration on gel el ectrophoresis reveals a typical ladder pattern of internucleosomal cle avage following MAM treatment, whereas gel electrophoresis of rRNA sho ws a conspicuous degradation of both 28S and 18S rRNAs. Ultrastructura l analysis has revealed the alterations of structures containing chrom atin and ribonucleoprotein (RNP) in dying cells of the EGL. The typica l granular beaded configuration of the condensed chromatin changes to a denser, more homogeneous texture suggesting nucleosomal disruption. The reorganization of RNP nuclear domains is reflected by the appearan ce of dispersed nucleoplasmic RNP particles and the formation of a coi led-body-like structure. However, typical nuclear domains involved in the splicing of RNAs, namely interchromatin granule clusters and typic al ''coiled bodies'', are not found in apoptotic cells. Intranuclear b undles of filaments have also been detected. In the cytoplasm, the pre sence of dispersed single ribosomes is an initial sign of apoptosis. T he massive dispersion and disruption of ribosomes detected after 24 h and 44 h of MAM treatment is reflected by the degradation of both 28S and 18s rRNAs. These results show that MAM treatment pro vides a usefu l experimental model for the study of apoptosis in the developing cent ral nervous system. The organization of the cell nucleus in cells unde rgoing apoptosis clearly reflects a disruption of the nuclear compartm ents involved in transcription and the processing and transport of RNA and is related to the patterns of DNA and rRNA degradation.