ELECTRON-MICROSCOPIC STUDY OF CELL DEGENERATION IN CHICK-EMBRYO SPINAL GANGLIA

Cell degeneration which normally occurs during development was studied under EM in thoracic spinal ganglia of chicken embryos. Cells undergoing degeneration were often identified as primitive neuroblasts or transitional cells between the primitive and intermediate neuroblast, and sometimes as intermediate neuroblasts. Degeneration seems to begin with nuclear alterations (chromatin condensation and segregation of the nucleolar components) and ribosomal changes (polysomal disaggregation, and displacement of many membrane-associated ribosomes). Soon the Golgi complex is no longer recognizable. Successively, large masses of finely granular material become prominent in the nucleus, the nuclear envelope breaks down, many mitochondria appear altered, many vacuoles with clear contents become evident and ribosome crystals appear. At the end of the process the degenerated cell appears as a shrunken mass of granular debris and membrane fragments. Some neuroblastic processes contain clumped filaments and microtubules and/or polymorphic dense bodies. These structural changes are not unique to this degenerative process, but closely resemble those following other types of injuries of the nerve cell. A drastic reduction in the nuclear synthesis of RNA, and a profound impairment in protein synthesis are probably the main biochemical events which occur in these degenerating cells. Degeneration, which probably affects the neuroblasts which fail to establish appropriate connections with the periphery, seems to be one of the major factors controlling cell number in the spinal ganglia.