Differential effects of axotomy on immature and mature hamster facial neurons: a time course study of initial nucleolar and nuclear changes.

The early nuclear and nucleolar responses at 0.5, 1, 2 and 4 days after axotomy were observed in neurons just before and after the completion of nuclear maturation. Axotomy of hamster facial motor neurons at a postnatal age of 15 days did not produce any changes within the nucleus that were significantly different from those of control cells. In addition, no significant changes were evident in the adults at 0.5 and 1 day after axotomy. At postoperative days 2 and 4, however, the adult neurons showed enlargement of the nucleus and nucleolus. Nucleolonemal strands became more rounded and distinct, and the large cluster of granules located centrally in the nucleolus disaggregated. The irregularly distributed clumps of nucleolus-associated chromatin dispersed to form a thin shell about the nucleolar periphery. In adults at postoperative day 4, the nucleoplasmic granules became more homogeneous and less distinctly outlined than normal. The peak of both nucleolar and nuclear responses coincided at 2 days after injury in the adult, i.e. 2 days before the previously documented chromatolytic peak at 4 days after injury. These studies on the ultrastructural level support our previous hypothesis that the 15-day neurons are synthesizing at peak capacity related to their rapid growth phase and cannot be stimulated further by axotomy. The adult neurons, however, do undergo a metabolic reorganization for regenerative synthesis, and the nucleolar and nuclear changes observed are indicative of transcriptive alterations involving the underlying genome.