Ultrastructural changes in the developing nucleolus following axotomy

Abstract

A large, basophilic, Feulgen-negative structure has been observed within the nucleoli of golden hamster facial motor neurons. This ‘intranucleolar body’ was seen, at the electron microscopic level, to be composed of granules which are thought to be ribonucleoprotein particles. At 15 days postnatal age, this aggregation of granules had not yet appeared. However, by 20–24 days, a small intranucleolar body had developed. At maturity, the intranucleolar body had a diameter of up to 2 μm. Facial nerve axotomy at 15 days did not appreciably alter the nucleolar morphology at 19 days; neither control (opposite side) nor experimental nucleoli contained intranucleolar bodies. Normal nucleoli developed intranucleolar bodies, however, between the ages of 20 and 24 days. After axotomy at 20 days and sacrifice at 24 days, a comparison of facial neuronal nucleoli demonstrated that while an intranucleolar body formed in the control, axotomy prevented this structure from forming at its normal time. Also, when the facial nerve of an adult hamster was axotomized and observed 4 days postoperatively, an intact intranucleolar body was not seen. Instead, the granular portions of the nucleolus appeared to have been dispersed into several small aggregates. We believe that the changes observed in granule distribution, both with development and after axotomy, are morphological indications of a qualitative and/ or a quantitative change in rRNA synthesis.