Neural degeneration and non-neuronal cellular reactions in the hypoglossal nucleus following an intraneural injection of toxic ricin.

The present study describes neuronal degeneration and its accompanying non-neuronal cellular reaction in the hypoglossal nucleus following an intraneural injection of Ricinus communis agglutinin-60 (RCA-60) into the hypoglossal nerve. The first noticeable structural changes were observed in neurons in hamsters killed 3 days after the RCA injection. Drastic alterations occurred in the period extending from the 5th to the 15th postoperative day. Two forms of neuronal degeneration were observed: light and dark types. In the light type, masses of free ribosomes were observed; other changes included the dilation of Golgi saccules and the presence of abnormal mitochondria. In the dark type of degeneration, the cells became condensed with vacuoles in their cytoplasm. Axon terminals presynaptic to the degenerating cells during this period appeared to be normal. A massive influx of mononuclear leucocytes by diapedesis occurred at the large venules. Some of the infiltrated cells were clearly lymphocytes, while others were monocytes which became indistinguishable from indigenous microglia once they were in the neuropil. Neural macrophages, most probably derived both from microglia and the infiltrated monocytes, were engaged in the phagocytosis of neuronal debris. A remarkable finding in the present study was the wide-spread occurrence of dark axon terminals in the neuropil in longer surviving animals (90 and 120 days). The structural alterations, e.g., clumping and swelling of some of the synaptic vesicles in the enhanced cytoplasmic density, suggest that these were undergoing atrophic changes resulting from the long period of dysfunction following the death of postsynaptic neurons induced by RCA.