An ultrastructural study of normal and injured hypoglossal nuclei after injection of horseradish peroxidase.

The left hypoglossal nuclei of normal rats, and rats which had received left hypoglossal nerve axotomies 7-21 days previously, were studied by electron microscopy after injection of horseradish peroxidase as a marker of extracellular space and pinocytosis. Quantitative analysis showed that the number of pinocytotically-derived structures in presynaptic boutons was significantly increased in rats at 7, 14, and 21 days after axotomy when compared with normal rats. It was suggested that presynaptic boutons which became detached from injured neurones retracted by a membrane cycling mechanism involving pinocytotic uptake of bouton plasmalemma, without compensatory membrane production elsewhere. It was confirmed that the channels in the microglial cells communicated with the extracellular space.