A quantitative analysis of boutons with different types of synapse in normal and injured hypoglossal nuclei

Abstract

Boutons were counted and classified according to their synaptic specializations on electron micrographs of left hypoglossal nuclei from rats, 7 to 84 days after left hypoglossal nerve axotomy. Results were compared statistically with those from normal rats. Somatic and neuropil boutons with symmetrical synapses were commoner than those with asymmetrical synapses in normal rats. A few somatic boutons had subsynaptic cisterns instead of conventional synapses. After axotomy, boutons with symmetrical synapses decreased in numbers from 7 to 35 days if somatic, or from 14 to 49 days if in the neuropil. Neuropil boutons recovered steadily, but somatic boutons erratically, as regeneration proceeded. Somatic boutons with subsynaptic cisterns decreased from 7 to 63 days postoperatively, followed by recovery. Somatic and neuropil boutons with asymmetrical synapses were unaffected by axotomy. It had previously been shown that only boutons containing clear spherical synaptic vesicles were numerically reduced after axotomy. After synaptic analysis, the present study suggests that the vast majority of these had symmetrical synapses, and a small minority of somatic boutons had subsynaptic cisterns.