Qualitative and quantitative study of synaptic displacement in chromatolyzed spinal motoneurons of the cat

Qualitative and quantitative cytological evidences show a reduction in number and percentage of coverage of terminals on chromatolyzed spinal motoneurons 4 to 90 days after severance of their axons in the ventral root. The reduction in number and percentage of boutons on the surface membrane is due to detachment of synapses. This detachment is associated with changes in specialized regions (synaptic complexes: Conradi, '69) of synapses which involve disappearance of both pre-and postsynaptic membrane thickenings and widening of the synaptic cleft (width of 400-1,400 A). The separation of terminals from surface membrane is also associated with the presence of reactive astrocytes and microglia. Most areas of surface membrane bared of synaptic contact are occupied by astrocytic processes. The astroglial responses coincide with the loss of synapses. Microglial cells, although extending to neuronal surface membrane, cover only a small portion of it. An extensive separation of boutons by microglia is not observed. Most microglia are seen located at some distance from surface membrane and are frequently separated by sheets of astrocytic processes. Synaptic restoration occurs by about 90 days after axotomy. Some motoneurons show good recovery, while others show only a limited amount.