Dendritic synapses of anterior horn neurons in amyotrophic lateral sclerosis: an ultrastructural study

This ultrastructural study deals with the synapses of primary dendrites of the anterior horn neurons in the lower lumbar spinal cords of seven patients with amyotrophic lateral sclerosis (ALS) who had mild neuronal depletion. Specimens from seven age-matched, neurologically normal individuals served as controls. In each instance, the autopsy was performed within 6 h after death. Our results indicate a statistically significant increase in degenerative changes in the dendrite presynapses of normal-appearing neurons of the ALS patients. The alterations included aggregation of electron-dense synaptic vesicles and dark mitochondria with dense cristae, and bundles of neurofilaments. However, we found no significant difference between controls and patients with respect to cross-sectional area and length of the dendrites, number of synapses per dendrite, and lengths of individual synapses and their active zones in the normal-appearing neurons, even though the patients' neurons had a smaller cross-sectional area. In chromatolytic neurons, the number of synapses and the length of the active zone of the primary dendrites were significantly diminished. These findings suggest that despite degenerative changes of the presynapses, the synapses in the primary dendrites of the anterior horn neurons are preserved at the early stage of ALS. The preservation of these synapses may be due to their relative resistance to degenerative processes, or may represent a compensatory mechanism of the synapses for diminished synaptic function in distal dendrites.