Stability of neuron and glial number in the parabigeminal nucleus of the ageing mouse.

The number of neurons and glia in the albino mouse parabigeminal nucleus remains constant between 6 and 31 months of age. The presence of a small number of mitotic and pyknotic cells in the parabigeminal nucleus indicates a slow turnover of the glial population throughout life. Examination of the superior colliculus showed that degenerating neurons were present from 6 months of age and neurons appeared to be much more sparsely scattered in the superficial layers of aged mice than in young adults. The stability of the neuron number in the parabigeminal nucleus may be due to the reciprocal innervation of neurons of the superior colliculi and parabigeminal nuclei protecting each other from transneuronal degeneration which might otherwise occur as a consequence of loss of retinal ganglion cells. Alternatively, since each neuron in the parabigeminal nucleus probably synapses with many tectal neurons, the loss of only some of its target neurons would probably not lead to transneuronal degeneration.