Loss of neurons from the motor nucleus of the facial nerve in the ageing mouse brain.

The number of neurons and neuronal nuclear diameter was estimated in the motor nucleus of the mouse facial nerve at 6, 25, 28 and 31 months of age. Neuronal number decreased significantly (P less than 0.01) from 2010 +/- 58 to 1699 +/- 42 and neuronal diameter (P less than 0.001) increased significantly (P less than 0.001) from 11.6 +/- 0.1 microns to 14.2 +/- 0.2 microns between 25 and 28 months of age. Facial motor neurons lost Nissl substance between 6 and 25 months of age and also showed an accumulation of lipofuscin at 28 and 31 months. A small number of degenerating neurons was present in the motor nucleus of the facial nerve at 28 and 31 months.