Stability of motor neuron number in the oculomotor and trochlear nuclei of the ageing mouse brain.

The total number of neurons in the oculomotor and trochlear nuclei of the mouse brain was estimated in three sets of serial sections at 6, 25, 28 and 31 months of age. Due to the oculomotor nucleus being completely intact in only one side of the brain, in each set of sections some counts were carried out in sagittal sections from the right half of the brain and other counts were carried out in coronal sections from the left half of the brain. It proved impossible to identify the boundary between the oculomotor nucleus and trochlear nucleus in the coronal sections so that in such sections the combined total of neurons in both nuclei was estimated. The oculomotor nucleus could be clearly identified in sagittal sections and therefore the total number of oculomotor neurons was estimated in eight brains (three at 6 months, two at 25 and 28 months and one at 31 months). The trochlear nucleus was intact in all but one of the sets of sagittal sections and trochlear neuron number was estimated separately in eleven brains. The total number of neurons in both nuclei did not vary significantly with age (mean 432). The mean number of oculomotor neurons in the eight brains was 341 and the mean number of trochlear neurons was 77. There was no variation in nuclear diameter at different ages nor was there any marked loss of Nissl substance or lipofuscin accumulation in the motor neurons with age.