Morphometric analysis of neuronal soma size within the motor nucleus of a transplanted murine skeletal muscle

Since the number of motor neurons supplying a muscle graft is reduced, the peripheral field for the surviving motor neurons would be enlarged. This possible change in motor unit size may result in morphologic changes in the size of motor neurons within the motor neuron pool of the graft. The extensor digitorum longus (EDL) muscle from 19 transplanted and 17 age-matched normal 129 ReJ female mice were injected with horseradish peroxidase in order to examine the cell size distributions of the motor neuron pools supplying these muscles. Computer-assisted morphometric analysis of cell sizes within the motor neuron pool to the transplants indicated a significant shift in cell size, with the largest areas ranging between 630 and 1250 micron2. A number of the alpha neurons supplying the grafts were twice the average cell size for the control population (means = 592 micron2). The increase in the number of large motor neurons indicates an hypertrophy of neurons reinnervating the grafts. The long-term graft is reinnervated by a decreased population of motor neurons (means = 7), which vary in size from small to very large, reflecting both changes in the possible source of the nerve reinnervating the graft as well as alteration in the size of the motor unit, respectively.