Loss of motor neurons from the median nerve motor nucleus of the mutant mouse 'wobbler'.

This paper describes the location and number of motor neurons in the median nerve pool of wobbler mice and normal littermates as determined by retrograde labelling of the cut median nerve with horseradish peroxidase (HRP) in animals from 3 weeks to 1-year-old. The median nerve motor nucleus is located in spinal segments C5-T1, and in normal animals contains 199 (6) (mean (SEM] motor neurons. Three-week-old wobbler mice have the same number of labelled neurons as control animals, and this number falls to 75% of normal values by 4 weeks of age, and to approximately 60% by 6 weeks of age and older. Numerous swollen, pale and frequently vacuolated perikarya are present in the same 3-6-week-old mice. In the 3-week-old mutants these comprise on average 17% of the total large (greater than 20 microns) neuronal cell bodies counted in segments C5-T1. By 6 weeks this figure has fallen to 10%, and to less than 4% in adult wobblers. We conclude that the most active period in the expression of the wobbler phenotype is from 3 to 6 weeks of age.