Postnatal development and cell death in the sciatic motor nucleus of the mouse

Motoneurons of the sciatic nerve were studied in the lumbar cord of the mouse, at various postnatal stages, 1-3 days, 6-7 days, three weeks, and 2-3 months. Horseradish peroxidase (HRP) was applied at the cut end of the distal sciatic nerve, and, after retrograde transport, transversal and longitudinal histological sections of the cord were made in order to compare various features of the motor pool such as cell grouping, position in gray matter, cell number, cell size, and gross dendritic patterns. As early as birth, sciatic nerve motoneurons were grouped in a longitudinal column in the homolateral gray matter. No labelled cells were found outside the column either ipsilaterally or contralaterally. The location of the column was constant throughout postnatal development, encompassing the same spinal root levels and occupying the same position in the gray matter, the lateral portion of the ventral horn. Cell volume showed a 4-5 fold increase. Dendrites were not visible at the early stages and became labelled at 2-3 months. The neuropil also displayed a 4-5 fold increase, causing considerable spacing of the motoneurons. Histograms of cell body volume showed the progressive differentiation of the initially uniform motoneuronal pool in different sub-populations. In each experimental animal, labelled cells were counted in all histological sections and, for statistical comparison, the value was corrected for mean cell size. A 31% decrease in the number of labelled cells was observed between birth and 2-3 months. The rate of cell death was not constant, since 17% disappeared during the first week and 14% thereafter. The 31% decrease was lower than that observed in previous studies in the rat, 50-80%. The difference in species may explain this discrepancy, however, technical differences must also be invoked. In these studies, HRP was injected into the biceps brachii muscle. An overestimation of cell death is possible with this technique because postnatal changes of the axonal endings permit easier HRP uptake in immature muscles than in mature muscles. The cut nerve technique however by-passes the axonal endings.