There is no loss of motor neurons in the rat spinal cord during postnatal maturation

Motor neurons are lost during embryonic development, but it remains controv ersial whether motor neuron cell death occurs during postnatal life. In thi s study we investigated the effect of postnatal maturation on the number of intact spinal motor neurons in the rat using retrograde labelling with mod el-based counting, and an unbiased stereological counting technique. To det ermine the number of motor neurons innervating a specific forelimb muscle i n rats of different postnatal ages FluoroGold was injected into the flexor carpi radialis. Before postnatal day 21 there were higher numbers of retrog radely labelled motor neurons than in adult rats, suggesting a 'loss' with postnatal maturation. This loss may be attributed to tracer diffusion to ad jacent muscles and to the permeability of the muscle spindle capsule in you nger animals. To obtain an unbiased estimate of the number of motor neurons in the C7 and C8 segments of the postnatal rat cervical spinal cord the fr actionator/optical disector counting technique was used. This method did no t show a loss of spinal motor neurons between birth and adulthood. The main conclusion from this study is that there is no loss of spinal motor neuron s during postnatal maturation.