The absence of significant postnatal motoneuron death in the brachial and lumbar spinal cord of the rat.

Motoneuron numbers in the brachial and lumbar lateral motor column (LMC) of the rat were examined quantitatively on embryonic day (E) 15 and postnatal days (P) 1-5. There was a significant (40-45%) loss of motoneurons in both regions between E15 and P1. No further loss was observed between P1 and P5. The decline of motoneurons between E15 and P1 is very likely due to cell death since there were considerable numbers of dying motoneurons present on E15. By contrast, few dying motoneurons were observed on P1-P5. Possible reasons for discrepancies between the present results and previous investigations are discussed. It is concluded that there is not a significant loss of either brachial or lumbar motoneurons in the postnatal rat and thus that postnatal rearragements of muscle innervation are most likely the result of axon elimination, not cell death.