Cell migration into the “established” lateral motor column in Rana pipiens larvae. II. Lumbosacral spinal cord and comparative aspects

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Abstract
Neuronal turnover has been demonstrated during the period of differentiation and maturation of motor neurons in the developing lateral motor columns of Rana pipiens larvae. As determined by thymidine‐3H labeling, cells continued to migrate into the lateral motor column (LMC) while there was a concurrent reduction in LMC cell numbers. Notably, cells in the lumbosacral LMC ceased to be labeled following thymidine‐3H injection at an earlier time than for the brachial LMC. This is consistent with a caudo‐cephalic direction of development among the two pairs of motor columns in the spinal cord. Since neuronal turnover occurs following the initial formation of the LMC and continues after the maximum number of cells has been attained, it is proposed that neuronal turnover may be a mechanism that allows for neural readjustment and refinement during spinal cord development.