Cell migration into the "established" lateral motor column in Rana pipiens larvae.

Neuronal turnover has been demonstrated during the period of differentiation and maturationof 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 forthe brachial LMC. This is consistent with a caudo-cephalic direction of development amongthe two pairs of motor columns in the spinal cord. Since neuronal turnover occurs followingthe initial formation of the LMC and continues after the maximum number of cells has beenattained, it is proposed that neuronal turnover may be a mechansim that allows for neuralreadjustment and refinement during spinal cord development.