Quantitative analyses of neuronal development in the lateral motor column of mouse spinal cord. III. Generation and settling patterns of large and small neurons.

The generation and settling patterns of large and small lateral motor column (LMC) neurons were compared in the spinal cords of three inbred strains of mice by means of tritiated thymidine autoradiography. No significant strain differences were observed for the number of large LMC cells (presumptive alpha motor neurons) that were heavily labeled on each injection day, although there were significant strain variations for this measure with regard to small LMC neurons (presumed gamma motor neurons and interneurons). The generation of both large and small LMC neurons began at the same time, but peak production of large cells preceded that of the small neurons. There were no strain differences observed for this relationship between the large and small cells. These findings indicate that the LMC, from the time of its initial formation, contains cells destined to become large and small neurons. The positions of large and small neurons within the adult LMC relative to their times of origin (settling patterns) were analyzed statistically. A significant ventrodorsal sequence for early-to-late generated cells was observed for both large and small LMC neurons. No significant strain differences were found in the analysis of settling patterns. A ventrodorsal settling pattern also has been described for amphibia (Prestige, '73) and, in conjunction with the proximodistal sequence of limb development described by other investigators, the ventrodorsal sequence could play a key role in the development of motor neuronal somatotopic organization.