Quantitative analyses of neuronal development in the lateral motor column of mouse spinal cord. I. Genetically associated variations in somal growth patterns.

The relative somal and nuclear sizes of neurons in the lateral motor column (LMC) of adult and embryonic mouse brachial spinal cord were determined by light microscopic morphometry. Three genetically varying mouse strains, previously shown to differ in the development of a forelimb reflex pathway, were studied. In adults, the size distribution of somata, nuclei, and nucleoli were bimodal for each strain, indicating that there are two distinct size classes of LMC neurons. The size division between large and small LMC neurons differed among strains with more large LMC neurons occurring in strain CBA/CaJ than in either LP/J or C57BL/6J. In embryos, the growth of LMC cells was studied by determining the average area of nuclear profiles for specimens ranging in age from embryonic day 11 (E11) to 16. The average nuclear profile area increased significantly during this period in all three strains, and differences were found in the initial size and apparent rate of growth among strains. Early in development (E11-12), strain differences in apparent cell size were: C57BL/6J greater than CBA/CaJ greater than LP/J, and this strain order corresponds to observed strain differences in the onset of reflexogenesis and synaptogenesis (Vaughn et al., '75). Later in development (E16), strain differences in apparent cell size were: CBA/CaJ greater than LP/J greater than or equal to C57BL/6J, and this relationship corresponds to a more rapid increase of presumptive afferent synapses in CBA/CaJ than in the other two strains between E15 and E16. Possible causal relationships among neuronal size, growth, and synaptogenesis are suggested by these strain differences.