Quantitative analyses of neuronal development in the lateral motor column of mouse spinal cord. II. Development of motor neuronal organelles.

The development of organelles within presumptive alpha motor neuronal somata was studied by electron microscopic morphometric analysis. Cells with large nuclei were selected for sampling from the lateral motor column of the brachial spinal cord of mouse embryos ranging in age from embryonic day 11 (E11) through E16. The first objective was to compare the cytodifferentiation of alpha motor neuronal somata among three genetically different strains of mice that differ in the development of forelimb reflex behavior and associated pathway synaptogenesis (Vaughn et al., '75). On the basis of multiple linear regression analyses, no significant differences were found among strains for either the initial levels or rates of cytodifferentiation. As a result, the data were combined for all three strains to analyze organelle changes during early neuronal development. The average areas of perikaryal cytoplasm and nuclei increased significantly. In addition, the relative areas of nucleoli, mitochondria, Golgi complexes, and rough endoplasmic reticulum increased, while the relative areas of heterochromatin and "free" ribosomes decreased. There were significant increases in the number of mitochondria and Golgi complexes per unit area of perikaryal cytoplasm. The average size of mitochondria appeared to increase during development, but was significantly smaller in adult alpha motor neurons than in embryonic specimens. In contrast, the average size of individual Golgi complexes was relatively constant throughout embryonic development, as well as in the adult. In general, the cytodifferentiation of alpha motor neurons appeared to progress in a relatively constant, linear fashion between E11 and E16.