Genetically associated similarities and differences in the generation of neurons comprising an early developing reflex pathway in mouse spinal cord.

Tritiated thymidine autoradiography has been used to study the generation of lateral motor neurons (LMNs), association interneurons (ANs) and dorsal root ganglion cells (DRGNs) in the spinal cords of genetically diverse strains of mice. The neuronal populations analyzed in this study form an early developing reflex pathway and the ontogeny of this circuit exhibits genetically associated variability. The strains of mice used in this investigation have been shown to differ in the embryonic age at which forelimb reflex movements are first manifest and in the timing of synapse formation within the reflex pathway. A precocious development of these reflex traits occurs in strain C57BL/6J in comparison to embryos of intermediate (CBA/CaJ) and late developing (LP/J) strains. All three inbred strains show the same basic generation sequence for the neuronal populations comprising the forelimb reflex pathway. The generation of LMNs precedes that of ANs, and the generation of ANs, in turn, precedes that of DRGNs. Since this is the same sequence as that observed for the formation of synaptic junctions in all three strains, it is suggested that synaptogenic sequences in reflex circuits may be determined by the generation sequence of the component neuronal populations. Although the strains all exhibit the same basic sequence of neuronal generation, the temporal relationships of the generation of each cell population within this sequence show significant strain dependent variations. C57BL/6J displays a larger temporal separation between the generation of each cell type than LP/J, and CBA/CaJ is intermediate to the other two strains in this respect. The fact that this strain order is identical to that observed for the development of reflex traits suggests that genetically associated differences in the timing of neuronal birthdays within a common generation sequence may have a substantial influence on the timing of synaptogenesis within the reflex pathway.