Genetically-associated variations in the development of hippocampal pyramidal neurons may produce differences in mossy fiber connectivity.

The neuronal generation patterns of hippocampal pyramids were analyzed in two inbred strains of house mice (SM/J and BALB/cJ) using triated thymidine radioautography. These two strains of mice possess markedly different patterns of mossy fiber synapses upon pyramidal neurons within a specific segment of regio inferior. The results of this study show that this same segment of regio inferior also displays markedly different patterns of pyramidal neuron generation in the two strains. Specifically, the pyramids of this segment of regio inferior are generated according to the typical "inside-out" sequence in SM/J mice, but this pattern is reversed into an "outside-in" sequence in BALB/cJ mice. Remaining segment of regio inferior, and all of regio superior, is formed in an inside-out sequence in both strains. These results strongly suggest that altered temporal patterns of pyramidal neuron generation may play a major role in determining mossy fiber connectivity patterns.