The genetic organization of neuron number in the pyramidal cell layer of hippocampal regio superior in house mice.

This report concerns variations in neuron number within the pyramidal cell layer of hippocampal regio superior in 18 inbred strains of house mice. There is a genetically associated variability in the total number of neurons in this pyramidal layer. Systematic strain variations in the orientation of the pyramidal cell layer are also present. Relations between the numbers of neurons in various experimenter-defined subdivisions of regio superior were examined following statistical corrections for the variations in orientation. This led to a preliminary delineation of 4 genetically-defined subdivisions of the regio superior pyramidal cell layer.

The genetic organization of neuron number in the granule cell layer of the area dentata in house mice.

This report concerns variations in neuron number within the granule cell layer of the area dentata that occur among inbred strains of house mice. There is genetically associated variability in the total number of neurons present, with a very substantial range of estimated values. Systematic strain variations in the orientation of the granule cell layer are also present. When statistical corrections for variations in orientation are made, associations between the neuron numbers of subdivisions of the granule cell layer are consistent with the presence of common genetic determination of neuron number throughout the entire lamina.

The genetic organization of neuron number in Ammon's horns of house mice.

This study of the Ammon's horns of 20 inbred strains of house mice had 3 primary objectives. The first was to determine whether there was genetically-associated variability in the number of neurons present within a dorsal and a ventral portion of this brain region. If genetically-associated variability proved to be present, there were two additional objectives which we wished to achieve. One was to determine the degree to which genetically-associated variations in neuron number are confined to specific neuronal regions, or shared between neuronal regions. The other was to identify specific extreme strains for further study. A section cut in a coronal plane was selected for a sampling within a dorsal portion of Ammon's horn, and a section cut in a horizontal plane was selected for a sampling within a ventral portion. Counts were made of neuronal nuclei in the granule cell layer of the dentate gyrus and in the pyramidal layers of regio inferior and regio superior of the hippocampus. Cross-sectional areas for these somal laminae were also measured and planimetric neuron densities were obtained by computing the ratios of nuclei counted to the areas of the appropriate somal laminae. Genetically-associated variability in counts of neuronal nuclei was observed for all neuronal regions within both the dorsal and the ventral sampled portions. Genetically-associated variability in planimetric neuron density was clearly present within the dorsal portion. The obtained counts of neuronal nuclei were adjusted statistically for variations in planimetric neuron density to provide the best indices of variation in neuron number presently available. Results for these adjusted neuron counts were as follows. First, there was an indication of substantial genetically-associated variability in neuron number. With the single exception of one neuronal region (dorsal regio superior), adjusted neuron counts exhibited significant genetically-associated variability for all neuronal regions within both portions sampled. Second, there was evidence for both separate and shared genetic determination of neuron number between neuronal regions. Strains were not uniformly high, intermediate, or low in their adjusted neuron counts. Instead, a complex patterning of high and low genetically-associated correlations was observed between neuronal regions and sampled portions. Third, extreme strains for each neuronal region were identified for the sampled dorsal and ventral portions. Differences between the neuron numbers of interconnected neuronal regions imply variations in connectivity. Such variations could provide a morphological basis for 'tuning' associated adaptive characteristics of populations of house mice to enhance the likelihood of their survival in environments with varying ecological requirements.