A qualitative solution with quantitative potential for the mouse hippocampal cortex flatmap problem

The hippocampal formation (HPF) is a focus of intense experimental investigation, particularly because of its roles in conscious memory consolidation, spatial navigation, emotion, and motivated behaviors. However, the HPF has a complex three-dimensional geometry resulting from extreme curvature of its layers, and this presents a challenge for investigators seeking to decipher hippocampal structure and function at cellular and molecular scales (neuronal circuitry, gene expression, and other properties). Previously, this problem was solved qualitatively for the rat by constructing a physical surface model of the HPF based on histological sections, and then deriving from the model a flatmap. Its usefulness is exemplified by previous studies that used it to display topological relationships between different components of intrahippocampal circuitry derived from experimental pathway-tracing experiments. Here the rat HPF flatmap was used as a starting point to construct an analogous flatmap for the mouse, where the great majority of experimental hippocampal research is currently performed. A detailed account of underlying knowledge and principles is provided, including for hippocampal terminology, and development from an embryonic nonfolded sheet into differentiated multiple adjacent cortical areas, giving rise to the adult shape. To demonstrate its utility, the mouse flatmap was used to display the results of pathway-tracing experiments showing the dentate gyrus mossy fiber projection, and its relationship to the intrahippocampal Purkinje cell protein 4 gene-expression pattern. Finally, requirements for constructing a computer graphics quantitative intrahippocampal flatmap, with accompanying intrahippocampal coordinate system, are presented; they should be applicable to all mammals, including human.