Redefining the boundaries of the hippocampal CA2 subfield in the mouse using gene expression and 3‐dimensional reconstruction

Abstract

The morphology of neurons in the main divisions of the hippocampal complex allow the easy identification of granule cells in the dentate gyrus and pyramidal cells in the CA1 and CA3 regions of Ammon's horn. However, neurons in the CA2 subfield have been much more difficult to reliably identify. We have recently identified a set of genes whose expression is restricted to either the dentate gyrus, CA1, CA2, or CA3. Here we show that these genes have an essentially nonoverlapping distribution throughout the entire septotemporal extent of the hippocampus. 3‐Dimensional reconstruction of serial sections processed for in situ hybridization of mannosidase 1, alpha (CA1), bcl‐2‐related ovarian killer protein (CA3), and Purkinje cell protein 4 (dentate gyrus + CA2) was used to define the boundaries of each subregion throughout the entire hippocampus. The boundaries observed for these three genes are recapitulated across a much larger set of genes similarly enriched in specific hippocampal subregions. The extent of CA2 defined on the basis of gene expression is somewhat larger than that previously described on the basis of structural anatomical criteria, particularly at the rostral pole of the hippocampus. These results indicate that, at least at the molecular level, there are robust, consistent genetic boundaries between hippocampal subregions CA1, CA2, CA3, and the dentate gyrus, allowing a redefinition of their boundaries in order to facilitate functional studies of different neuronal subtypes in the hippocampus. J. Comp. Neurol. 485:1–10, 2005. © 2005 Wiley‐Liss, Inc.