Spatial Coordinates of the Rostral Junction Points of the Genu of the Corpus Callosum and the Habenular Commissure in Tree Shrew, Mouse and Rat Brains: A Preliminary Study.

Spatial coordinate systems are essential for precise brain localization and atlas-based comparison, yet current stereotaxic frameworks mainly rely on cranial landmarks such as bregma-lambda (BL) or anterior commissure-posterior commissure (AP). Here, we defined the rostral junction point of the genu of the corpus callosum (GCCrj) that refers to the central convergence point of bilateral genu of the corpus callosum structures at the rostral part on coronal histological slices. The GCCrj and rostral junction point of the habenular commissure (HBCrj) constituted the new GCCrj-HBCrj (GH) coordinate plane, which is located at the midline of the cerebral hemispheres. We described and delineated the GCCrj and HBCrj reference points across mouse, rat and tree shrew brains. We performed conversions between the GH points and the points of the existing BL and AP coordinate systems in standard brain atlases for each species. The height values for the GH system's G and H points were -0.55, +0.15 and -0.05 mm across the three species, respectively. The horizontal plane of the GH system showed superior agreement with the horizontal plane of the BL coordinate system compared to that of the AP system within existing atlases. In summary, this study provides the spatial coordinates of the GCCrj and HBCrj as new reference coordinates for a multi-species brain atlas. The two landmarks GCCrj and HBCrj are positioned on both the horizontal plane and cerebral mid-sagittal vertical plane, running parallel to the BL reference plane. The novel GH coordinate system is compatible with established standard stereotaxic brain coordinate systems and may provide an alternative for a mammalian brain coordinate atlas.