Anatomic localization of the transentorhinal region of the perirhinal cortex

Brain imaging measures of entorhinal cortex and hippocampus volumes provide valid and reliable markers of Alzheimer's disease (AD). Since AD neurofibrillary pathology begins more laterally in the transentorhinal region (TR) of the perirhinal cortex, volumetric measures of this structure might provide more sensitive preclinical markers of AD, provided its anatomic location is known. The purpose of this study was therefore to define the anatomic location of the TR with respect to the collateral sulcus. Gallyas-stained coronal temporal lobe sections of consecutively autopsied patients were inspected and included in this study if the staining clearly marked the TR (n = 64). The number and depths of the collateral sulci were related to the lengths and placement of the TRs. Two patterns emerged: (1) if two discontinuous collateral sulci were present, the TR straddled the more medial and shallow collateral sulcus (8%); (2) if one collateral sulcus was present (91%), the TR included its medial bank. We provide more detailed descriptions of the lateral and medial TR borders for use in volumetric imaging studies.