Pigmentarchitectonic subfields of the entorhinal region as revealed in tangential sections

The entorhinal region is an important center of the limbic system involved in many dementing disorders. The boundaries of the entorhinal region of the left and right hemispheres were investigated in tangential sections (5 individuals, age range 21 to 29 years). This method preserves the rostral portion of the entorhinal region which is usually lost in coronal sectioning. The sections were stained with the pigment-Nissl-method. The superficial cellular layer of the centromedial part of the entorhinal region consists of large heavily pigmented neurons forming islands clearly separated from each other. The anterior and posterior parts of the entorhinal region display an opposite pattern consisting of small islands and stripes with ill-defined boundaries. The islands contain small and sparsely pigmented neurons surrounded by large and well pigmented cells. Close to the adjacent proisocortex, the small cell containing islands confluent while the large and well pigmented neurons disappear. Hence, the medial side of the entorhinal region extends up to the uncus and the lateral side into the main branch of the rhinal sulcus. The entorhinal region covers the frontal portion of the parahippocampal gyrus up to the periamygdaloid cortex and the posterior part ends acute-angled within the medial portion of the parahippocampal gyrus.