Medial cortex of the lizard Gekko gecko: a hodological study with emphasis on regional specialization

There is increasing evidence that the archicortex in mammals and reptiles is not a homogeneous structure. However, little is known about the regional specialization of this cortical area in reptiles. Therefore, the efferent connections of the medial cortex of the lizard Gekko gecko were studied with the anterograde tracer Phaseolus vulgaris-leucoagglutinin. The small-celled part of the medial cortex (Cxms) projects to various parts of the septum in a topological way: the rostral part projects to the anterior septal nucleus, whereas the caudal part projects to the lateral septal nucleus and the nucleus septi impar. In addition, Cxms projects to the large-celled part of the medial cortex (Cxml). Axons that originate from the dorsal part of Cxms terminate at the proximal parts of the apical and basal dendrites of the neurons of Cxml caudal to the injection site. In contrast, fibers originating from the ventral part terminate on more distal parts of the dendrites of neurons of Cxml rostral to the injection site. Other projections from Cxms to the dorsal cortex (Cxd) and the external amygdaloid nucleus were found. The Cxml projects bilaterally to Cxms. These projections terminate in the superficial and deep plexiform layers. In addition, projections to the cell plate of Unger, Cxd, and to the lateral septal nucleus were found. It appears, on the basis of the efferent connections, that Cxms can be divided into a rostral and caudal part, while hodological differences also exist between the dorsal and ventral parts of Cxms. The results of the present study do not suggest a subdivision of Cxml. The regional variations of the medial cortex in the lizard Gekko gecko differ from the regional variations described in other reptilian species.