The topographical and laminar organization of a commissural-associational entorhino-entorhinal projection in the guinea pig.

A commissural-associational entorhino-entorhinal projection in the guinea pig was analyzed using anterograde and retrograde axonal tracing techniques. The projection originated in layer II and terminated in layer Ia of both the medial entorhinal area (MEA) and the lateral entorhinal area (LEA). A few cells in other layers, especially layer III, also contributed to the commissural system. The projection was largely homotopic with the exception of the most medial MEA, which projected ventrally like previously described projections from the para- and presubiculum to the superficial layers of the entorhinal area. The commissural fibers crossed the midline in the dorsal hippocampal commissure. The antero-posterior position of the fibers within the commissure reflected the ventrodorsal position of their origin in the entorhinal area.