Intrinsic and commissural connections within the entorhinal cortex. An anterograde and retrograde tract-tracing study in the cat

Intrinsic and commissural connections within the entorhinal cortex (EC) were examined in the cat by the anterograde and retrograde tract-tracing methods with Phaseolus vulgaris leucoagglutinin and cholera toxin B subunit. Intrinsic axons to the superficial layers (layers I–III) arose mainly from layers II, III, Vd (deep part of layer V), and VI, were distributed more widely in the superficial layers than in the deep layers, and terminated progressively more densely in more superficial layers; most densely in layer I. In the medial entorhinal area (MEA) and the ventromedial and the ventrolateral divisions of the lateral entorhinal area (VMEA and VLEA), the longitudinal connections through the intrinsic fibers to the superficial layers is often more restricted in rostral direction than in caudal direction. In the dorsolateral division of the lateral EC (DLEA), the longitudinal connections through the intrinsic fibers to the superficial layers extended distantly in both rostral and caudal directions. Intrinsic fibers to the deep layers (layers IV–VI) originated mainly from layers IV and Vs (superficial part of layer V) and were distributed rather sparsely and diffusely; they were distributed more widely in the deep layers than in the superficial layers. Commissural axons to the homotopic EC regions originated from layers II and III of the MEA and DLEA and terminated in all EC layers, most densely in layer I.