A GOLGI-STUDY OF THE PRINCIPAL PROJECTION NEURONS OF THE MEDIAL CORTEX OF THE LIZARD PODARCIS-HISPANICA

The medial cortex of lizards is a simple three-layered brain region di splaying many characteristics that parallel the hippocampal fascia den tata of mammals. Its principal neurons form a morphologically diverse population, partly as a result of the prominent continuous growth of t his nervous center. By using the classic Golgi impregnation method, we describe here the morphology of the principal neurons populating the medial cortex of Podarcis hispanica. These were projection neurons giv ing off descending axons. These axons displayed deep collateral branch es provided with prominent axonal boutons, while the main axonal branc h reached adjacent cortical areas and the bilateral septum. According to three main classification criteria, dendritic tree pattern, dendrit ic spine covering, and some size, we have distinguished eight differen t types of projection neurons. Five of them, ''heavily spiny granular' ' (monotufted, medium-sized), ''heavily spiny bitufted'' (large), ''sp iny bitufted'' (medium-sized), ''sparsely spiny bitufted'' (small), an d ''superficial multipolar'' (small), were found in the cell layer, wh ereas the three others lay outside this layer and were regarded as ect opic types (''outer plexiform ectopic bitufted,'' ''inner plexiform ec topic bitufted'', and ''inner plexiform monotufted''). Additional seco ndary criteria, soma position and shape, allowed us to further classif y bitufted neurons into three distinct subtypes each: ''superficial-ro und,'' ''intermediate-fusiform,'' and ''deep-pyramidal.'' Moreover, a variety of small impregnated cells were observed; they probably repres ented newly generated immature neurons that had not yet completed thei r development. These cell types were compared with those reported prec iously in Golgi, immunocytochemical, and electron-microscopy studies, both in the reptilian medial cortex and in the mammalian dentate area. Presumably age-related changes and synaptic relationships of these pr ojection cells in the medial cortex circuitry were analyzed. (C) 1997 Wiley-Liss, Inc.