A Golgi study of the principal projection neurons of the medial cortex of the lizard Podarcis hispanica

Abstract

The medial cortex of lizards is a simple three-layered brain region displaying many characteristics that parallel the hippocampal fascia dentata of mammals. Its principal neurons form a morphologically diverse population, partly as a result of the prominent continuous growth of this 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 giving off descending axons. These axons displayed deep collateral branches provided with prominent axonal boutons, while the main axonal branch reached adjacent cortical areas and the bilateral septum. According to three main classification criteria, dendritic tree pattern, dendritic spine covering, and soma size, we have distinguished eight different types of projection neurons. Five of them, "heavily spiny granular" (monotufted, medium-sized), "heavily spiny bitufted" (large), "spiny bitufted" (medium-sized), "sparsely spiny bitufted" (small), and "superficial multipolar" (small), were found in the cell layer, whereas the three others lay outside this layer and were regarded as ectopic types ("outer plexiform ectopic bitufted," "inner plexiform ectopic bitufted", and "inner plexiform monotufted"). Additional secondary criteria, soma position and shape, allowed us to further classify bitufted neurons into three distinct subtypes each: "superficial-round," "intermediate-fusiform," and "deep-pyramidal." Moreover, a variety of small impregnated cells were observed; they probably represented newly generated immature neurons that had not yet completed their development. These cell types were compared with those reported previously 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 projection cells in the medial cortex circuitry were analyzed.