MORPHOLOGICAL PROPERTIES OF INTRACELLULARLY LABELED LAYER-I NEURONS IN RAT NEOCORTEX

The morphology of neurons in layer I of rat neocortex, including Cajal -Retzius (CR) cells, was studied by using intracellular biocytin stain ing in brain slices obtained from rats during the first 22 postnatal d ays. Within the first postnatal week, horizontal bipolar neurons or CR cells were prominent in layer I. Typically, CR cells had one main den drite and one axon originating from opposite poles of the somata. Even though the main dendrites and axons could be quite long, complex dend ritic or axonal arbors were not observed. Starting around postnatal da y 6 (PN 6), CR cells were less frequently observed. From PN 10 to PN 2 1, nonpyramidal neurons with diverse morphologies became the main neur onal component in layer I. The somata of layer I nonpyramidal neurons were quite variable in size and shape. Dendrites were smooth or sparse ly spiny, and the dendritic trees were mainly restricted to layer I, c overing an area with a diameter of about 200 mu m. Axon collaterals of these cells formed elaborate arbors with diameters of around 700 mu m in layer I and extending, in many cases, to layer II/III and even lay er IV. This extensive axonal plexus provides a rich anatomical base on which layer I neurons, functioning as local circuit elements, may int eract with each other and with neurons in other layers. (C) 1996 Wiley -Liss, Inc.