INVERTED PYRAMIDAL NEURONS AND INTERNEURONS IN CAT CORTICAL SUBPLATE ZONE ARE LABELED BY MONOCLONAL-ANTIBODY SP1

During development, the subplate zone of the cat neocortex contains ne uronal populations with distinct morphological and neurochemical pheno types. A subset of those are specifically recognized by a mouse monocl onal antibody termed SUBPLATE-1 (SP1), which was generated against tis sue homogenates of kitten cortical white matter. SP1 stains cell bodie s and proximal dendrites, but rarely distal dendrites, axonal arbors o r spines. In order to characterize morphologically the SP1 immunoreact ive subplate cell types, we combined SP1 immunohistochemistry with int racellular iontophoretic injections of Lucifer yellow. The majority of double-labelled neurons were inverted pyramids with a single thicker spine-covered dendrite that descended into the white matter and a tuft of thinner spinous dendrites that ascended from the upper somatic pol e, but generally remained confined to the white matter. Other double-l abelled neurons were multipolar to bitufted, although often equipped w ith one thicker descending dendrite. In inverted pyramidal cells, the axons originated from the descending dendrite or, more rarely, from th e lower portion of the soma, and descended into the white matter. They formed collaterals recurring toward the grey matter. The presence of dendritic spines on double-labelled pyramidal cells and the axonal arb orization patterns were two novel features not revealed previously by SP1 immunohistochemistry alone. The inverted pyramidal morphology was typical for double-labelled neurons located in the subplate zone below the apices of the gyri, whereas those located below the flanks or sul ci or deep in the white matter often displayed a bitufted or multipola r spinous morphology. A minority of the double-labelled neurons were m ultipolar with smooth dendrites and locally branching axons. These res ults suggest that in the cat subplate zone, a majority of the cells ex pressing the SP1 antigen are spinous, and we termed the spinous subpla te cells 'subplate pyramidal neurons'.