Labeling of pyramidal and nonpyramidal neurons with lectin Vicia villosa during postnatal development of the guinea pig.

Labeling of cortical neurons with a lectin, Vicia villosa (VVA), was investigated in guinea pigs aged 1 day old to adult. Lectin histochemistry revealed a perineuronal sheath, which outlined the cell bodies, apical dendrites, and axon initial segments, in distinct populations of pyramidal and nonpyramidal neurons. Their laminar positions were segregated, with the pyramidal neurons confined to layer 5 and the nonpyramidal neurons distributed mainly in layers 3-5. The VVA-labeled substance(s) was detected at the interfaces between neurons and cellular elements present in the perisynaptic region, including glial processes and fine axons. However, it was excluded from synaptic clefts of presynaptic terminals. Double immunohistochemistry revealed that most of the VVA-labeled neurons were also labeled perineuronally with a monoclonal antibody, Cat 301, and vice versa. Dendritic patterns of the VVA-labeled pyramidal neurons were studied further by intracellular injection of Lucifer yellow into fixed slices. Apical dendrites had a considerable thickness before arborizing into a few daughter branches in layer 3 or 4, suggesting a morphological resemblance to intrinsic, bursting pyramidal neurons defined physiologically in vitro. During postnatal development, there was a global spatiotemporal pattern in the onset of VVA labeling of the cortical neurons. The labeling progressed from medial and posterior cortical areas, which are closely related to the hippocampal formation, to more lateral and anterior areas, which are less closely related. The labeling patter thus tends to follow the order of the phylogenetical development of the isocortex.