Expression of monoclonal antibody Q113 immunoreactivity in the rat cerebral cortex: Unique differential sublayering of layer I. Staining of radial glia

Monoclonal antibody mabQ113 recognizes a 120‐kilodalton polypeptide which, in the cerebellar cortex, is confined exclusively to a subset of Purkinje cells which are organized in parasagittal bands (Hawkes et al.: Brain Research 333:359–365, 1985). In all other areas of the adult rat brain examined the localization of the mabQ113 epitope was marked by regional neuronal and glial coexpression (Plioplys and Hawkes: Brain Research 375:1–12, 1986). Similar neuronal‐glial co‐expression was characteristic of the adult rat cerebral cortex. Intriguingly, mabQ113 revealed a unique differential sublamination of layer I. In the neocortex, layer I was split into two sublayers, with the more superficial sublayer weakly stained and the deeper sublayer stained more intensely, whereas in the pyriform cortex, layer I was split into three. These sublaminations do not correspond to previously described subdivisions of layer I. In the developing cortex, the mabQ113 epitope is found in radial glial fibers. Stained radial fibers are first seen beginning at E17, reach a maximum at P4 and finally disappear between P12 and P14. The laminar distribution of mabQ113‐immunoreactivity emerges earlier in the pyriform cortex than the neocortex: the sublamination of layer I is seen at P4 in the pyriform cortex but not until P8 in the neocortex. The significance of these observations is discussed.