A novel monoclonal antibody K1 recognises early neurones in the rat cortex

The aim of this study was to produce monoclonal antibodies specific for neurones that are generated earliest in the rat neocortex. One of the established clones, K1, showed a strong immunoreactivity in the marginal zone at the 19th day of gestation (E19). The immunoreactivity of K1 initially appeared in the primordial plexiform layer at E15, in the subplate at E16, and in the marginal zone by E17. It became undetectable in the first postnatal week. The immunoreactivity was not detected in the neocortex of adults or elderly. Western blotting analysis revealed reactive bands at positions corresponding to proteins of 290 and 280 kDa for the neocortical membrane fractions prepared from E16 to E21 embryos. In these stages, smears of bands were also found at positions corresponding to higher molecular weights. A single band of protein of 280 kDa was detected for the soluble fractions prepared from the embryos at E19 and E21. These reactivities were susceptible to protease, but not to enzymatic or chemical destruction of carbohydrate residues. Electron microscopic analysis showed that the K1 immunoreactivity was detected primarily on the cellular membranes of neurites. In the marginal zone at E19, the K1 immunoreactivity was localised where neurites make contact with other neurites or somata. A portion of these contact points had typical features of synapses. In the cortical plate of the same stage, arrays of tiny K1-immunoreactive puncta were observed on a subset of radial processes. These results suggest that monoclonal antibody K1 is a marker recognising neurites of subplate neurones that extend radially and make neuronal contacts in the marginal zone.