Ultrastructural localization of stage-specific neurite-associated proteins in the developing rat cerebral and cerebellar cortices.

SNAP/TAG-1 is a glycoprotein of 135 kDa and is expressed on the surface of a subset of growing axons in the developing rodent CNS. The ultrastructural localization of this antigen was analysed in embryonic day 17 cerebral cortex and postnatal days 4 and 8 cerebellar cortex of rats using immunoelectron microscopy with a monoclonal antibody which recognizes SNAP/TAG-1 (4D7), and peroxidase-conjugated secondary antibody. In the embryonic cortex, immunoreactivity was associated with the plasma membranes of restricted groups of axons, neuronal somata and their leading processes located in the intermediate zone, subplate and cortical plate. Immunoreactive axons were bundled together in groups of 10-20 and were separated from non-immunoreactive axons. Some growth cones were immunoreactive; however, not all growth cones of 4D7-immunoreactive axons showed staining. In the postnatal cerebellum, immunoreactivity was associated with the somata and axons of granule cells that are located in the most internal portion of the external granule cell layer. In cerebral and cerebellar cortices, immunoreactivity appeared in corresponding points of adjacent cell membranes in punctuate fashion and with a regular periodicity of 100-200 nm. The possibility that SNAP/TAG-1 is acting as an adhesion molecule among specific subgroups of axons in the developing CNS is discussed.