EMBRYONIC FORM OF N-CAM AND DEVELOPMENT OF THE RAT CORTICOSPINAL TRACT - IMMUNOELECTRON MICROSCOPIC LOCALIZATION DURING SPINAL WHITE-MATTERINGROWTH

The neural cell adhesion molecule N-CAM has been proposed to function in the guidance of outgrowing axons in the peripheral and central nerv ous system. Light microscopic observations have shown that the embryon ic form of N-CAM (200-230 kDa) is present in the ventralmost part of t he dorsal funiculus during corticospinal tract (CST) ingrowth in the f irst postnatal week (Joosten, Dev. Brain Res., 78 (1994) 226-236). Her e, the subcellular localization of the embryonic form of N-CAM (E-NCAM ) is determined by pre-embedding staining on vibratome sections and by postembedding immunogold-labelling on Epon embedded spinal cord secti ons. The electron microscopical observations indicate that E-NCAM is p resent on the outer membrane of CST growth cones as well as other unmy elinated axons which are present in the ventralmost part of the dorsal funiculus. Furthermore, E-NCAM is localized in an irregular patchy wa y on the outer side of the axonal membrane of small unmyelinated, late r arriving CST axons. From these results it may be deduced that E-NCAM is involved in CST tract formation through guidance of outgrowing pio neer CST growth cones along other unmyelinated axons and through media tion of axon fasciculation of later arriving CST axons.