Expression of a novel N-CAM glycoform (NOC-1) on axon tracts in embryonic Xenopus brain

The aim of the present study was to characterize the expression pattern of a novel N-CAM glycoform, called NOC-1, within axon tracts of the developing Xenopus brain. Double-label immunostaining of wholemount embryonic brains revealed that only a subpopulation of axons express NOC-1. Axons in the supra-optic tract (SOT) express NOC-1 and course ventrally from the presumptive telencephalon to join the t tract of the post-optic commissure (TPOC), the principal longitudinal tract of the forebrain. NOC-1 was also expressed by axons that form discrete fascicles in the ventral portion of the TPOC. Growth cones in the SOT appeared to only turn caudally upon contacting the NOC-1 expressing axons in the ventral region of the TPOC. Thus, all NOC- 1 expressing axons within the TPOC follow a common ventral pathway. These observations demonstrate that there is selective partitioning of axons which express a novel N-CAM glycoform within the developing Xenopus forebrain. Moreover, these results indicate that the SOT-TPOC junction is an important choice point for ventrally growing SOT axons and that selective interactions occur between subpopulations of axons expressing NOC-1 in the SOT and TPOC