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

The aim of the present study was to characterise 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 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. © 1996 Wiley‐Liss, Inc.