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 patter n of a novel N-CAM glycoform, called NOC-1, within axon tracts of the developing Xenopus brain. Double-label immunostaining of wholemount em bryonic brains revealed that only a subpopulation of axons express NOC -1. Axons in the supra-optic tract (SOT) express NOC-1 and course vent rally from the presumptive telencephalon to join the tract of the post -optic commissure (TPOC), the principal longitudinal tract of the fore brain, NOC-1 was also expressed by axons that form discrete fascicles in the ventral portion of the TPOC. Growth cones in the SOT appeared t o only turn caudally upon contacting the NOC-1 expressing axons in the ventral region of the TPOC. Thus, all NOC-1 expressing axone within t he TPOC follow a common ventral pathway. These observations demonstrat e that there is selective partitioning of axons which express a novel N-CAM glycoform within the developing Xenopus forebrain. Moreover, the se 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. (C) 1996 Wiley-Liss, Inc.