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Rin, a novel cell-surface protein that labels reticular neurons early in chick neurogenesis.

Rin is a large cell-surface glycoprotein that we have recently purified from chick brain, with a molecular weight of approximately 200 kD. Protein microsequence obtained from immunopurified rin does not match any sequences in the Genbank data base. Based on the sequence information and on its localization in the early chick embryo, rin is a novel cell-surface protein. Rin is expressed on the surface of many, but not all, axons in the developing chick nervous system. In the chick hindbrain, rin is expressed on reticular neurons, the first neurons to extend axons within the brain. Cranial motoneurons, which extend axons just a few stages later, do not express rin. Rin-positive axons pioneer the caudal section of the medial longitudinal fasciculus. The very first rin-positive axons that reach the floorplate do not enter the floorplate, but remain ipsilateral. Some of the next immunopositive axons to reach the floorplate do cross the midline, often with an alteration in trajectory, and often extending within the floorplate for some distance before reaching the other side. The failure of the very first rin-positive axons to cross the floorplate, and the changes in trajectory observed when the next axons extend onto the floorplate, suggests that early differentiating neurons cross the midline with some difficulty.