Immunocytochemical detection of Reissner's fiber-like glycoproteins in thesubcommissural organ and the floor plate of wildtype and cyclops mutant zebrafish larvae

The subcommissural organ (SCO) and the floor plate (FP) secrete high molecu lar weight glycoproteins that polymerize in the form of the Reissner's fibe r (RF). To study to what extent the absence of the FP affects the expressio n of these glycoproteins, we have investigated the brain and spinal cord of 48-h and 72-h wildtype and cyclops (cyc) mutant zebrafish larvae by using a polyclonal antiserum against bovine RE Wildtype larvae showed immunoreact ivity in the SCO at the dorsal forebrain-midbrain boundary. In the ventricl e, over the SCO surface, thin immunoreactive fibers aggregated into an RF t hat ran along the third and fourth ventricles and the central canal of the spinal cord until, at its caudal end, the fiber disintegrated and formed a strongly immunoreactive massa caudalis that left the neural tube and invade d the surrounding tissues of the tail fin. The rostral end of the FP, linin g the pontine flexure, was also strongly immunoreactive, as was the caudal third of the FR Cyc mutants showed an immunoreactive SCO and fibrous materi al in the ventricle, but an RF was missing. There was no label in the ventr al midline of the neural tube except in some specimens in which the caudal FP persisted and was immunoreactive. It is concluded that the product of th e cyc gene is not required for the expression of SCO glycoproteins but for their polymerization into an RF in the brain ventricles.