SPATIOTEMPORAL PATTERN OF RETINAL GANGLION-CELL DIFFERENTIATION REVEALED BY THE EXPRESSION OF NEUROLIN IN EMBRYONIC ZEBRAFISH

The expression of neurolin, the fish homologue of the cell adhesion mo lecule DM-GRASP/BEN/SC-1, is dynamically regulated. Here we demonstrat e that the expression of neurolin correlates with early events of reti nal ganglion cell (RGC) differentiation in zebrafish embryos, Neurolin mRNA first appears [28 h postfertilization, (PF)] in nasoventral cell s, representing the first RGCs, then in dorsal, central (34 to 40 h PF ) and temporal RGCs, After differentiation of RGCs in the central port ion of the retina, RGCs exhibiting neurolin mRNA form rings, These rin gs move toward the retinal periphery and encompass older (central) RGC s, Thereafter, such as at 3.5 days PF, neurolin mRNA expressing RGCs a re confined to the annular growth zone at the retinal peripheral margi n. Two hours after onset of mRNA expression, RGCs acquire antineurolin immunoreactivity on the surface of their somata and on their axons as they extend to the tectum. The mRNA signal in RGCs decreases signific antly within 20 h after its appearance, which correlates with the arri val of axons in the tectum, This is followed by weakening of neurolin immunoreactivity on RGCs and axons. This pattern of RGC differentiatio n in zebrafish revealed by the expression of neurolin is unique among vertebrates, The spatiotemporal expression pattern of neurolin suggest s a functional significance of this cell adhesion molecule in RGC reco gnition and RGC axon growth. (C) 1996 John Wiley & Sons, Inc.