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 molecule DM-GRASP/BEN/SC-1, is dynamically regulated. Here we demonstrate that the expression of neurolin correlates with early events of retinal ganglion cell (RGC) differentiation in zebrafish embryos. Neurolin mRNA first appears [28 h postfertilization, (PF)] in nasoventral cells, representing the first RGCs, then in dorsal, central (34 to 40 h PF) and temporal RGCs. After differentiation of RGCs in the central portion of the retina, RGCs exhibiting neurolin mRNA form rings. These rings move toward the retinal periphery and encompass older (central) RGCs. Thereafter, such as at 3.5 days PF, neurolin mRNA expressing RGCs are confined to the annular growth zone at the retinal peripheral margin. 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 significantly within 20 h after its appearance, which correlates with the arrival of axons in the tectum. This is followed by weakening of neurolin immunoreactivity on RGCs and axons. This pattern of RGC differentiation in zebrafish revealed by the expression of neurolin is unique among vertebrates. The spatiotemporal expression pattern of neurolin suggests a functional significance of this cell adhesion molecule in RGC recognition and RGC axon growth.