Investigation of the migration path for new rod photoreceptors in the adult cichlid fish retina.

In the retina of adult teleost, precursor cells divide in the outer nuclear layer and give rise to new rod photoreceptors. These new rods migrate from the outer limiting membrane to the inner edge of the outer nuclear layer (ONL) before differentiating. In order to understand which cues these cells use during migration and insertion at the appropriate location we combined cell-specific stains in the retina of the cichlid fish Haplochromis burtoni, viewed with confocal laserscan microscopy: Dividing cells were labeled with bromodeoxyuridine (BrdU), Müller glial cells, cone photoreceptors, and horizontal cells were detected by specific antibodies. During the migration phase (24 to 48 h after BrdU uptake) up to 46% of BrdU-labeled cells were spindle shaped and radially oriented. Most of them were in direct proximity to Müller cell processes. Four days after BrdU-uptake, most labeled cells (91%) were found in the inner portion of the ONL and displayed a spherical shape. This marks the end of the movement of the new rods. At this stage, the labeled cells showed no preference to lie near glial fibers but were often found close to the pedicles of double cones. The leading edge of migrating cells reached into the outer plexiform layer (OPL) but not further than processes of horizontal cells. This is beyond the location of mature rods. We hypothesize that the cells are repelled in the OPL and insert back in the ONL to differentiate as rods.