Early development of photoreceptors in the ventral retina of the zebrafish embryo.

The first photoreceptor outer segments in the retina of the zebrafish Brachydanio rerio appear in the embryo 2.5 days after fertilization, as revealed by scanning and transmission electron microscopy. These outer segments arise in a small region ventral to the exit of the optic nerve. Ultrastructural features of the developing photoreceptor cells, especially those that distinguish the rods and cones, are described. By 3 days after fertilization, the time of hatching, photoreceptor outer segments are widespread in the retina. However, at this time and in the young larva the early developing ventral region remains distinctive because of its conspicuous population of rods.