Cone and rods in the retina of the zebrafish.

A: Zebrafish retinas at 3 days-post-fertilization (dpf). The neural retina consists of six major classes of neurons including two types of photoreceptors, rods and cones. B: Rod and cone photoreceptors function under the dim and normal light condition, respectively. They have a very specialized apical membrane structures called the outer segment (OS), in which phototransduction molecules function. Genetic defects cause photoreceptor degeneration, which are majorly classified into rod-cone dystrophy called retinitis pigmentosa (rods initially degenerate, followed by cone degeneration), cone-rod dystrophy (cone degeneration, followed by rod degeneration), and Leber congenital amaurosis (LCA). LCA is a severe disease with congenital loss of both cones and rods at early stage. Degenerated photoreceptors are shown here in dark grey.