Development of the neural retina in the zebrafish, Brachydanio rerio (neurogenesis)

The embryonic zebrafish eye consists of two epithelia, neural and pigmented. Neuroepithelial cells are elongated with processes extending to both vitreal and scleral surfaces. Mitotic figures of M (mitotic) phase cells are positioned at the scleral (ventricular) surface. Elongated nuclei are present in S (DNA synthesis) phase cells in the middle of the neuroepithelium as seen in autoradiographs recording ('3)H-thymidine incorporation. Nuclei of cells in G (gap) phases are positioned between M and S phase cells. A cell cycle time of 10hr was calculated in the embryonic neuroepithelium at 1.0d (days postfertilization). During neuroepithelial cell proliferation, ('3)H-thymidine incorporation studies present evidence for two simultaneous gradients of cell birthdays. One gradient lies in the plane of the retina with the oldest cells in the center with cell age decreasing toward the peripheral margin. The other gradient lies perpendicular to the first across the retinal layers; the oldest cells are vitreally positioned and the progression of ages follows through to the youngest cells at the scleral surface. Postmitotic cells accumulate at the vitreal surface, while scleral cells continue to divide until all the cells of the central retina are produced. Before hatching at 3d, neuroepithelial cells of the central retina cease to proliferate. Differentiation is first observed with the formation of the optic nerve and later by the segragation of cell bodies into three nuclear layers, while their processes interconnect to form two plexiform layers. After this time, the oldest cells can be identified as ganglion cells. Interneurons and glial cells have intermediate birthdays, and photoreceptors are the last cells to arise. In the central retina, birthdays of all four cone photoreceptor types are the same, and most cones are postmitotic before the photoreceptor layer has formed. Each cone type then matures morphologically and arrives at its adult level in the photoreceptor layer. Short single cones become positioned at 6d, long single cones become positioned at 10d, while double cones remain in the most scleral cone position. In the ventral retina, rod photoreceptors have been identified, and their birthdays are the same as neighboring cones.