Eye See You
• Fish eyes continue to grow larger throughout their lives because of stem cells that are concentrated at the ends of the red arcs (nerve tissue) seen in this zebrafish eye. This allows the visual cells of the retina to be repaired and regenerated continuously. The retina is seen wrapped around the lens (green circle with black center) in this cross-section.
• The eye is really an outgrowth of the brain formed during embryo development. Take a look at the orange cells in the eyefield (inset A; ef) pushed to form two lateral bulges by the advancing midline (A-B; blue).
• Humans (and other mammals) lack stem cells in the adult eye although research is focusing on Müller cells, a type of glial cell that may be able to regenerate neurons and photoreceptors lost to disease and injury.
Image source: http://bpod.mrc.ac.uk/archive/2013/3/7
Inset diagram: http://www.ucl.ac.uk/zebrafish-group/research/eye.php