Fish eye development

In the surface-dwelling fish, eyes form by a process quite similar to that in other vertebrates. First, the optic vesicle buds out from the brain and contacts the overlying ectoderm, which forms the lens precursor. The optic vesicle folds to become the optic cup, while the lens develops and becomes internalized. The optic cup then differentiates into the retina and the ectoderm overlying the lens becomes the cornea. In the cave fish, the lens and optic cup develop normally for the first 24 hours; after this, however, the lens cells undergo programmed cell death or apoptosis. The cornea and iris do not form and the retina is extremely disorganized. Growth of the eye stops and photoreceptor cells never develop. The rudimentary eye eventually sinks back and is covered by skin.The authors proposed two hypotheses for eye degeneration in the cave fish. In one, lens apoptosis is responsible for eye degeneration. In the other, the optic cup, which is diminished in the cave fish, produces insufficient amounts of some necessary signal, leading to the deterioration of the eye and lens.Yamamoto and Jeffrey transplanted embryonic lenses from eyed fish into the optic cups of eyeless fish and vice versa. They found that a lens from a surface fish could induce normal-looking eyes in the blind cave fish, whereas the reciprocal transplantation caused a degeneration in the eye of the surface fish. In each case, the unmanipulated eye on the other side served as a control. To determine whether programmed cell death was responsible for this deterioration, the authors looked for apoptosis in the donor lenses. They found that the cave fish lens still underwent apoptosis when implanted in a surface fish embryo and the resulting eye was small and disorganized. In the opposite experiment, the surface fish lens did not undergo apoptosis and was able to stimulate normal eye development in the blind cave fish host, including differentiation of a lens, retina, iris and cornea. The restored cave f