Modulation of cell proliferation in the embryonic retina of zebrafish (Danio rerio)

We describe light-microscopically the development of the embryonic zebrafish eye with particular attention to cell number, cell proliferation, and cell death. The period from 16 to 36 hr post fertilization (hpf) comprises two phases; during the first (16-27 hpf) the optic vesicle becomes the eye cup, and during the second (27-36 hpf) the eye cup begins to differentiate into the neural retina and pigmented epithelium. All cells in the eye primordium are proliferative prior to 28 hpf, and the length of the cell cycle has been estimated to be 10 hr at 24-28 hpf (Nawrocki, 1985). Our cell counts are consistent with that estimate at that age, but not at earlier ages. A 10-hr cell cycle predicts that the cell number should increase by 7% per hr, but during 16-24 hpf the cell number increased by only 1.5% per hr. Despite the low rate of increase, all cells labeled with bromo-deoxyuridine, so all were proliferative. We considered three possible explanations for the nearly-constant cell number in the first phase: proliferation balanced by cell emigration from the eye, proliferation balanced by cell death, and low proliferation caused by a transient prolongation of the cell cycle. We excluded the first two, and found direct support for the third. Previous examinations of the cell cycle length in vertebrate central nervous system have concluded that it increases monotonically, in contrast to the modulation that we have shown. Modulation of the cell cycle length is well-known in flies, but it is generally effected by a prolonged arrest at one phase, in contrast to the general deceleration that we have shown.