Retinal neurogenesis: The formation of the initial central patch of postmitotic cells

We have investigated the relationship between the birthdate and the onset o f differentiation of neurons in the embryonic zebrafish neural retina. Birt hdates were established by a single injection of bromodeoxyuridine into emb ryos of closely spaced ages. Differentiation was revealed in the same embry os with a neuron-specific antibody, zn12. The first bromodeoxyuridine-negat ive (postmitotic) cells occupied the ganglion cell layer of ventronasal ret ina, where they formed a small cluster of 10 cells or less that included th e first zn12-positive cells (neurons). New cells were recruited to both pop ulations (bromodeoxyuridine-negative and zn12-positive) along the same fron t, similar to the unfolding of a fan, to produce circular central patch of hundreds of cells in the ganglion cell layer about 9 h later. Thus the form ation of this central patch, previously considered as the start of retinal neurogenesis, was actually a secondary event, with a developmental history of its own. The first neurons outside the ganglion cell layer also appeared in ventronasal retina, indicating that the ventronasal region was the site of initiation of all retinal neurogenesis. Within a column (a small cluste r of neuroepithelial cells), postmitotic cells appeared first in the gangli on cell layer, then the inner nuclear layer, and then the outer nuclear lay er, so cell birthday and fell fate were correlated within a column. The ter minal mitoses occurred in three bursts separated by two 10-h intervals duri ng which proliferation continued without terminal mitoses. (C) 1999 Academi c Press.