An in situ screen for genes controlling cell proliferation in the optic tectum of the medaka (Oryzias latipes).

The optic tectum is a dorsal, prominent and well corticalised structure of the fish brain. It grows according to a pattern exceptional in the vertebrate central nervous system, by addition of radial columns of cells at its periphery. We took advantage of this peculiar feature to readily identify genes differentially expressed in the tectal proliferative (marginal) vs. post-mitotic (central) zones. Out of 500 medaka cDNA clones screened by WMISH, more than 100 were expressed in one or the other of these zones. Unexpectedly, we also identified a small class of genes expressed between these two zones. All the characterised genes of this class encode down regulators of the cell cycle. Therefore, such a screening strategy allows in particular cases to raise testable hypotheses on the involvement of genes in the control of the cell cycle, in addition to characterising unknown genes with patterned expression related to cell proliferation.