Morphogenesis of the optic tectum in the medaka (Oryzias latipes): A morphological and molecular study, with special emphasis on cell proliferation

We analyzed the medaka optic tectum (OT) morphogenesis by using 5-bromo-2'- deoxyuridine (BrdU) immunohistochemistry (with a new method we developed fo r pulse-labeling embryos) and in situ hybridization with three probes, two for recently cloned homeobox genes (Ol-Prx3 [Paired-Related-Homeobox3] and Ol-Gsh1 [Genetic-Screen-Homeobox1:]) and one for Ol-tailless. The tectal an lage first appears as a sheet of proliferating cells expressing Ol-Gsh1 and Ol-tailless but not Ol-Prx3. Cells subsequently cease to proliferate in a superficial and rostral zone and begin to express Ol-Prx3. When tectal lami nation begins, the proliferative zone (mpz) becomes restricted to a crescen t at the OT medial, caudal, and lateral margin. This mpz functions througho ut the fish's entire life. It produces cells that are added at the OT's edg e as radial rows, spanning every layer of the OT. The cells of the mpz cont inue to express Ol tailless in the adult, whereas Ol-Gsh1 expression is tur ned off. When superficial layers form, Ol-Prx3 expression becomes restricte d to the underlying deep layer, where it persists in the adult. Ol-Prx3 see ms to be a marker for the differentiation of a subset of deep cells and all ows analysis of tectal lamination, whereas Ol-tailless and Ol-Gsh1 could be involved in the control of tectal cell proliferation. This study constitut es a first step toward molecular approach to OT development in anamniotes. We compare and discuss the expression patterns of the homologs of the genes studied, and more generally the morphogenetic patterns of the medaka tectu m, with those encountered in other cortical structures and in other vertebr ate groups. (C) 1999 Wiley-Liss, Inc.