Evidence for neural stem cells in the medaka optic tectum proliferation zones.

Few adult neural stem cells have been characterized in vertebrates. Although teleosts continually generate new neurons in many regions of the brain after embryogenesis, only two types of neural stem cells (NSCs) have been reported in zebrafish: glial cells in the forebrain resembling mammalian NSCs, and neuroepithelial cells in the cerebellum. Here, following our previous studies on dividing progenitors (Nguyen et al. [1999]: J Comp Neurol 413:385-404.), we further evidenced NSCs in the optic tectum (OT) of juvenile and adult in the medaka, Oryzias latipes. To detect very slowly cycling progenitors, we did not use the commonly used BrdU/PCNA protocol, in which PCNA may not be present during a transiently quiescent state. Instead, we report the optimizations of several protocols involving long subsequent incubations with two thymidine analogs (IdU and CldU) interspaced with long chase times between incubations. These protocols allowed us to discriminate and localize fast and slow cycling cells in OT of juvenile and adult in the medaka. Furthermore, we showed that adult OT progenitors are not glia, as they express neither brain lipid-binding protein (BLBP) nor glial fibrillary acidic protein (GFAP). We also showed that expression of pluripotency-associated markers (Sox2, Musashi1 and Bmi1) colocalized with OT progenitors. Finally, we described the spatio-temporally ordered population of NSCs and progenitors in the medaka OT. Hence, the medaka appears as an invaluable model for studying neural progenitors that will open the way to further exciting comparative studies of neural stem cells in vertebrates.