Figure 1. Transition from radial glial cells to neurons.

A-B) Xenopus laevis tadpole head (A) and enlarged view of the midbrain (B) with the right lobe of the optic tectum (ot) outlined. The dotted line represents the medial edge of the dorsal tectum. Box width in A = 0.5 mm. C-F) Projections of 2-photon Z-stacks of Timer fluorescent protein expression in the optic tectum. Images were acquired from the same tadpole over time. Immature Fluorescent Timer emits a green fluorescent signal, but as the protein matures the emission spectrum changes to red fluorescent signal (shown as magenta). Coexpression of both immature and mature Timer protein fluorophores is shown as white. Initially the majority of the transfected cells are radial glia, identified by radial processes and pial endfeet (arrows), but over development there are fewer radial glia and most Timer-expressing cells are neurons, identified by elaborate dendritic arbors (final image, F). Asterisks indicate site of intertectal axons in the anterior dorsal commissure. Scale bar = 50 μm.