Micrographs of D-V sections of ED4.5 midbrains (H-E stained). A. Control. B. GliA electroporated. C. Shh non-ventralized electroporated. D. Shh partially ventralized electroporated, The intertectal sulcus (arrowhead) that separates the left and right tectal hemispheres tend to disappear in GliA electroporation and disappears in Shh electroporated specimens. E. Detail of the roof plate (the thinner neuroepithelial band at the bottom of the intertectal sulcus) alterations produced by GliA and Shh electroporations. From top to bottom: Control, GliA electroporated, Shh (non-ventralized) electroporated and Shh (partially ventralized) electroporated dorsal midbrains. Graded alterations of the roof plate (medial zones indicated with dotted lines) can be observed in electroporated specimens. In GliA treated the roof plate can still be recognized at the bottom of the intertectal sulcus; in Shh (non-ventralized) specimens the roof plate is poorly demarcated and difficult to identify; in Shh (partially ventralized) midbrains the roof plate is no longer recognized. Arrows: indicate the position of the original sulcus limitans in a partially ventralized midbrain. SL: sulcus limitans. SL’: additional sulcus limitans typical of partially ventralized specimens. Scale bars: 50 μm.