Layer-restricted Brn3b expression in the dorsal midbrain.

(A, B) Schematic diagrams of the midbrain after CTB injection into the contralateral eye (A); detailed layer distribution in the coronal section of the dorsal midbrain (top) and a sagittal image showing the location (dotted line) of the imaged coronal section (bottom) (B). (C, D) Brn3b expression (green), visualized by anti-Brn3b antibody is detected in the sSC, iSC, dSC, and PAG at P11 (C). Magnified view of the boxed area (D) shows Brn3b expression confined to the bottom layer of the sSC (SO). CTB labeling (red) divides the superficial SC into the SGS and SO. The aqueduct (Aq) is also shown. DAPI (blue). (E) Schematic diagram of the conditional Brn3b allele. Conditional deletion (by En1-Cre) removes an open reading frame of Brn3b and places a human placental AP under the control of the Brn3b promoter. (F) Dorsal midbrain section from Brn3bflox/+:: En1-Cre mouse histochemically processed for AP signals at P24. Brackets represent the layer segregation. (G-K) Axonal projections from the dorsal midbrain to adPBG and Pn (G), CnF (H), central lateral nucleus of the thalamus (CL) (I), lateral posterior nucleus (LP) and hypothalamus (HY) (J), vLGN and PRC (K). Each area was identified by its anatomical position (see Methods). (L) Schematic diagram of the midbrain Brn3b+ neuronal projections. Scale bars: 250 μm. adPBG, adjacent parabigeminal nucleus; AP, alkaline phosphatase; CnF, cuneiform nucleus; CTB, cholera toxin B; dSC, deep SC; iSC, intermediate SC; PAG, periaqueductal gray; Pn, pons; PRC, precommisural nucleus; SC, superior colliculus; SGS, stratum griseum superficiale; SO, stratum opticum; sSC, superficial SC; vLGN, ventral lateral geniculate nucleus.