mbl Mutants Have Parapineal Migration Defects and Bilaterally Symmetric Habenulae (A–H and A′–H′) Dorsal or frontal views of the epithalamus (anterior to the top) of (A, A′, D, and D′) 2-day-, (B and B′) 2.5-day-, (F and F′) 3-day-, and (E, E′, G, G′, H, and H′) 4-day-old wild-type and mbl mutant embryos. All markers used in the panels are indicated on the left (the text color matches the expression domain color in double labelings). (A–A″ and C) gfi is expressed exclusively in the parapineal (midline is indicated by the red dotted line). Of the two mbl embryos shown, one shows normally migrated parapineal cells, and the other shows parapineal cells at the midline. (B and B′) Embryos carried a foxD3:GFP transgene [mbltm213 x Tg(foxD3:GFP)]. Double labeling shows that, even in the presence of migrating parapineal cells, lov gene expression remains low in the left habenula of the mutant (B′). (H and H′) The arrows mark the neuropil of the medial left habenula, which is reduced in the mbl mutant. (I and I′) Dorsal views of 3D reconstructions of confocal images of habenular axon terminals in the target IPN nucleus labeled with lipophilic dyes as indicated. (A″ and D″–H″) Graphs illustrate the percentage of embryos with wild-type (WT) left, reversed (rev) right, medial (med), bilateral (bil), or not visible (nv) gene expression or neuropil formation. Bilateral right (bil-right) indicates that both habenulae exhibit the profile of gene expression or neuropil formation characteristic for the right habenula of wild-type embryos. The graph in (I″) shows that in nearly all mbl embryos the axonal projections coming from the habenulae intermingle in the ventral IPN.
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