wanderer mutants misroute TCA fascicles onto the ventral surface of the forebrain. (A) Dorsal views of a normal littermate (left) and wanderer mutant (right) show the wanderer mutant has reduced cortex (ctx) and olfactory bulb (ob) size, and reduced blue TCA staining in cortex. mb, midbrain. (B, C) All TCAs extend dorsally into the cortex of a wild-type brain, but in a wanderer brain, a subset of TCAs turn ventrally (red arrow) after crossing the DTB. dTh, dorsal thalamus; ot, optic tract. (D, E) Neurofilament-M (NFM) staining confirms that wanderer mutants have a thick bundle of axons extending ventrally from the internal capsule (ic) near the DTB. Scale bar, 0.5 mm. (F) Ventral views show aberrant TCA fascicles on the ventral surface of the wanderer mutant forebrain (red arrows), but the pontocerebellar projection and the optic tract (ot) appear normal. (The proximal optic nerve was torn off during dissection.) Dashed lines show approximate position of DTB. (G) Close-up of wanderer brain ventral surface in (F). Some axons grew all the way to the olfactory bulbs (ob) and appeared to make terminal boutons (long red arrows), while others extended shorter distances in a rostral trajectory (short red arrows), and one misrouted TCA fascicle grew along the optic tract (wide red arrow). (H, I) The caudal cortex overlying the thalamus was removed to show a lateral view of the pathways of TCAs and optic tract. A control brain (H) shows the TCAs traveling from dorsal thalamus (dTh) to neocortex (NCx), and the optic tract axons (ot, black arrows) coursing up the side of the diencephalon from the optic chiasm to the dLG nucleus of dTh. The lateral olfactory tract (LOT) projects caudally from the olfactory bulb. A similar view of a wanderer mutant brain (I) reveals a normal optic tract, but a misrouted bundle of TCAs (red arrows) derailed ventrally from the internal capsule, onto the lateral ventral surface of the forebrain toward the olfactory bulb.