Fig. 7. Patterning of the chiasm at E13.5. Sections are horizontal, anterior is upwards. Immunohistochemistry was performed using antibodies for SSEA-1 (A-F) and Nkx2.2 (G,H). (A) Wild-type section through the optic chiasm showing SSEA-1-positive neurons arrayed in an inverted `V'-shape. Arrow indicates the tip of the `V'. SSEA-1 expression is also evident in the optic nerves (on). (B) A Foxg1^-/- section equivalent to (A) showing SSEA-1 expression in the mutant correlate of the optic nerve (on^*). Note that the plane of section is slightly tilted such that the left side is ventral to the right. The SSEA-1-positive `arm' of the `V' is visible on the right side, but is less clear on the left, which is further ventral. (C,D) Immediately dorsal to the optic chiasm, wild-type and Foxg1^-/- embryos show bilaterally symmetrical SSEA-1 expression posterior to the third ventricle. SSEA-1 expression extends posteriorly and laterally in both C and D. (E,F) High magnification showing the morphology of SSEA-1 neurons in the wild type and Foxg1^-/- mutant. (G,H) In the wild type and Foxg1^-/- mutant, Nkx2.2 is expressed at the posterior optic chiasm, where it overlaps partly with SSEA-1 expression. Broken lines indicate the position of the midline. Scale bar: 200 µm in A,B; 100 µm in C,D,G,H; 50 µm in E,F.