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Fig. 4. Regulation of other homeobox genes by Sax1. (A-C) Lateral view of
brains at HH18, stained for Emx2 mRNA in blue and GFP mRNA
in red. After electroporation of the control construct (A), the single
Emx2 stripe in the ventral mesencephalon (the dorsal stripe only
appears at HH20) is evident. Ectopic expression of Sax1 represses
Emx2 expression in the mesencephalon (B, arrow), while expression of
VP16Sax1 leads to an enlargement of the ventral Emx2 stripe
(C, arrow), and also induces ectopic spots of Emx2 in the dorsal
mesencephalon and diencephalon (arrowheads). (D-F) Expression of Six3
in blue and GFP in red, following electroporation. The pattern of
Six3 is normal after electroporation of the control construct (D),
while ectopic expression of Sax1 abolishes the expression of
Six3 in the mesencephalon (E, arrow). VP16Sax1 has no effect
on Six3 expression (F, arrow). (G-I) Phox2a expression in an
HH21 embryo, 2 days after electroporation. In a control embryo,
Phox2a signals label the trochlear and oculomotor nuclei (G).
Unilateral ectopic expression of Sax1 leads to a rostral expansion of
the Phox2a expression in the oculomotor nucleus (H, arrow) compared
with the control side (arrowheads marks the normal rostral limit of
Phox2a expression), while VP16Sax1 represses Phox2a
expression in the oculomotor and trochlear nuclei (I, arrows). (J-L)
Pax6 expression in an HH21 embryo, 2 days after electroporation. The
Pax6 stripe in the ventral mesencephalon is largely unaffected by
Sax1 (K, arrowhead) or VP16Sax1 (L) expression. By contrast,
the ventral Pax6 domain in the pretectum, prominent in embryos
expressing the control construct (J), is lost following ectopic expression of
Sax1 (K, arrow). mes, mesencephalon; nIII, oculomotor nucleus; nIV,
trochlear nucleus; pt, pretectum.
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