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Fig. 7. RA signalling is required and is sufficient for the specification of
intermediate cells. (A) Lateral views of stage 20-22 chick embryos generated
in New Culture from stage 10 embryos grafted with either control beads (upper
panel) or BMS453 beads (lower panel) into the prospective telencephalon. (B,D)
Consecutive transversal sections of New Culture embryos, showing the
expression of transcription factors in the telencephalon. The broken circle
indicates the position of the bead. (B) Embryos (n=10) grafted with
control beads in the prospective telencephalon (upper panel) expressed
Meis2+ cells in the intermediate region at mRNA level,
PAX6+ cells in the dorsal region and NKX2.1+ in the
ventral region of the telencephalon. Embryos (n=12) grafted with
BMS453 beads into the prospective telencephalon (lower panel) had an
asymmetric expression pattern of Meis2, being expressed only at one
lateral side of the developing telencephalon. EMX1+,
PAX6+ cells were located dorsally, while NKX2.1+ cells
were positioned in the ventral region of the telencephalon. (C) Lateral views
of stage 20-22 chick embryos generated in New Culture from stage 10 embryos
grafted with either control beads (upper panel) or RA beads (lower panel) into
the prospective telencephalon. The heads of the RA-treated embryos were normal
in size but the eyes were enlarged. (D) Embryos (n=10) grafted with
control beads in the prospective telencephalon (upper panel) did not generate
MEIS2+ cells, but generated PAX6+ and EMX1+
cells in the dorsal region and NKX2.1+ in the ventral region of the
telencephalon. Embryos (n=4) grafted with RA beads into the
prospective telencephalon (lower panel) generated a large number of
MEIS2+ cells in the dorsal and intermediate regions of the
telencephalon, a reduced number of PAX6+ cells, no EMX1+
cells and a normal number of NKX.2.1+ cells.
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