Fig. 6. Testing commitment to anteroposterior (AP) fates by reciprocal manipulation of RA signalling with RA and BMS493, a RA pan-antagonist. All pictures were taken with the same magnification. Control hearts display a central ventricular chamber flanked by bilateral limbs formed by posterior precursors. (A) BMS493 at 10^–4 M at stages HH4-7 induced atrophy of the cardiac inflow compartment and increased the ventricular chamber. Treatment at stages beyond HH7 failed to affect cardiac morphology, indicating that posterior precursors commit to their fates between HH7-8. (B) RA at 10^–5 to 10^–4 M at HH4-7 inhibited ventricular development. RA treatment beyond HH7 failed to affect chamber morphology, indicating that ventricular and conotruncal precursors commit to their fates between stages HH7-8. (C) Amhc1 expression after BMS493 and RA. (D) Codes for dots outlining cardiac structures.