Effect of verapamil on embryonic SP. (A) Coincubation of tissues with Hoechst and 50 μM verapamil dramatically reduces the percentage of SP cells in yolk sac and embryonic tissue isolated 8.0 dpc by a factor of 14.8 and 5.9, respectively. (B) Coincubation of tissues isolated 10.5 dpc with Hoechst and 50 μM verapamil decreases the percentage of SP cells in yolk sac and embryo by a factor of 11.4 and 1.4, respectively. All verapamil experiments were performed twice.