Fig. 1

The initiation of contraction of the rat heart primordium. A Time courses of the beginning of contraction of heart primordia between E 9.88 and E 10.19 days (n = 31). Each circle represents the time at which an embryo was removed from one pregnant rat. The time courses with open circles represent data from embryos that began to contract more than 16 min after transfer to the incubator. The time courses with closed circles represent data from embryos that began to contract within 16 min of transfer to the incubator or that did not begin to contract. The thin line represents the period when the heart primordium did not contract. The bold line represents the period when the heart primordium exhibited beating. The left edge of each bold line indicates the time when beating became apparent. B Representative ventral view of a rat embryo just removed from the uterus. This example corresponds to the time course marked with the asterisk in A. C The expanded picture at the time when the embryo was observed in a small incubator. This picture corresponds to the dotted rectangle in B. D The change in beating rate. E The recording of tissue motion corresponding to the position shown as a black line in C and to the time shown in D. The data were obtained using ImageJ software. Arrowheads indicate the timing of contractions. Direction of the contraction, cr (cranial), corresponds to that in C