Regional Organization of Pre-pacemaker Cells in the Cardiac Primordia of the Early Chick Embryo

ABSTRACT The lateral cardiac primorida of the early chick embryo were cut into anterior, middle and posterior fragments. Each fragment was incubated at 37°C in culture medium, for 48 hours. At the middle and end of the incubation period each culture was examined for the presence of spontaneously beating heart tissue, and its intrinsic pulsation rate was determined. The posterior fragments, which in the intact embryo form atrial and sinus tissue, gain the capacity to produce spontaneously contractile heart vesicles earliest (stage 4+), and exhibit the highest level of heart-forming potency. The middle fragments are first able to form beating hearts at stage 5, but do so in a lower percentage of the cases. The cells in these fragments normally form ventricle. The anteriormost fragments, which represent the presumptive conus and cono-ventricular regions of the heart, are not able to form beating cardiac vesicles until stage 7, and then do so with very low frequency. Vesicles from all three fragments increase in spontaneous pulsation rate with age of the donor. The posterior fragments produce heart tissue which beats at from 90 to 135 beats/min., with a mean of 115 beats/min. Vesicles from the middle pieces range from 40 to 85 beats/min., averaging 65 beats/min. The anterior fragments increase from 10 to 40 beats/min., with an average rate of 36 beats/min. It may be concluded from these results that the various parts of the heart (sinus, atrium, ventricle and conus) are represented in specific, localized portions of the heart-forming regions in the early embryo (stages 5-9), and that each portion contains pre-pacemaker cells which develop an appropriate level of intrinsic rhythmicity. The antero-posterior rate gradient observed in the formed functional heart is already defined in the heart-forming regions of the head-process stage embryo. Whether this early regionalization of pacemaker activity is due to the distribution in the cardiac primordia of pre-pacemaker cells already determined as to their future pulsation rate, or is the result of a gradually emerging, epigenetic pattern of localized interactions between the pre-heart mesoderm and surrounding tissues, cannot be decided on the basis of the present experiments. A test of this question is suggested, and other points of relevance to these results are discussed.