Conotruncal myocardium arises from a secondary heart field

The primary heart tube is an endocardial tube, ensheathed by myocardial cel ls, that develops from bilateral primary heart fields located in the latera l plate mesoderm. Earlier mapping studies of the heart fields performed in whole embryo cultures indicate that all of the myocardium of the developed heart originates from the primary heart fields. In contrast, marking experi ments in ovo suggest that the atrioventricular canal, atria and conotruncus are added secondarily to the straight heart tube during looping. The resul ts we present resolve this issue by showing that the heart tube elongates d uring looping, concomitant with accretion of new myocardium. The atria are added progressively from the caudal primary heart fields bilaterally, while the myocardium. of the conotruncus is elongated from a midline secondary h eart field of splanchnic mesoderm beneath the floor of the foregut. Cells i n the secondary heart field express Nkx2.5 and Gata-4, as do the cells of t he primary heart fields. Induction of myocardium appears to be unnecessary at the inflow pole, while it occurs at the outflow pole of the heart. Accre tion of myocardium at the junction of the inflow myocardium with dorsal mes ocardium is completed at stage 12 and later (stage 18) from the secondary h eart field just caudal to the outflow tract. Induction of myocardium. appea rs to move in a caudal direction as the outflow tract translocates caudally relative to the pharyngeal arches. As the cells in the secondary heart fie ld begin to move into the outflow or inflow myocardium, they express HNK-1 initially and then MF-20, a marker for myosin heavy chain. FGF-8 and BMP-2 are present in the ventral pharynx and secondary heart field/outflow myocar dium, respectively, and appear to effect induction of the cells in a manner that mimics induction of the primary myocardium from the primary heart fie lds. Neither FGF-8 nor BMP-2 is present as inflow myocardium is added from the primary heart fields. The addition of a secondary myocardium. to the pr imary heart tube provides a new framework for understanding several null mu tations in mice that cause defective heart development.