INDUCTION OF AVIAN CARDIAC MYOGENESIS BY ANTERIOR ENDODERM

An experimental system was devised to study the mechanisms by which ce lls become committed to the cardiac myocyte lineage during avian devel opment. Chick tissues from outside the fate map of the heart (in the p osterior primitive streak {PPS} of a Hamburger & Hamilton stage 4 embr yo) were combined with potential inducing tissues from quail embryos a nd cultured in vitro. Species-specific RT-PCR was employed to detect t he appearance of the cardiac muscle markers chick Nkx-2.5 (cNkx-2.5), cardiac troponin C and ventricular myosin heavy chain in the chick res ponder tissues, Using this procedure, we found that stage 4-5 anterior lateral (AL) endoderm and anterior central (AC) mesendoderm, but not AL mesoderm or posterior lateral mesendoderm, induced cells of the PPS to differentiate as cardiac myocytes, Induction of cardiogenesis was accompanied by a marked decrease in the expression of rho-globin, impl ying that PPS cells were being induced by anterior endoderm to become cardiac myocytes instead of blood-forming tissue, These results sugges t that anterior endoderm contains signaling molecules that can induce cardiac myocyte specification of early primitive streak cells. One of the cardiac muscle markers induced by anterior endoderm, cNkx-2.5, is here described for the first time. cNkx-2.5 is a chick homeobox-contai ning gene that shares extensive sequence similarity with the Drosophil a gene tinman, which is required for Drosophila heart formation, The m esodermal component of cNkx-2.5 expression from stage 5 onward, as det ermined by in situ hybridization, is strikingly in accord with the fat e map of the avian heart. By the time the myocardium and endocardium f orm distinct layers, cNkx-2.5 is found only in the myocardium, cNkx-2. 5 thus appears to be the earliest described marker of avian mesoderm f ated to give rise to cardiac muscle.