DIFFERENTIATION OF MYOENDOCRINE CARDIAC-CELLS FROM PRESUMPTIVE HEART MESODERM EXPLANTS OF BUFO-ARENARUM

To investigate which factors are involved in the differentiation of Bu fo arenarum heart myoendocrine cells, we studied the distribution of A trial Natriuretic Peptide (ANP) immunoreactivity in hearts formed from presumptive cardiac mesoderm explanted at early embryo stages. Explan ts isolated from different embryonic stages throughout neurulation wer e cultured in vitro with or without the pharyngeal endoderm, and in ot her cases transplanted to the caudal region of embryos at similar stag es. We utilized immunohistochemical and morphological techniques to as sess myoendocrine cardiac cell differentiation. Development of heart b eat and positive tropomyosin immunolabeling were considered signs of c ardiac tissue differentiation. Our results confirm that explants of ca rdiac mesoderm cultured with endoderm showed a greater and more comple te level of cardiac differentiation than those of mesoderm alone, and this coincides with the staining pattern of tropomyosin. ANP immunosta ining revealed that cardiac cells containing ANP were scarce in those cultures without endoderm. On the contrary, in both cultured and graft ed explants containing endoderm, ANP immunostaining was intensive and well-distributed in the cardiac tube, and in some cases restricted to one side of the formed heart. We conclude that the endoderm regulates cardiac cell differentiation, and in this way, is involved in the deve lopment of the heart myoendocrine system.