J. Embryol. exp. Morph. 84,1-17 (1984) Printed in Great Britain © The Company of Biologists Limited 1984 Accumulation and localization of troponin-T in developing hearts of Ambystoma mexicanum By REBECCA A. FULDNER 1 , SOO-SIANG LIM, MARION L. GREASER AND LARRY F. LEMANSKI Department of Anatomy and the Muscle Biology Laboratory The University of Wisconsin, 1300 University Avenue, Madison, Wisconsin 53706, U.S.A. SUMMARY Troponin-T (Tn-T) expression in developing hearts of axolotls, Ambystoma mexicanum, was studied with the use of polyclonal and monoclonal antibodies and SDS-polyacrylamide gel electrophoresis. In precontractile hearts (stage 32/33), Tn-T was present in addition to myosin, actin and tropomyosin as evidenced by the presence of the protein bands in SDS- gels and by indirect immunofluorescence. Tn-T was localized in amorphous collections at the peripheries of these precontractile cells. Hearts of normal and cardiac lethal mutant siblings were also analysed for Tn-T expression. No detectable differences in the quantity of protein present was observed by gel electrophoresis or by indirect immuno-fluorescence. The most striking difference concerned the localization of the protein. In normal hearts, Tn- T was primarily localized in the I-bands of organized myofibrils; however, in mutant cells the Tn-T was localized in amorphous collections at the cell peripheries suggesting a reduction of myofibrillar organization in these cells. No differences were observed in the contractile protein composition between normal and mutant em-bryonic hearts by gel electrophoresis experiments. INTRODUCTION The temporal appearance of contractile proteins in differentiating myoblasts has received a great deal of attention in recent years. Results of various studies performed on skeletal myoblasts in vitro concur that a mechanism exists regulating the coordinate expression of both structural and regulatory contrac- tile proteins (Devlin & Emerson, 1978,1979; Allen, Stromer, Goll & Robson, 1979; Hastings & Emerson, 1982). Synthesis of these proteins is concurrent with myoblast commitment (irreversible withdrawal from the cell cycle) and fusion of myoblasts into myotubes, although these latter events are not required for the induction of contractile protein gene expression (Nguyen, Medford & Nadal-Ginard, 1983). The differentiation of cardiac myocytes in vivo has not been studied as extensively. In differentiating cardiac myocytes, there is not a fusion of the cell ^ o whom reprint requests should be addressed.