Abstract
Cardiac myofibrillogenesis was examined in cultured chick cardiac cells by immunofluorescence using antibodies against titin, actin, tropomyosin, and myosin. Primitive cardiomyocytes initially contained stress fiber-like structures (SFLS) that stained positively for alpha actin and/or muscle tropomyosin. In some cases the staining for muscle tropomyosin and alpha actin was disproportionate; this suggests that the synthesis and/or assembly of these two isoforms into the SFLS may not be stoichiometric. The alpha actin containing SFLS in these myocytes could be classified as either central or peripheral; central SFLS showed developing sarcomeric titin while peripheral SFLS had weak titin fluorescence and a more uniform stain distribution. Sarcomeric patterns of titin and myosin were present at multiple sites on these structures. A pair of titin staining bands was clearly associated with each developing A band even at the two or three sarcomere stage, although occasional examples of a titin band being associated with a half sarcomere were noted. The appearance of sarcomeric titin patterns coincided or preceded sarcomere periodicity of either alpha actin or muscle tropomyosin. The early appearance of titin in myofibrillogenesis suggests it may have a role in filament alignment during sarcomere assembly.
Highlights
Cardiac myofibrillogenesis was examined in cultured chick cardiac cells by immunofluorescence using antibodies against titin, actin, tropomyosin, and myosin
U 'LTRASTRUCTURAL studies on cultured rat cardiomyocytes and embryonic hamster, rat, or sheep hearts have led to the development of a model for cardiac myofibrillogenesis in which Z ?band-like materials are the initiation sites of myofilament assembly [2, 13, 16]
H zones and M lines are not seen until late in organogenesis in the rat and after organogenesis in the hamster [16]. Another model for cardiac myofibrillogenesis is based on observations of the assembly of myofibrils in isolated chick cardiomyocytes where myofibrils apparently begin to form from structures resembling stress fibers [4, 12]
Summary
Cardiac myofibrillogenesis was examined in cultured chick cardiac cells by immunofluorescence using antibodies against titin, actin, tropomyosin, and myosin. Primitive cardiomyocytes initially contained stress fiber-like structures (SFLS) that stained positively for a actin and/or muscle tropomyosin. H zones and M lines are not seen until late in organogenesis in the rat and after organogenesis in the hamster [16] Another model for cardiac myofibrillogenesis is based on observations of the assembly of myofibrils in isolated chick cardiomyocytes where myofibrils apparently begin to form from structures resembling stress fibers [4, 12]. Information on the structural organization of titin molecules in sarcomeres is limited at present, a model has been proposed in which titin is a major constituent of a set of longitudinal filaments running through the sarcomere [25, 26]. Little is known about the mechanism and timing of titin assembly in the sarcomere in relation to the other major myofibrillar proteins
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