The topographical relationship between stress fiber-like structures (SFLS) and nascent myofibrils was examined in cultured chick cardiac myocytes by immunofluorescence microscopy. Antibodies against muscle-specific light meromyosin (anti-LMM) and desmin were used to distinguish cardiac myocytes from fibroblastic cells. By various combinations of staining with rhodamine-labeled phalloidin, anti-LMM, and antibodies against chick brain myosin and smooth muscle alpha-actinin, we observed the following relationships between transitory SFLS and nascent and mature myofibrils: (a) more SFLS were present in immature than mature myocytes; (b) in immature myocytes a single fluorescent fiber would stain as a SFLS distally and as a striated myofibril proximally, towards the center of the cell; (c) in regions of a myocyte not yet penetrated by the elongating myofibrils, SFLS were abundant; and (d) in regions of a myocyte with numerous mature myofibrils, SFLS had totally disappeared. Spontaneously contracting striated myofibrils with definitive Z-band regions were present long before anti-desmin localized in the I-Z-band region and long before morphologically recognizable structures periodically link Z-bands to the sarcolemma. These results suggest a transient one-on-one relationship between individual SFLS and newly emerging individual nascent myofibrils. Based on these and other relevant data, a complex, multistage molecular model is presented for myofibrillar assembly and maturation. Lastly, it is of considerable theoretical interest to note that mature cardiac myocytes, like mature skeletal myotubes, lack readily detectable stress fibers.

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