I-bands of striated muscle contain lateral struts

In electron micrographs of striated muscle, the I-band often shows a distinct cross-striation. The periodicity of this striation is near 40 nm and has been attributed to troponin, which is localized along the thin filament. However, the cross-striation is often so prominent as to be suggestive of physical structures running transversely across the I-band. We examined I-band ultrastructure using three independent methods: thin sections of chemically fixed specimens; freeze-fracture; and freeze-substitution. With all three methods we found transverse structures distributed throughout the I-band, many of which bridged the gap between neighboring filaments. Such structures were observed in each of the several species studied. In fish muscle in particular, which has a highly regular lattice, it was obvious that these structures gave rise to the observed periodicity.