Ronald A. Bergman, Ph.D., Adel K. Afifi, M.D., Paul M. Heidger,
Peer Review Status: Externally Peer Reviewed
Human, Helly's fluid, H. & E., 1416 x.
The names given to the two major transverse striations of skeletal and cardiac muscle are derived from the studies of Brücke* (1858). With routine light microscopic techniques, alternating dark and light bands are seen within striated muscle fibers (Plates 66, 67 and 68). Polarization microscopy reverses the appearance of the dark band, which becomes bright, and the light band, which appears dark. The dark band of routine light microscopy, exhibiting birefringence with polarized light, is anisotropic and is called the A band. The light band of routine light microscopy is poorly refractile and relatively isotropic and is called the I band.
Muscle fibers: Showing cross striations formed by alternating segments of high and low refractive index resulting from their submicroscopic structure, which is revealed by electron microscopy.
A band: Anisotropic band.
I band: Isotropic band. Note the birefringence or anisotropy of the Z line in the center of the I band.
*Brücke was a nineteenth-century Viennese physiologist.
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