|MadSci Network: Science History|
What do the abbreviations mean in muscle histology?
Anyone who as looked at the labeled parts of muscle tissue viewed under a microscope, such as seen here http://www.ks.uiuc.edu/~ericlee/Telethonin/, will run across the names: I Band, A Band, Z Line (or Band or Disk), H Band and M Line. It’s quite an alphabet soup of abbreviations. Although the letters that start these names do stand for something, the original names are not commonly used. Therefore, they might as well be random letters.
But, for the record:
The names I Band and A Band refer to light properties when the muscle is viewed under a microscope. The I stands for Isotropic and the A stands for Anisotropic. Isotropic means “equal measure” and anisotropic means “not equal measure.” When muscle is viewed under a standard light microscope the light is refracted. When muscle is viewed under a polarized light, the A Band part is double refracted, but the I Band is still single refracted. This means that the I Band doesn’t change with polarized light, and is therefore the equal measure band. Bottom line, it means the A Band is the dark part and the I Band is the light part.
In the middle of the I Band is a dark line – the Z line. The sarcomere (the functional unit of muscular contraction) runs from Z Line to Z Line. So, the Z Line is “between” the sarcomeres. But in this case, the Z stands for zwischen, which is the German word for between.
If you look closely at the A Band, you will see that there is a lighter region in its center. This region is the H Band. The H here comes from the German word Heller, which means “bright.” So, the H Band is the bright part of the A Band.
And finally, in the middle of the H Band is another darker region, the M Line. The M here also comes from German, the word mittel for middle – in this case it could have been English, but it’s not. So, the M Line is in the middle of the A Band.
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