Muscle cell appearance

[joeyjo100]

What causes the A-band to be dark in a sarcomere?
Conversly, what causes the I-Band to be light?

[mishrashubham]

http://upload.wikimedia.org/wikipedia/commons/a/a4/Sarcomere.gif

The 'A band' consists of thick filaments (myosin protein) overlapped with thin filaments (actin protein). And this increased density of filaments makes it appear dark. Notice that the 'H zone' in the middle of the 'A band' does not have overlapping thin filaments making it comparatively lighter.

There is nothing special in the darkness, just density of filaments.