Skeletal muscle fibres. Light micrograph of a longitudinal section through skeletal muscle. The muscle is composed of many long fibres, containing contractile proteins arranged in structures called myofibrils. The regularly arranged proteins cause the myofibrils to have a cross- striated (banded) appearance. They lie in parallel along the fibre, giving the appearance of regular cross-striations along the length of the fibre. The light (I) bands and the dark (A) bands can be seen here as alternating light and dark pink vertical stripes. The many flattened muscle cell nuclei (purple) can also be seen. Haematoxylin and eosin stain. Magnification: x160 at 35mm size.