Skeletal muscle and nerve. Coloured transmission electron micrograph (TEM) of a section through skeletal (striated) muscle. The muscle myofibrils are split into contractile units called sarcomeres by the discontinuous Z lines. A sarcomere contains protein filaments which can slide over each other to allow the muscle to contract. In the connective tissue are nerve fibres surrounded by the myelin sheaths of Schwann cells. Schwann cell nuclei are also visible. Bottom left is part of a blood vessel containing a single red blood cell. Magnification: x2000 when printed at 10cm.