Muscle fibre. Coloured scanning electron micrograph (SEM) of a freeze-fractured skeletal (or striated) muscle fibre. The fracturing of the fibre has revealed that it consists of a bundle of smaller fibres called myofibrils. The myofibrils are crossed by transverse tubules (horizontal lines), that mark the division of the myofibrils into contractile units (sarcomeres). Magnification: x8000 when printed at 10 centimetres wide.