Tendon. Coloured transmission electron micrograph (TEM). A tendon is made up of parallel bundles of collagen fibres. fFbroblasts (red) lie between the collagen bundles.Fibroblasts secrete collagen fibres, the main component of tendons. Tendons are inelastic, though with a degree of flexibility, and attach a muscle to a bone. Magnification: x20, 000 at 10 cm wide