Collagen fibrils. Coloured scanning electron micrograph (SEM) of a bundle of collagen fibrils. The tropocollagen molecules can be seen as bands running parallel to the long axis. Fibrils form bundles called fibres. Collagen, a type of connective tissue, is the main structural protein in the body. It has high tensile strength, providing structure and elasticity to skin, tendons, ligaments and bones. Magnification: x50MB,000 when printed 10cm wide.