Ligaments are another type of connective tissue that attach two bones and consequently hold the joints together. Tendons are similar but different type of tissue that attach the muscles to the bones. All of these tissues, the bones, ligaments, tendons and the skeletal muscles themselves, are made up of proteins. One of the most predominant proteins is called collagen.Collagen is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content.