Connective tissue disease refers to a group of disorders involving the protein-rich tissue that supports organs and other parts of the body. Examples of connective tissue are fat, bone, and cartilage. These disorders often involve the joints, muscles, and skin, but they can also involve other organs and organ systems, including the eyes, heart, lungs, kidneys, gastrointestinal tract, and blood vessels. There are more than 200 disorders that affect the connective tissue. Causes and specific symptoms vary by the different types.