Since most connective tissue diseases involve the musculo-articular skeleton, the relevant content of the Merck Manual of Treatment is given here for your reference. The human skeletal system is made up of numerous pieces of bone. Bone is a human tissue that is constantly in dynamic change and has multiple functions. Bone provides the body with a scaffold for strength, stability and muscle movement, as well as protection for vital internal organs. There are two main shapes of bone: flat (e.g., skull and vertebrae) and long tubular (e.g., femur and upper limb bones). The internal structure of bone is essentially the same. Its hard outer shell is composed mostly of proteins, such as collagen, and a substance called hydroxyapatite, which is composed mainly of calcium and some other minerals. Hydroxyapatite stores most of the body’s calcium and is the main material basis for the strength of the bones. In the middle of the bone is the soft bone marrow, which is less dense than bone and contains hematopoietic cells. Blood vessels run through the middle of the bone and nerves are located around the bone. The joints are formed at the joints between bone and bone. The shape of the joint determines the magnitude and direction of movement. Some joints, such as those between the bones of the skull, called sutures, cannot produce movement in adulthood, while others can produce varying degrees of movement, such as the shoulder joint, called the pestle and mortar joint, which produces internal, external, forward, backward, and lateral movements of the upper extremity. In the elbow, fingers, and toes, the gibbous joints produce only flexion and extension movements. The other components of the joint provide stability and protection against injury due to continuous use. Within the joint, the ends of the bone are covered by a layer of cartilage. Cartilage is a smooth, tough, protective tissue that absorbs shock and reduces friction during joint movement. Inside the joint is synovial tissue that surrounds the joint and forms the joint capsule. The cells of the synovial tissue produce a clear, bright synovial fluid that fills the joint cavity, which reduces the friction generated during joint movement (Editor’s note: The synovial cartilage tissue involved in rheumatoid arthritis refers to these two parts). Muscles are bundles of fibers that produce contractions. Skeletal muscles are the tissues that maintain the posture of the limb and produce movement. Antagonistic muscles are distributed around the bones and joints, such as the biceps antagonizing the triceps at the front during elbow flexion. Tendons and ligaments are tough connective tissues. (Editor’s note: Tendon lesions such as those involved in ankylosing spondylitis can manifest as heel pain and discomfort. It is also possible to understand why ankylosing spondylitis is also a connective tissue disease). Tendons are found at the ends of all the muscles that attach to the bone, while ligaments wrap around the joints. They serve to stabilize and strengthen the joint, allowing movement to take place in a certain direction. Bursae exist between adjacent structures, such as bone and ligaments, and are filled with synovial fluid to provide a cushion that would otherwise rub against adjacent tissues during motion, causing wear and tear and rupture. The various parts that make up the joint work in harmony to help balance movement and prevent injury. When you are walking or running, the cord muscles at the back of the thigh contract to pull the calf into flexion; at the same time, the quadriceps at the front of the knee relax to facilitate knee flexion; the cartilage and synovial fluid within the knee joint reduce friction from movement; the five ligaments surrounding the knee maintain the anatomical position of the bone; and the bursa provides a cushion between the tibia and patellar tendon.