The human skeletal system consists of 206 bones that form the framework for the body and the tendons, ligaments and cartilage that connect the bones together. The skeletal system provides structure, protects the internal organs, produces blood cells, stores minerals and allows for movement. Diseases of the skeletal system may affect the bones, or the surrounding tissues holding the bones together.
Influence of Age
Although most think of bones as being solid, hard structures, actually only the outer bone, known as cortical bone, is solid while the inner bone, known as trabecular bone, consists of a honeycomb-like structure. All bone consists of living cells, which continually undergo a process of breaking down and rebuilding known as remodeling. During childhood and adolescence, as bones grow, the body builds new bone tissue faster than it breaks down old tissue. By the age of 20, however, bone can be lost faster than new tissue forms, according to MedLine Plus and the National Institutes of Health. This makes older bones more susceptible to certain types of diseases.
Cells in Bones
Skeletal system diseases can affect any of the three types of cells in bones. Osteoclasts reside on the outer surface of bones and function to dissolve old bone tissue. These cells, made up of two or more cells fused together, have more than one nucleus and are therefore large in size. Osteoblasts contain only one nucleus and come from the bone marrow. These cells work in groups to build new bone, known as osteoid, from collagen and proteins. Osteoblasts also reside on the surface of bones. Osteocytes come from osteoblasts and make up the inside of bones.
Genetic Diseases
Genetic diseases occur as a result of a mutation, or change, in a gene---a specific piece of deoxyribose nucleic acid. One major genetic bone disease, osteogenesis imperfect affects 25,000 to 50,000 Americans. Also known as brittle bone disease, OI causes fragile bones that break easily as well as weakness, hearing loss, fatigue, joint laxity, curved bones and blue sclera, according to the Osteogenesis Imperfecta Foundation. Osteopetrosis, another genetic bone disease causes thickening of the bones.
Bone Degradation
A major skeletal system disease, known as osteoporosis or porous bone disease, affects 10 million people in the United States, with another 34 million at risk, according to the National Osteoporosis Foundation. Although it can occur at any age, those over the age of 50 have a higher risk since the bones lose more mass as people age. Osteoporosis occurs when the osteoclasts break down bone faster than the osteoblasts can build new bone.
Surrounding Tissue
Where two bones meet, known as a joint, and the tendons and ligaments holding them together can also be affected by skeletal diseases. Common joint diseases include arthritis, a group of more than 100 conditions characterized by inflammation of the joints. Arthritis diseases affect more than 46 million Americans and causes chronic pain, stiffness and joint damage that can lead to debilitation, according to the Arthritis Foundation.
