The skeletal system is composed of tissue called bone that helps with movement, provides support for organs, and synthesizes blood cells. The outer layer of bone is composed of a matrix made of collagen and minerals that gives bones their strength and rigidity. The matrix is formed from functional units called osteons that include layers of compact bone called lamellae. The lamellae surround a cavity called the Haversian canal, which houses the bone’s blood supply. These canals are in turn connected to the periosteum, the bone’s outermost membrane, by another series of channels called Volkmann’s canals.
Within osteons are blood cells called osteoblasts, mononucleate cells that produce bone tissue. When the bone tissue hardens around these cells, the cells are known as osteocytes, and the space they occupy within the bone tissue is known as lacunae. The lacunae are connected by a series of channels called canaliculi. Osteoclasts, a third type of bone cell, are responsible for breaking down bone tissue. They are located on the surface of bones and help balance the body’s calcium levels by degrading bone to release stored calcium. The fourth type of bone cell, lining cells are flatted osteoblasts that protect the bone and also help balance calcium levels.
Within the hard outer layer of bone is the spongy layer called cancellous bone, which is made up of support structures called trabeculae. Within this layer is the bone marrow, which houses cells that produce red blood cells in a process called hematopoiesis. Bone marrow also produces many of the lymphocytes that play an important role in the immune system.
Bones are divided into four main categories. Long bones, such as the femur and humerus, are longer than they are wide. Short bones, in contrast, are wider than they are long. These include the clavicle and carpals. Flat bones are wide and flat, and usually provide protection. Examples of flat bones include the bones of the skull, pelvis, and rib cage. Irregular bones, as the name suggests, have an irregular shape that doesn’t fit into the other categories. These bones include the vertebrae and bones of the jaw.
Bones are held together (articulated) at joints by connective tissue called ligaments. Joints can be classified based on the tissue that connects the bone. Fibrous joints are connected by dense fibers, while cartilaginous joints are joined by cartilage. Cartilage is more flexible than bone but denser than muscles. In addition to joining together bone, it also helps hold open passageways and provides support in structures like the nose and ears. Finally, synovial joints are joined by synovial fluid, which lubricates the joint and allows for movement.
Types of Synovial Joints
movement through one plane of motion as flexion/extension
elbows, knees, fingers
range of motion through multiple planes and rotation about an axis
movement through multiple planes, but cannot rotate about an axis
sliding movement in the plane of the bones’ surfaces
vertebrae, small bones in the wrists and ankles
movement through two planes as flexion/extension and abduction/adduction, but cannot rotate about an axis
Only movement is rotation about an axis
Important pathologies of the skeletal system include osteoporosis, which occurs when minerals are leached from the bone, making bones more likely to break. Broken bones can also be caused by brittle bone disease, which results from a genetic defect that affects collagen production. Joint pain can be caused by osteoarthritis, which is the breakdown of cartilage in joints, and rheumatoid arthritis, which is an autoimmune disease that affects synovial membranes.