Basics of the Skeletal System

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OpenStax Anatomy and Physiology

The skeletal system functions as a support and protection, body movement, storage for minerals, and formed element production. The organs that compose the skeletal system includes bones, ligaments, cartilages and joints.

A soft connective tissue called bone marrow fills the interior of a bone. There are two types of bone marrow: red marrow and yellow marrow. Red marrow involves in the production of red blood cells, white blood cells, and platelets. Yellow marrow is composed of fats and adipose tissues.

Bones are classified by its shape namely long, short, flat, irregular, and sesamoid. Long bones are cylindrical in shape and usually long by length. Examples of long bone include the femur, tibia, fibula, humerus, radius, and ulna. Short bones are shaped like cubes and provide support and stability. Some short bones include the carpals and tarsals. Flat bones are curved and thin. These bones serve as an attachment for muscles. Flat bones include sternum, ribs, scapula, and cranial bones. Irregular bones have complex shape and usually protect the internal organs. Spinal columns and facial bones are considered irregular bones. Sesamoid bones are small and round, and can be found embedded in tendons. Patella is an example of sesamoid bone.

A long bone has two main parts: diaphysis and epiphysis. Diaphysis is the hollow shaft between the two ends of a long bone and the hollow area which is filled with yellow marrow is called medullary cavity. Each wide end of a long bone is called epiphysis and filled with spongy bone and red marrow. The epiphyseal plate is a layer of cartilage involved in bone growth. When a person reaches approximately 18-21 years old, the epiphyseal plate is replaced by a bone tissue and becomes an epiphyseal line.

The endosteum is the membranous lining found in the medullary cavity. This is where bone remodeling, growth, and repair occur. The periosteum is the membrane found on the surface of the bone which composes the blood vessel, nerves, and lymph vessels that sustain the compact bone.

There are four types of cell that can be found in bone tissue: osteogenic cell, osteoblast, osterocyte, and osteoclast. Osteogenic cells are bone stem cells that can differentiate into osteoblast. Osteoblast is responsible the formation of bone. As osteoblast moves around to build new bones, eventually, osteoblast gets trapped and becomes osteocyte. Osteocyte is the primary cell that maintains bone tissue. Osteoclast forms from white blood cells and is responsible for breaking down and reabsorbing bone tissues.

Osteon is the structural unit of compact bone and composed of calcified matrix called lamella oriented in concentric rings. The center of osteon is called the central canal which houses the blood vessels, nerves, and lymph vessels.

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Betts, J. G., Young, K. A., Wise, J. A., Johnson, E., Poe, B., Kruse, D. H., … DeSaix, P. (n.d.). Anatomy and Physiology. Houston, Texas: OpenStax. Access for free at:


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