27 Introduction to the Skeletal System

Did you know that there are approximately 206 bones in an adult body? The smallest bones are in your middle ear (malleus, incus, and stapes) and the largest bone is in the thigh (femur). These bones form the bulk of the skeletal system and have many important functions. They are critical for maintaining our structure and body shape. Skeletal muscles insert on bones and pull them for movement.

Bones are living tissues formed from cells, proteins (mainly collagen), and minerals (calcium and phosphate). The skeletal system consists of bones, but also cartilage and ligaments. Bones will grow longitudinally (lengthwise) until approximately ages 18 (for women) and 21 (for men), after that people generally won’t get any taller. But bone tissue is active and becomes stronger when stress is placed on it. Exercise stimulates a type of bone growth that causes thickening of the bones (appositional growth), which makes bones stronger. If you are a couch potato, your bones actually become weaker!

Bones also protect organs. Think about how easy trauma could damage your brain, spinal cord, heart, and lungs if they were not protected by bone. Bone is very strong yet typically has some flexibility, which makes it the perfect tissue for this function.

One other important role of bones is in hemopoiesis (hematopoeisis). Bone tissue contains red bone marrow which contains the stem cells for the formation of all blood cells. The stem cell is known as the hemocytoblast and ultimately forms all red blood cells, white blood cells, and platelets. Red blood cells are critical for transporting oxygen and carbon dioxide in the blood. White blood cells are the main cellular component of our immune response, and platelets are necessary for hemostasis (blood clotting).

Components of the Skeletal System

The skeletal system consists of bones, cartilage, and ligaments, which all fall into the category of connective tissue. Bones are classified by their shape. Some are long, some are short, others are flat, and the rest have an irregular shape. There are two types of bone (osseous) tissue. One is cancellous (nicknamed spongy bone due to its sponge-like appearance) and the other is more dense and known as compact bone. Both types of bone are found in every bone (note there is a difference between bone tissue and a bone), but compact bone (a tissue) is stronger and always found on the outside of the bone.

All bone consists of bone cells of various functions. These are osteocytes, osteoblasts, osteoclasts, and osteoprogenitor cells. Notice they all have the prefix “osteo”. Osteo is derived from the Greek word (osteon) for bone. The suffix “blast” means “to make” while the suffix “clast” means “to break down”. Osteoblasts are important for forming bone while osteoclasts break down bone and remove calcium. These cells will be studied in additional detail in this unit.

Besides bones, the other two structures considered as parts of the skeletal system are cartilage and ligaments. A specific type of cartilage, hyaline cartilage, is found covering the ends of bones that move with respect to one another. The areas of the bone surfaces that interact in these movements are called the articular surfaces (articulate means “to join”) and the hyaline cartilage in these same areas is called articular cartilage. Here it reduces friction and cushions the surfaces of the joint. The degeneration of this cartilage is the cause of osteoarthritis.

Hyaline cartilage is also important for longitudinal bone growth. In this case it is not hyaline cartilage, as it is not located on the surface of the bone, but instead is a thin band of hyaline cartilage within the end of the long bone, in an area known as the epiphyseal plate (or growth plate). During growth, the layer of cartilage first expands, before developing into new bone tissue.

Ligaments are a type of dense regular connective tissue that is very strong, and inelastic. Ligaments join bones together to provide stability to the joint. A sprain occurs when a ligament is damaged. Ligaments consist mainly of the structural protein collagen and have a white appearance.