Your browser is no longer supported

For the best possible experience using our website we recommend you upgrade to a newer version or another browser.

Your browser appears to have cookies disabled. For the best experience of this website, please enable cookies in your browser

We'll assume we have your consent to use cookies, for example so you won't need to log in each time you visit our site.
Learn more

The skeletal system: Bone structure

  • Comment

Brendan Docherty

PGCE, MSc, RN, is clinical stream manager, cardiology and critical care, South Eastern Sydney and Illawarra Area Health Service, Australia

This article, the first of a four-part series, explores the core anatomy and physiology of bones, including their purpose and function. Part two covers bone growth; part three looks at the axial skeletal system; and part four examines the appendicular skeletal system.

 

This article, the first of a four-part series, explores the core anatomy and physiology of bones, including their purpose and function. Part two covers bone growth; part three looks at the axial skeletal system; and part four examines the appendicular skeletal system.

 

 

MICROSCOPIC STRUCTURE

 

 

There are three core cells involved in the forming, maintenance and recycling of bone components (called osteogenesis): osteoblasts, osteocytes and osteoclasts.

 

 

 Osteoblasts are bone-forming cells. They are located on the surface of bone and are responsible for synthesising the organic components of the bone matrix, including type I collagen, proteoglycans and glycoproteins. The formation of bone involves two processes Ò matrix formation and mineralisation (calcium and phosphate ions arranged to create bone salts).

 

 

 Osteocytes are the most numerous type of bone cells. An osteoblast becomes an osteocyte when it no longer synthesises collagen. Osteocytes also assist in the control of bone mineralisation. In certain disease states, such as hyperparathyroidism, PagetÌs disease, or disuse osteoporosis, osteocytes may be involved in the process of excessive bone resorption.

 

 

 Osteoclasts are derived from a monocytic-macrophage system and are responsible for bone resorption (collagen hydrolysis and bone mineral dissolution). They are multinucleated cells with fine finger-like cytoplasmic processes and are rich in lysosomes. Osteoclast dysfunction can lead to osteopetrosis (dense, thickened bone with lack of marrow) (McRae, 2006).

 

 

BLOOD CELL FORMATION

 

 

Red bone marrow is a fatty network of connective tissue and is found in foetal developing bones, and some adult bones (for example ribs, pelvis and long bones such as the femur).

 

 

The process of haematopoiesis (from Ancient Greek: haima blood; poiesis to make) is the production of red (erythrocyte) and white (leukocyte) blood cells and platelets (thrombocytes) and is critical to the functioning of the human body (Fig 1).

 

 

Haematopoietic stem cells have the ability to develop into any type of blood cell (pluripotent) giving rise to the many that are required and produced from these stem cells (Farhi et al, 2004).

 

 

FUNCTION

 

 

There are five main functions of the skeleton:

 

 

 Protection for internal organs and support to soft tissues Ò for example the ribcage supports breathing;

 

 

 Movement Ò in conjunction with muscles, the bones of the upper and lower limbs pull and push, acting like levers;

 

 

 Production of blood cells (haematopoiesis) in red bone marrow;

 

 

 Storage of fats (triglycerides) in yellow bone marrow;

 

 

 Mineral storage and release (in conjunction with homeostatic regulation). For example, 97% of calcium is stored in the bones (Tortora and Grabowski, 2003).

 

 

Based on texture of cross sections, bone tissue can be classified as:

 

 

 Compact bone (dense bone, cortical bone) Ò compact bone is ivory-like and is dense in texture without cavities. Some medullary cavities (for example, the humerus) contain yellow bone marrow in adults;

 

 

 Sponge bone (trabecular bone, cancellous bone) Ò sponge bone is sponge-like with numerous cavities and is located within the medullary cavity. This is where red bone marrow is contained;

 

 

 Periosteum Ò connective tissue that surrounds bone and contains bone-forming cells (allows thickness growth only) and is essential for fracture repair and for the attachment of tendons or ligaments (Tortora and Grabowski, 2003).

 

 

Bone can also be classified according to its anatomical presentation (see Box 1).

 

 

JOINTS

 

 

Bones are connected to each other by joints to form the skeletal system Ò and these joints will normally have no capacity, or limited capacity to move, specific movement or a large range of movements (Agur and Dalley, 2004).

 

 

For example, the arm has three types of joint (Fig 2).

 

 

No capacity or limited

 

 

Non-movable (synostosis) joints allow no mobility and are connected by bone tissue or dense connective tissue, for example skull bones. Joints with very limited movement (synchondrosis) would include those joints between the ribs and the sternum or between the vertebral discs Ò where bones are connected by hyaline cartilage.

 

 

Specific movement

 

 

Hinge or pivot joints, such as in the knee and elbow, enable movement to occur similar to the opening and closing of a hinged door. The pivot joint in the neck allows the head to turn from side to side. Another joint with a specific range of movement is the saddle joint, which is only found in thumbs. The bones in a saddle joint can rock back and forth and from side to side but they have limited rotation.

 

 

Large range of mobility

 

 

Ball and socket (diarthrosis) joints allow good mobility. These joints consist of a capsule, an articular cavity containing synovial fluid and articular cartilage covering the adjoining two bones. Gliding joints occur between the surfaces of two flat bones that are held together by ligaments. Some of the bones in the wrists and ankles move by gliding against each other.

 

 

References:

 

 

Agur, A.M.R., Dalley, A.F. (2004) GrantÌs Atlas of Anatomy (11th ed). Philadelphia, PA: Lippincott Williams and Wilkins.

 

 

Farhi, D.C. et al (2004) Pathology of Bone Marrow and Blood Cells. Philadelphia, PA: Lippincott Williams and Wilkins.

 

 

McRae, R. (2006) Pocket Book of Orthopaedics and Fractures (2nd ed). Edinburgh: Churchill Livingstone.

 

 

Tortora, G.J., Grabowski, S.R. (2003) Principles of Anatomy and Physiology (10th ed). New York, NY: John Wiley and Sons.

  • Comment

Have your say

You must sign in to make a comment

Please remember that the submission of any material is governed by our Terms and Conditions and by submitting material you confirm your agreement to these Terms and Conditions. Links may be included in your comments but HTML is not permitted.

Related Jobs