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Bone
Article by: David M. Findlay, Department of Orthopaedics and Trauma, University of Adelaide, Adelaide, Australia
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The hard connective tissue that, together with cartilage, forms
the skeleton of humans and other vertebrates. Bone (Fig. 1) is
made of calcium phosphate crystals arranged on a protein scaf-
fold and performs a variety of functions. For example, bone has
a structural and mechanical role, protects vital organs, provides
a site for the production of blood cells, and serves as a reserve
of calcium. Bone tissue is constantly renewing itself, and it has
great capacity to respond to altered stresses and loads and
to repair fractures. Specifically, in the 19th century, German
anatomist and surgeon Julius Wolff proposed that bone tissue
will remodel itself in response to the mechanical demands and
stresses placed on it (Wolff 's law). If a bone is subjected to in-
creased stress, it will become denser and stronger. Conversely,
if stress on a bone is reduced, with time the bone may become
weaker. Wolff 's law provides a foundational concept that is
still widely relevant, however modern research has revealed
that bone adaptation is influenced by various factors beyond
just mechanical loading, including genetics, hormones, and
systemic health. The formation and remodeling of bone are ac-
complished by the action of bone cells of the osteoblast (bone-
forming cell) and osteoclast (bone-resorbing cell) lineages. The
activities of these cells are directed by hormones and other
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Key Concepts
• Bone is the hard connective tissue that serves as the chief component of the vertebrate skeleton.
• There are two types of skeletal bones: flat bones (for example, the skull and ribs) and long bones (for example,
the femur).
• Formed of calcium phosphate crystals arranged on a protein scaffold, bone protects vital organs, provides a site
for the production of blood cells, and serves as a reserve of calcium.
• Bone is a dynamic tissue and is constantly being remodeled by the actions of osteoclasts (bone-resorbing cells)
and osteoblasts (bone-forming cells).
• The most common bone disease is osteoporosis, in which there is a net loss of bone due to osteoclastic bone
resorption that is not completely matched by new bone formation.
Fig. 1: Diagrammatic representation of a long bone (in this case, a femur).
(Copyright © McGraw Hill)
