Bone

The vertebrate body is supported by an endoskeleton made of cartilage and bone. (Sharks and their relatives use only cartilage.)

The bones of the human skeleton perform several functions:





Structure

The diagram below depicts the structure of a typical long bone such as the femur.

Physiology

Looking at a skeleton, bone seems an inert thing. But in the living body it is anything but.

The size and shape of bones not only changes during growth, but for the rest one's life it is continuously being remodeled in response to the stresses put on it. Approximately 10% of your bone mass is removed and replaced each year.

The remodeling of bone requires the coordinated activity of two types of cells:

Osteoclasts are derived from stem cells in the bone marrow — the same ones produce monocytes and macrophages. [Link]

Excess activity of osteoclasts (common after menopause in women) produces osteoporosis. The bones become weakened as cortical bone gets thinner and the spaces in spongy bone get larger.

Reduced activity of osteoclasts produces osteopetrosis. This occurs when a person inherits a mutant version(s) of one or another of the genes needed for normal osteoclast function. Inhibition of osteoclast function causes too much bone to form leading to

Hormones and Bone

Many hormones affect bone growth and remodeling.

The links above will take you to descriptions of how these hormones act.

Bone also produces hormones thus is itself an endocrine organ.

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3 October 2013