Bone Structure and Function

Functions of the skeleton
Bone architecture
Mineral Reservoir
Material properties of bone
Changes with aging
How to measure bone density

Functions of the skeleton

Bone, the material that makes vertebrates distinct from other animals, has evolved over several hundred million years to become a remarkable tissue. Bone is a material that has the same strength as cast iron, but achieves this while remaining as light as wood.

The front leg of a horse can withstand the loads generated while this 1500-pound animal travels at 30 miles per hour. The upper arm is able to keep birds aloft through entire migrations, sometimes over 10,000 miles without landing. The antlers of deer, used as weapons in territorial clashes with other deer, undergo tremendous impacts without fracturing, ready to fight another day.

At some point, unfortunately, forces of impact exceed even bone's ability to hold up. Falling on the ice, suffering a collision in a car or a tumble on the ski slopes can cause the bone to fail. While fractures are disastrous, bone - because it is a live tissue - almost instantly begins a healing process. Without question, bone is the ultimate biomaterial. It is light, strong, can adapt to its functional demands, and repair itself.
Functions of the skeleton
* Structural support for heart, lungs and marrow
* Protection for brain, uterus, and other internal organs
* Attachment sites for muscles allowing movement of limbs
* Mineral reservoir for calcium and phosphorus
* Defense against acidosis
* Trap for some dangerous minerals such as lead

Photo by Paul Crompton
©University of Wales College of Medicine

Bone architecture

There are two major kinds of bone, trabecular (spongy) and cortical. Trabecular bone gives supporting strength to the ends of the weight-bearing bone. The cortical (solid) bone on the outside forms the shaft of the long bone.

This xray of a femur shows the thick cortical bone, and the trabecular bone which is arranged to withstand the stresses from usual standing and walking. Compressive stresses are those of the body weight pushing the bone down, and tensile stresses are from the muscles, pulling the bone apart.

Photo courtesy of Clint Rubin
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Magnified view of a cut surface of bone, showing the cortical bone and trabecular bone surrounded by marrow tissue (M). This is from the iliac crest, part of the pelvic bone. The actual width is about 1 cm.

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Further magnification demonstrates the organization of the cortical bone into Haversian systems, consisting of concentric layers of bone and a central canal which supplies blood. The small black dots are spaces that contain osteocytes. The boundaries between Haversian systems are the cement lines.

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Mineral reservoir

In addition to its mechanical functions, the bone is a reservoir for minerals (a "metabolic" function). The bone stores 99% of the body's calcium and 85% of the phosphorus. It is very important to keep the blood level of calcium within a narrow range. If blood calcium gets too high or too low, the muscles and nerves will not function. In times of need, for example, during pregnancy, calcium can be removed from the bones. This process is carefully regulated by hormones and is discussed more completely in the section about hormones.

Bone material properties

Components of bone:

* The organic matrix is compsed primarily of the protein collagen which provides flexibility. 10% of adult bone mass is collagen. This is discussed further in the section on collagen and bone matrix
* The mineral component is composed of hydroxyapatite, which is an insoluble salt of calcium and phosphorus. About 65% of adult bone mass is hydroxyapatite.
* Bone also contains small amounts of magnesium, sodium, and bicarbonate.
* Water comprises approximately 25% of adult bone mass.

Changes with aging

This graph shows values for bone mineral density at the hip in Caucasian men and women and African-American men and women. With aging, bone density decreases in all groups. This inevitable bone loss is frequently the cause of osteoporosis. Data are from a study by Looker, Osteoporosis International 1998
This image shows trabecular bone structure in the lower spine of a young adult compared to an osteoporotic elderly adult.

Image courtesy of Ralph Mueller, Ph.D., Switzerland
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Young adult Elderly Adult with osteoporosis

How to measure bone mineral density

If you take a sample of bone and wash out the liquid bone marrow, the remaining bone is about half mineral and half protein. The weight of mineral per volume of bone is the Bone Mineral Density. This is determined by two things: how many mineral atoms are deposited within the bone matrix, and how porous the matrix is. The strength of the bone is largely determined by the bone mineral density.

©2004 by Jane Lian, Jeffrey Gorski and Susan Ott
Updated 1/16/04

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