By Elizabeth A. Murray, Mount St. Joseph University
Let’s talk a bit about the construction of bone. An organism’s skeleton system is made up of bones, cartilages, and ligament. Together, the entire framework facilitates the movement of an organism and takes part in many of its vital physiological functions.
Structure of Bones
Bone has widely scattered living cells that are surrounded by and embedded within a connective tissue matrix. For the most part, the cells produce this matrix bed, and then they lie in it. The matrix is mostly composed of a variety of calcium phosphate salts, collectively called hydroxyapatite. These minerals are crystallized around strands of the protein collagen.
Depending on age and health, the overall chemical breakdown of bone is generally about 50% inorganic minerals, around 25% organic compounds—mostly the collagen—and about 25% water—much of which is inside the bone cells.
Brittle yet Flexible
A good analogy is that the makeup of bone is similar to fiberglass. Bone’s collagen fibers, like the fibers in fiberglass, give bone some flexibility and resiliency.
The mineral part of the matrix, on the other hand, gives bone its strength—just like the glass in fiberglass—but it’s brittle. It’s the combination of the flexible collagen and strong minerals that give bone its remarkable properties.
Experiment to Understand Bone’s Flexibility
To understand bone’s makeup and resulting properties, we can conduct an experiment. Get a pair of chicken leg or thigh bones; both bones should be from the same animal and be about the same size. Remove the meat and then put one of the bones in a container of vinegar, for maybe a week or so. Cover the container with a lid to avoid the pungent smell.
After a while, when you take it out, the bone will have the same shape, but it will be flexible—you might even be able to tie it in a knot! This is because the acids in the vinegar demineralize the bone—in other words, the vinegar leaches out the calcium in the bone matrix, leaving all the flexibility of bone, but without the strength.
Experiment to Understand Bone’s Brittleness
Take the other bone of the pair and put it in a 400° oven for an hour or so. When it cools off, you’ll find that the bone is really brittle, and might even scratch easily with your fingernail or a fork.
This is because the heat denatured the collagen proteins and cooked off all the water, leaving only the brittle matrix behind. Remember, the minerals give bone its strength, while the collagen provides some flexibility—and it’s the combination of the two that gives support with resiliency.
This article comes directly from content in the video series How We Move: The Gross Anatomy of Motion. Watch it now, on Wondrium.
Remodeling of Bones
Bones majorly comprise and keep stocks of calcium. If we go without dairy products, without sardines or canned salmon with its small edible bones, or if we eat no dark, leafy greens—then blood calcium levels fall.
In this case, we have hormones that tell other bone cells, called osteoclasts, to get some of the calcium out of bone storage and put it back into the blood. So, they take just what they need to keep the blood calcium levels at their optimal range for all of the other body processes that need it.
This cycling of minerals is called bone remodeling, and it’s an important part of bone maintenance. About 10% of bone mineral is turned over each year—so your skeleton is constantly renewed by the process of remodeling.
Bone Density Loss
If we don’t take in enough dietary calcium to offset our body’s daily needs, then more calcium is taken from our bones than we replace through our diet. Hence, bones steadily lose their mineral content.
And while we can limit or slow bone loss to some degree with weight-bearing exercise and a good diet, a certain amount of bone loss is inevitable, especially since the ability to build new bone starts a steady decline past around age 40.
Factors Affecting Bone Loss
There are factors and habits that can negatively impact bone strength, and even accelerate bone loss. These include smoking, excessive alcohol consumption, poor diet, and a sedentary lifestyle. But even without any of those negative factors, at some point, bone mineral loss is inevitable.
Women, in particular, have accelerated bone loss after they hit menopause, which on a worldwide average happens at about age 50—occurring at slightly older ages in developed countries, and a bit earlier in less-developed parts of the world.
Osteoporosis and Pathological Fractures
The postmenopausal bone loss is because the female hormone estrogen has protective effects that keep bone strong, but estrogen is no longer produced after menopause. At that point, women begin to lose bone minerals at an increased rate compared to before menopause, or when compared to men of the same age.
Given that women can spend about a third of their lives in a postmenopausal state, osteoporosis is a global health concern for them. Men also have bone loss, and by about age 65 or 70, their rates of skeletal demineralization are about the same as in women, leading to osteoporosis.
Since women started their bone loss sooner, however, they are at greater risk of more advanced osteoporosis. And when bone is demineralized by osteoporosis, someone’s own body weight can be enough to cause a fracture.
This is called a pathological fracture, because the bone was already weakened by another condition, as opposed to a fracture caused by trauma. When bones are that weak, they have difficulty healing, especially if the blood supply to the bone is disrupted by the fracture.
Common Questions about Understanding Bone Structure, Remodeling, and Bone Density
The bone matrix is mostly composed of a variety of calcium phosphate salts, collectively called hydroxyapatite.
A fracture that is caused when the bone was already weakened by another condition, as opposed to a fracture caused by trauma, is called pathological fracture.
The factors affecting bone loss include smoking, excessive alcohol consumption, poor diet, and a sedentary lifestyle. But even without any of those negative factors, at some point, bone mineral loss is inevitable.
