Osteoblasts, Osteoclasts, Calcium, and Bone Remodeling
Impressive Structures With Multiple Functions
Bones are impressive structures that are even more amazing than many people realize. They provide attachment sites for muscles and enable us to move. Some, such as the skull and ribs, protect vital organs. They also make our blood cells, store minerals such as calcium and release them when necessary, and store lipids, which are an energy reserve.
One very important function of bone is to send calcium into the bloodstream when the body needs it. Calcium is a vital chemical in our bodies. It's necessary for muscle contraction, blood clotting, nerve conduction, and other functions. It also provides strength to bones and teeth.
Specialized cells called osteoclasts break down bone to free the calcium. Cells known as osteoblasts deposit calcium into bone, remaking it. The process of replacing old bone with new bone is known as remodeling.
Types of Bone Tissue
There are two types of bone tissue. The outer layer of a bone is composed of compact or cortical tissue. This is a dense material with low porosity. Spongy tissue (also called cancellous or trabecular tissue) forms the inner part of bones. It's made of a network of solid bone enclosing many pores. Marrow is located in these pores.
Bone marrow is red or yellow in color. The red type makes blood cells and the yellow type stores lipids (fats). Bones in different areas of the body have different proportions of compact and spongy tissue as well as different types of marrow.
Compact or Cortical Bone
The unit or building block of compact bone is a cylindrical structure called an osteon. The name comes from the Greek word for bone.
- An osteon contains a central canal called the Haversian canal. Blood vessels and nerves run through this structure.
- A Haversian canal is surrounded by circular, concentric layers of tissue called lamellae. The lamellae are made of a material called bone matrix.
- Bone matrix is made of a mineral called hydroxyapatite. This mineral contains calcium and phosphorus as well as a protein called collagen.
- Extending from the Haversian canal and through the lamellae are small horizontal canals called canaliculi.
- Lacunae are small cavities or chambers located between one lamella and the next. (The dark purple structures in the diagram above are the lacunae.) The osteocytes or mature bone cells are located in the lacunae.
- Osteocytes are star-shaped cells. They have long extensions that project into the canaliculi.
- The membrane that covers the outer surface of the bone is called the periosteum.
Spongy, Cancellous, or Trabecular Bone
Spongy bone looks like a honeycomb or latticework. Each rod of bone is called a trabecula or a spicule. Trabeculae don't contain osteons or Haversian canals. They do contain lamellae, or layers of bone matrix, but the lamellae are parallel to each other. The matrix contains lacunae and canaliculi, as well as osteocytes, osteoblasts, and osteoclasts. Nutrients move from the marrow in the pores of the lattice into the trabeculae. The nutrients nourish the cells in the bone.
When the amount of bone manufacture equals the amount of disintegration, the mass of a bone remains the same. At certain stages of our lives or under certain conditions, however, the amounts are different. In these cases a bone's mass will change.
Functions of Osteoblasts and Osteoclasts
Osteoblasts build new bone matrix and osteoclasts break it down. (I remember the difference in the words' meanings by the fact that the letter b in "osteoblast" is also the first letter of the word "build".)
The creation and destruction of bone, the communication between its cells, and the signaling processes that occur are complex activities. Scientists have discovered that osteoblasts make a protein hormone, which is known as osteocalcin. The functions of this hormone and the activities that take place in bone are still being investigated.
Osteoblasts are cuboidal cells that work as a group to form new bone. They contain a large quantity of rough endoplasmic reticulum, which makes and transports proteins. They also have a large Golgi complex, which acts as a packaging area for products manufactured by the cell.
The osteoblasts move over the matrix of a bone and deposit a protein mixture called osteoid. The osteoid contains a protein called collagen as its major component. Then the osteoblasts deposit minerals—including calcium—into the osteoid to make bone. The new material fills in the cavity formed by osteoclasts.
Some osteoblasts become trapped in the bone matrix and are transformed into osteocytes inside lacunae. Osteocytes are thought to be sensory cells that are involved in signaling processes inside the bone. They connect to other osteocytes through their projections, which extend through the canaliculi. They are the most abundant cells in bone and also appear to have the longest lifespan.
Some osteoblasts become flattened and turn into lining cells that cover the surface of the matrix instead of being transformed into osteocytes.
Unlike osteoblasts, osteoclasts contain more than one nucleus. They are large cells produced by the fusion of several smaller ones. Osteoclasts travel over the surface of the bone matrix and secrete acids and enzymes to disintegrate it, forming a little pit on the surface of the bone.
As an osteoclast becomes active, the surface that is contact with bone becomes ruffled. This increases the surface area for absorption of minerals. The minerals (in their ionic form) are absorbed into the osteoclast, which later releases them into the tissue fluid located between cells. From there the ions enter the blood. The process of bone breakdown and mineral uptake by the osteoclasts is known as resorption.
Hormonal Control of Calcium Deposition and Release
The parathyroid glands make a hormone called parathyroid hormone (also known as PTH or parathormone, which stimulates the action of osteoclasts when the amount of calcium in the blood falls. The hormone causes the transfer of calcium from bone to blood. On the other hand, the thyroid gland makes a hormone called calcitonin which slows the activity of osteoclasts, decreasing bone breakdown. Parathyroid hormone seems to be the more significant of the two hormones.
Estrogen in females and testosterone in males help to maintain bone strength. Other hormones that have an influence on bone mass are growth hormone, which is made by the pituitary gland, and cortisol, which is made by the adrenal gland. Growth hormone increases bone mass while excess cortisol decreases it.
Bone Production and Resorption
In general, when someone performs regular weight-bearing exercise, the amount of bone production exceeds the amount of resorption and bones increase in size. On the other hand, if someone is bedridden, production falls and the net effect is bone loss.
Our stage of life also influences the behavior of our bones. Bone production predominates during growth while resorption tends to predominate as we age. Researchers have found that the amount of resorption becomes larger than the amount of production in our mid thirties, although the difference doesn't become significant until our forties or fifties. A nutritious diet, moderate exercise of the correct type, and a healthy lifestyle can slow resorption and stimulate the production of new bone as we age.
Worldwide, osteoporosis causes more than 8.9 million fractures annually, resulting in an osteoporotic fracture every 3 seconds— International Osteoporosis Foundation
What Is Osteoporosis?
Osteoporosis is a disorder in which the bones become unusually porous and brittle and bone density decreases. The condition generally appears in older people, although it occurs in young ones as well. In osteoporosis the amount of bone resorption is much higher than the amount of bone production.
Osteoporosis can occur in both males and females, but it's most common in post-menopausal women. After menopause the amount of estrogen in a women's body decreases significantly, increasing the risk of weakened bones.
In the video below, a doctor describes osteoporosis. If you are concerned about the disorder, a visit to your own doctor would be a good idea. He or she will be able to diagnose the condition. The doctor will also be able to give you suggestions for dealing with the disorder, reducing the chance that osteoporosis will develop, and decreasing its damage if it does appear.
How to Help Keep Bones Strong
Some common tips for keeping bones strong are described below. They are also common suggestions for helping us to stay healthy in other respects. If you already have osteoporosis, however, don't do any exercises without consulting your doctor.
In order to strengthen bones and reduce the chance of osteoporosis development, good nutrition and sufficient (but not extreme) exercise are important. An adequate intake of both calcium and vitamin D is necessary. Vitamin D is needed for calcium absorption through the lining of the small intestine. Other nutrients are also needed for bone health, so a varied diet with lots of nutritious foods should be followed. Smoking should be avoided, since plenty of research shows that it weakens bones. Excessive alcohol intake does the same thing.
Someone who wants to use exercise as a tactic to prevent osteoporosis should do some research. Some types of exercise are great for maintaining general health but don't stimulate bone growth significantly. It's also important to do exercises that strengthen the high-risk bones for osteoporosis, which are the hips, spine, and wrists.
With care and effort, we can fight our body's tendency to reduce bone mass as we age. We can reduce the chance of developing osteoporosis and slow its progression if it's already developed. Medications that can treat osteoporosis exist, but prevention is better than treatment.
- Information about bone remodeling from the University of Washington
- Biology of bone tissue from the US National Library of Medicine
- The biology of bones from the International Osteoporosis Foundation
- Facts about maintaining bone strength from the National Osteoporosis Foundation
- Tips to keep your bones healthy from the Mayo Clinic
This content is accurate and true to the best of the author’s knowledge and does not substitute for diagnosis, prognosis, treatment, prescription, and/or dietary advice from a licensed health professional. Drugs, supplements, and natural remedies may have dangerous side effects. If pregnant or nursing, consult with a qualified provider on an individual basis. Seek immediate help if you are experiencing a medical emergency.
Questions & Answers
What are your thoughts on Xgeva?
I'm not the right person to ask, since I'm not a doctor. There are many factors involved in choosing a prescribed medication. The likelihood of a benefit for the patient's particular condition, the potential side effects, the potential for harm, the patient's other ailments, and their general state of health all play a role in the choice. Your family doctor or a specialist who is familiar with your case would be the best person to consult.Helpful 2
© 2013 Linda Crampton